Modders guide to Advanced Engine Porting

Discussion in 'General Tuning' started by spectroplasm, Jan 11, 2008.

  1. spectroplasm

    spectroplasm Member

    pug309 GTI / PGO bmx
    I thought i'd contribute to our ever wonderful forums by writing up a small article to those who would like to know more or are just curious on how the engine works and how the engine kit makers rake in your dough through trade secrets :), it's not a complete list but i'll add to it as i get the time to write them so bear with me folks :)

    What is compression you might ask?, well in short words it is the fruit of compacting or compressing something in a smaller form than it already is

    in engine terms compression is the pressure obtain from compacting the air and fuel in the engine (the barrel).
    Compression varies from scoot to scoot hence a standard 50cc barrel will have lower compression than a tuned barrel. this is because compression is one key to having a faster stronger scooter, the areas that affect compression are:

    - air density (more compact air give higher compression).
    - compression ratio (this is the level of compression eg: how many times it is compressed compared to it's original size 20:1 means 20 times it's size)
    - cylinder bore (a bigger bore can eat more gas and air)
    - stroke (how far up the barrel the crank can push the piston head, the farther the more compression you have, but be careful a too high a stroke will

    produce more heat due to ring friction and hightened compression, sometimes you will detonate your piston or blow your engine)
    - squish clearance (this determines the area above the piston head and your barrel head, lower squish give more compression and vice versa)
    - exhaust port timing (from the moment the exhaust closes to the moment the spark lights up this is your compression time or build phase)

    so now that we know what compression is some might want to ask how to measure engine compression?, well if you're a guy who relies on your senses to test

    your engine then the easiest way is to remove your spark plug and put your finger in the hole (now lets use common sense here people if you got small fingers

    then use the biggest one you have till you jam it it the hole, i don't wanna hear someone say "spec! you cunt you told me to put my finger in and the piston bashed it to bits") next have some one kick the kick or just run your starter very briefly, if your finger flies out of the hole your engine is it good

    condition, if your finger moves out and the engine makes a "pffft pffft pffft" sound from your finger you need to check your engine for wear, if you finger

    don't move or you just feel a hump you need new rings or piston/ring.

    if you have access to a probe then screw it in place of the spark plug and kick the engine or start your starter, read the compression gauge it should give you a nominal read of 20~34psi depending on your engine make and bore size etc.

    another way is to calculate manually by taking the geometry of your engine and multiply it by 15.

    and now on with the game :) after shedding light on what compression is and what it does we will now look into the compression ratio
    in an engine there is 2 types of compression ratio
    1: the distance from the BDC (bottom dead center) all the way to your TDC (top dead center) of your cylinder, this is called the geometric compression ratio or GCR for short.
    2: the distance from the exhaust port lock (where it closes) to your TDC, this is called the Trapped compression ratio or TCR for short.

    compression ratio varies upon these factors:
    - bore size
    - stroke length
    - squish clearance height
    - combustion volume
    - piston type dome/flat
    - exhaust port height

    a general rule of thumb by which most people measure the compression is by adding 1psi for every .002mm removed from you TDC barrel

    compression gives you torque
    for drag racers you need all the torque you can get to propulse you bike as fast as possible, but in the process of doing so your will also have a faster heat build up (heat=eeeeviiil).

    circuit racers don't need important torque but instead a higher rpm.

    well that's the first part of our engine porting thread m8s i'm getting the second set ready this afternoon and will post it up asap :)

    here is the second part

    Blow Down Time (BDT)
    Blow Down Time is the distance in degrees between the opening of the exhaust port to the opening of the transfer ports. BDT (in degrees) controls the time

    that a cylinder has to empty itself of exhaust before the transfer ports open and allow the fresh fuel-air mixture into the cylinder.
    In order for the fuel air mixture to move into the cylinder, the pressure inside the cylinder must drop below that of the crankcase. For example, a fully tuned engine may have peak cylinder pressure of approximately 700~775 psi and that pressure reduces as the piston travels downward. At 15,000 RPM's the engines BDTiming has only .000244 seconds to empty the remaining cylinder pressure to less then approx. 20psi .in order for the transfers to start flowing fresh air mixture into the cylinder when the ports open again. As RPM's increase, shorter BDT's reduce the possibility that the exhaust will have enough time to leave the cylinder before the transfer ports open and increase the possibility of the exhaust and fresh charge mixing in the cylinder and decreasing the engines power potential.

    Increases in BDT extends an engines RPM potential by allowing more time for the exhaust and pressure to leave the cylinder before the incoming fresh charge enters the cylinder. But you would think that increasing the BDT would be the easiest way to increase your power. The power you gain will also affect your power stroke greatly (The power stroke is the distance from (peak pressure) TDC to EO (exhaust port open),is is the pressure from expanding combustion, pushing down on the piston, creating the torque transmitted by the crankshaft to your variator to clutch), this is due to raising the exhaust port. One should never go about without thorough testing each change (a dyno is strongly recommended, if you do not have access to a dyno you can still simulate the test by having a spare rim and tire for the rear wheel, fill the tire with lead bits and sand it should weigh approx 40~50kg this will simulate the drag from the bike and you can get a rough approximation of the bike in a real racing situation), even the slightest change can make a great difference (always test by half degrees, NEVER do a full degree mod unless you are absolutely sure of the outcome).

    Areas that [affect/affected by] BDT are:
    - exhaust port opening
    - transfer port opening
    - power stroke
    - crankcase compression
    - type of exhaust system.

    all in all the BDT has some control over what kind of power the cylinder will produce. Lower BDT's will make more torque and have a lower RPM potential. and the opposite will produce more peak horsepower and have a greater RPM potential.Reducing the power stroke by small amounts at first may help the power. On engines that produce very little power to begin with small changes to BDT can have big repercussions. Reducing the BDT a little more and you may loose bottom end power. Raising the exhaust port even more may decrease the power through out the RPM range.

    **part three finished below :)

    Crankcase Compression Ratio (CCR)

    never thought there would be so much compression in your bike eh? :) don't worry it's all there for a cause, CCR has the role of sucking the fuel and air mixture from your reeds and pushes them to your transfer ports when your bike hits BDC. your CCR is never the same (this is especially true if you are moving from stock to street/race kits) depending on the changes you have made to your barrel, your CCR changes too. depending on the compression Ratio(CR),Blow Down Time(BDT), and your exhaust port area, the pressure made by your cylinder may overtake that of your crankcase. In such an event this will cause a feedback or a delay in the scavenging charge going to the cylinder. a common remedy would be to increase CCR, BDT, and finally exhaust port timing. Though there is always a trade off because too much CCR will hurt the intake ports flow of air/fuel mixture from the carb due to insufficient port velocity to overcome the back flow from the crankcase. this is also caused if you have exaggerated intake port timings.

    Factors that control your CCR:

    - Bore size
    - Stroke length
    - Type of Crank (half moon, full moon , pork chop, etc
    - Crankcase volume

    Crank case compression ratio is measured by taking all the area under the piston crown at TDC divided by the the area under the piston crown at BDC, this is approximative because not all engines have the same crankcase volume.

    Crankcase Pressure Time (CPT)

    CPT is the amount of time in degrees, that the engine has to build up enough pressure to send the fresh fuel-air mixture through the transfer ports and into the cylinder when the ports open. CPT is measured from the "closing of the intake port" (IC), to the "opening of the transfer ports"(TO). Depending on how the engine is ported and what type of stroke is being used, a <100cc scooter engine may have as little as 30 degrees and a >100cc scooter can have up to 65 degrees to build up the pressure to force the scavenging flow out the transfer ports when they open. Since our piston port engines have symmetrical timing on the transfer port and intake port. Any increases of either intake or transfer port timing, will decrease CPT. Examples of this would be, cutting the intake skirt or installing a stroker crank will decrease CPT. Normal ranges for CPT is 38' to 48', lower degrees can give more peak power and the opposite will produce more torque.

    Factors affecting CPT are:

    - intake port duration
    - transfer port duration
    - crankcase compression

    **part four finished below :)

    Exhaust Port Duration (EPD)

    The exhaust port duration or EPD for short is another major factor in the power potential of the engine. Like the transfer ports, the exhaust port needs enough area to allow pressure from the power stroke to escape before the transfer ports open, this is also the first thing tunners will modify when porting an engine. A full kit 70/80cc engine's exhaust port @ 15,000rpms will open and close in a mere .0017 seconds (that's 3 times faster than the exhaust duration of a Porsche 911 GT turbo!!!) and ~.0010 /.0011 seconds @ 18,000rpms (F1 class engines :eek: woot!)

    Factors affecting EPD:
    - port opening
    - blow down time.

    Exhaust Port Shape and Short Circuiting (aka fuel feedback cancellation)

    The exhaust port shape is the key element that controls both your engine's noise (high pitch, low pitch), your power band, and the amount of short circuiting you'll get. It is also, in little part, responsible for your engine's power delivery.

    A faster port clearance (port that opens quickly) will produce a higher pitched noise thus making you have a louder engine, and also will give you a hard hitting power band (the type that tugs you forward).

    A slower port clearance (aka gradual port clearance) will produce a sluggish engine and a more pleasant sound (all stock scooter engines have gradual clearance <---this is a must since this is the only way to have your bike street legal.

    Exhaust ports that are very wide or have been lowered to the piston crown @ BDC will have a greater tendency to short circuit the fresh fuel-air flows from the transfer ports, out the exhaust port (this is the nastiest thing to happen since you will think it's you pipe that is at fault).

    **Part five coming up later today :)
    **Part five done :)

    Intake port duration or IPD

    The intake port consumes the fresh fuel air mixture the engine uses to produce its power. The port duration determines the power and RPM potential of the engine. The problem with piston port intake systems is the timing is symmetrical. The sooner you open the port, the later it closes.

    The later the engine closes the intake port, the shorter the time (in degrees) the engine has to build up pressure to send the fuel through the transfer ports. This is "Crankcase Pressure Time". Increases in intake port timing decreases the CPT. Some engines have such low intake port timing and high CPT that small changes with intake port timing can make a significant difference. The intake port must be a balance of the need for power @ a given RPM vs. torque required for a satisfactory low & mid range power.

    70~100cc ported engines that have a port opening around say 90' TDC (the piston is on the up stroke) and has had sufficient time to build up vacuum to suck in the fuel air mixture, will have problems on the down stroke. This is because the engine has had 90' of time to build up pressure inside the crankcase and it wants to release it all. With the intake port still open, a back flow through the intake port at low and possibly the mid RPM range. The back flow will stop when the incoming fuel air mixture velocity is greater than the back flow. Any engine with that much intake timing would have terrible bottom end power, some engines may not even be able to start running. If this happens to you check your CCR, jet size, carb size as these are the first things at fault.

    As a general rule of thumb: timings with openings less than 68' TDC make good torque, Openings from 68' to 75' TDC have good overall power and openings from 76' to 82' TDC can have substantially better top end with a corresponding loss of bottom end.

    factors affecting the Intake port duration IPD:
    - crankcase compression
    - carb size and type
    - intake manifold
    - air filter

    **stay tuned for more :)
    **last part here :p
    Short circuiting
    Refers to the fresh scavenging flow that enters the cylinder and the possibility that a portion of that fresh fuel air mixture may by-pass its normal looping flow and exit out the exhaust port. Fresh fuel air mixture that has short circuited and remains unburned is power lost. When modifying an engine, it is wise to be aware of the possibility of short circuits. Race kit barrels have been meticulously designed to prevent this as much as possible. this can be remedied by adjusting your transfer port angles :).

    Factors affecting short Circuiting:
    - exhaust port shape and width
    - transfer port angles

    Squish Clearances

    Decreasing squish clearances is normally a quick and easy way to pick up more power. By decreasing the clearance, it promotes better combustion and reduces the possibility of detonation.
    .020" clearance works well on 70~100cc barrels (on larger engines .030" to .040" is better). Clearance of some kind is required because as engine components heat up, they grow and the connecting rod stretches with RPM. Without the clearance, at high RPM the piston will hit the head.

    One fast clean way to measure the clearance is with a piece of rosin core solder (try a small diameter piece first, like 1/32" , then 1/16" if it doesn't touch) bent to an "L" shape and inserted through the spark plug hole, directly above the piston pin and against the cylinder wall. Then turn the engine over (use your hand to turn the variator), pull out the solder and measure the thinnest part and that will be your clearance, easy as pie :).

    Transfer Port Duration (TPD)

    Transfer port duration is another of the main controlling factors for type of power and RPM potential of an engine. As engine RPM and power increases, so does the need for fuel to supply that need.The time that the transfer ports can deliver the fuel decreases with RPM.

    Let's take an example: an engine with 120' of transfer port duration has only .004 seconds to deliver it's fresh fuel air charge at 5,000 RPM. At 10,000 RPM, naturally it's 1/2 that time at .002 seconds and at 15,000 RPM's it's only .00133 seconds. In that time the port is only wide open for exactly .0008 sec at 10,000RPM. Damn... crazy stuff indeed :eek: :eek: :eek: .Since time is the problem, transfer ports have a certain amount of "Effective Area" or "Time-Area" to obtain a given horsepower and RPM potential. Increases in area, multiplied by the number of ports and duration are part of the answer.

    Transfer Port Angles

    The transfer port angles have a relationship with the kind of power an engine will produce. Transfer port angles that are close to right angles of the exhaust port, produce more mid to top end power. Ports that are angled back towards the intake side of the cylinder will produce a broader range of power

    Power Stroke

    Power stroke is the duration of the pressure applied to the piston during combustion, in degrees from peak pressure to the exhaust port opening. power stroke is an important factor to consider for scooters. Increases in exhaust port duration and blow down time will significantly reduce the power stroke, while stroker cranks and decreases in deck clearances can increase the power stroke.

    A thing to avoid is to copy big engine setups (125+cc) to our scooters (50 < 125cc) as such changes will have dangerous effects on the bike.

    Factors affecting Power Stroke:
    - exhaust port opening
    - stroke and ignition timing
    - blow down time
    - squish clearance
    - deck height

    Rod length

    Rod length plays the variable element in trigonometric equations to determine a ports opening in degrees from TDC. It is also used to change or compliment how an engine delivers its power. The shorter the connecting rod is, the less time the piston spends at TDC, your engine tends to build primary and secondary compression faster, will effect the port timing and will accelerate faster off a corner. A long rod will produce a more "tractable" power band, the piston will spend more time at TDC and BDC and will affect the port timing.

    This is the secret to the 2fast and Cristofolini barrels, they have been honed to make use of the longer rod they came with :)

    Port chamfer

    A port chamfer is the rounded edge all ports must have to prevent the ring or piston from catching the port. Chamfering the ports is necessary when any of the ports have been cut to change the shape or timing. There are many techniques used by porting technicians to chamfer a cylinder, many depend on how mild or extreme a cylinder has been ported.

    ***warning*** Many race organizations have written limits to chamfering in to their rule books for classes that run stock cylinders, because exaggerated port chamfers can change a ports opening and duration, thus giving you an unfair advantage over the rest of the riders(but who gives a crap anyhow ;), i don't attend races or only attend street races for the fun of it :p ).

    Piston overlap

    Piston Overlap is a rare thing to hear :) no one really knows it except for the serious makes, but it is a measurement that is built into every engine. It refers to the amount of overlap the pistons bottom edge has over the exhaust port at TDC. Pistons skirts are designed to be long enough to prevent the exhaust port from opening at TDC. Typical overlaps are in the 1mm or .040" range. Normally piston overlap isn't an issue, until you install a 2 mm stroker crank. Small exhaust port openings at TDC are acceptable in the .005" to .010" range, but excessive openings may lead to a blown piston head (air cooled engines) or detonation (liquid cooled engines).

    Piston pin offset

    Piston pin offset is designed into a piston to prevent excessive wear to the exhaust port side of the cylinder and piston of high output engines. Offset normally falls within the range of .5 to 2mm, towards the intake side of the piston. Scooters have centered piston pins because of their small engine size but certain makes like Polini,MHR,Hebo Manston follow the .2mm guidlines to an extent depending on their cylinders :).

    Loop porting

    Loop Porting refers to a type of cylinder port arrangement that aims the scavenging flows to "loop" through the cylinder and combustion chamber to help push the spent gasses (exhaust) out the exhaust port. There are 2 major types of 2 stroke engines, schnurle(aka loop port) and cross flow. Cross flow are old and outdated by now they are used in chain saws and model airplane engines. Many cross flow engine have approximately 4 to 5 small transfer ports on the intake side of the cylinder and 4 to 5 small exhaust ports on the opposite side. The cross flow engine uses a piston with a deflector that aims the scavenging flows upward to help chase the spent gases out the many exhaust ports. But the pistons were heavy, the engines were very inefficient, and farts a lot too.


    This is our final subject in the article folks :), Friction tho underviewed by most people is the major factor of heat creation, power consumption (power as in hp not volts), and parts wear. all moving parts of the scooter is under the influence of frictional forces. bearings, rings, shafts, barrel, belt, weights, brake/clutch shoes.

    Friction in the barrel is the key note to power loss and sadly this loss makes a gap for every boost in rpm you gain, this is why mono ring pistons exist :) to lesses the gap in power loss for every extra rpm gained. since nothing is perfect you should take into account that even with a reinforced crank you piston is never straight in it's path in the cylinder. to remedy this it is always better to check you central bearings, on the contrary as some people may think your stock crank can take ~14,000rpms if you care enough for your bearings :), changing the stock crank's central bearings to a harder one will also give you a mid race class crank (so forget buying those expensive cranks, you know it's not worth it and they pray hard you never know about this free upgrade). So in closing end to our long article I would like to point out some small tips to those who wish to enter the fun world of engine porting.

    - when doing port jobs always move in .5mm measures
    - test after each job, note down your changes.
    - only make single changes at a time, this will save you HUGE amounts of time if ever something goes wrong.
    - if you don't have the money to invest but would like to have a go at engine porting do/change the following.
    * central bearings, press out the cental rod, take out bearings and switch with a harder one: my fav brands are SKF and FORD.
    * main bearings, NEVER use the stock ones if you wish to port engines, those things are just plain useless, switch with new ones.
    * Remove the guts of your stock pipe (no where as near as a tuned pipe but you will feel and hear the difference.
    * surface you cylinder head mount (the flat thing that you spark hooks up to)
    * slightly sand you variator plate by 10mm, this will steepen the curve giving you a faster grip
    * get bigger jets for your carb (or enlarge your stock ones with a needle drill, don't over do it tho only widen a little bit and test test test).

    - if you do have the dough to spare then the following will help you out a LOT
    * adjustable CDI (better types are those which have a serial interface)
    * Irridium point sparks (this sparks have the brightest spark and is also needed it you are thinking of hooking up a heat probe)
    * noiseless spark wire and cap, i use NGK racing spark cap and wire any good make will do with a total or better nominal filter level of ~7ohms.
    * internal Rotor ignition
    * a direct admission pipe, if you can't find one you can make one out of an aluminum block:
    +take the block and cut it to have 35 degree's angle. next up unscrew you existing admission pipe and mark the countour of your reed cage top with blue sealant, firmly and correctly place the aluminum block to the top of the reed cage and press for a second and pull it up straight (to avoid smugging the blue sealant), you should have the imprints of the countour on the aluminum block mark it over with permanent marker and cut through it. repeat for the top side of the aluminum block. once you finish this you will have your own homemade aluminum block admission :).

    here is my favorite scooter mod ever made by team cristofolini :), we can all do the same now if we have enough love and time and erm yeah money. this is what still drives me further to mod my scooter :) tho i strongly doubt id be able to ride that thing in an open road
    it's a beast of a scooter indeedy specs below

    Malaguti F12 with
    - 4x TCR 80cc cylinders
    - 4x 28mm Carburator
    - 4x TCR custom pipes w/o silencers
    - custom subframe
    - 350BHP
    - 0-100kmh in 2.056 seconds (a farking rocket :drool: )
    - Top speed 276kmh NOS less setup

    Happy modding mates :)
    Kieran and Aaron420e like this.
  2. capri dave

    capri dave Active Member

    Good article there...very informative :tup:
  3. spectroplasm

    spectroplasm Member

    pug309 GTI / PGO bmx
    thanx mate :) i'm still working of part four and so on until it's all finished up then i will clean it to make it more tidy. stay tuned for more
  4. Dec

    Dec Active Member

    nothing ugm
    spot on mate really helpfull :beer:
  5. shiny red

    shiny red New Member

    really good mate can not wait for the rest ??
  6. wobbly_trials

    wobbly_trials Active Member

    RD350, SR125, ICE125
    gotta say this is very informative and i respect you for taking the time to put this on the forum.

    hopefully it will encourage people to look further than bolt-on-bits.
  7. wobbly_trials

    wobbly_trials Active Member

    RD350, SR125, ICE125
    ok, start off by taking small ammounts from the exhaust port, initially don't make it any higher, jut begin to flatten it off to male it more of a square shape (still need some curve, maybe 1.5mm drop either side) then attempt to widen the port slightly, maybe 1/2 a mm to start with

    the trick is knowing where and how far to go with ports. and remember, catching the nickasil will cost about £80.

    the theory for all 2 stroke engines is the same but obviously the figures change with ccs and stroke.

    i can't give figures because i don't know and probably wouldn't if i did do.
  8. Haresign

    Haresign New Member

    GPR50 + Peugeot 307
    Thats a damn good article,

    You do this stuff for a living? you know your stuff

    p.s. wobbly, make it sticky! :pimp:
  9. wobbly_trials

    wobbly_trials Active Member

    RD350, SR125, ICE125
    i'll make it sticky once it's complete
  10. spectroplasm

    spectroplasm Member

    pug309 GTI / PGO bmx
    hey mate thanx for the rating :) i don't do it for a living i just do it for the passion, 2stroke engines are too good to let go of :) i started at the age of 19 and am 28 now still holding on to the dear little toys i have :p, parts four and five will be up today i promise :) stay tuned for more guys.
  11. wobbly_trials

    wobbly_trials Active Member

    RD350, SR125, ICE125
    all i can say is thank's for your input to the forum, when your'e happy that the content is complete i will sticky it as it's looking extremely informative so far.
  12. spectroplasm

    spectroplasm Member

    pug309 GTI / PGO bmx
    ok it's all done guys :) wobbly you can sticky now if it's ok :)
  13. Jamie

    Jamie Active Member

    xt660x, 200sx s14a

    wow. :wtf:

  14. Anonymous

    Anonymous Guest

    cheers mate helps a lot might save me a few £££ aswel very good and thanks for putting it up on the site :tup:
  15. webber

    webber New Member

    i really want to try this porting stuff.... i've read through this twice and searched other things on the net but yet it still makes no sense to me (i'm blonde? lol)... where do i start, is it worth doing it to a malossi MHR REP.? :wtf: :wtf:

    other than that a very detailed bit of information.


  16. waxhead

    waxhead Member

    Good interesting article

    Um im not sure that that the bike is really making 380 hp
    that would be 95hp from each 80cc cylinder
    Would be very impressive to say the least
  17. nibbles

    nibbles Active Member

    i thought it was running 120 n great article im starting my porting very soon so this will be very handy
  18. Kezie

    Kezie New Member

    01 Rox
    Very Very nice write up dude i wish i wasn't shaky Stevens or would do my own porting until that stops my parts will go to Stan Stevens
  19. MiNoR cOnFuSiOn

    MiNoR cOnFuSiOn Administrator Staff Member

    et2 monster
    good luck, he discribes scooters as "those shitty little things" and wont go near them....

    stan stevens is a gp/crosser tunner... why the hell would he bother with a scooter? this isnt a guess, ive spent many an hour chin wagging with him about 2t's (back befor i got into scooters in the first place)
  20. markymark2

    markymark2 Member

    excellent post , explains things fairly simply for those that have some knowledge already of how a 2t engine works , I always find it amazing how many young riders decide to tune / modify their scoots without any knowledge whatsoever and to be honest there is virtually nothing that can just be bolted on to produce more power without something else also needing changing / upgrading. A perfect example of this is the replacement of the stock pipe for a decent quality power pipe , the adverts would have you believe in many cases that a straight swop is all there is to it , if only ! , generally speaking , different / lighter rollers need fitting and the carb up-jetting , so therefore a knowledge of the engine , transmission and carburation are needed , not just the ability to undo approx. 5 nuts and bolts to swop pipes .That leads me to another point if the new pipe bought is "e" marked then it will be restricted and unless this restriction is removed very little gain in power will be made over the stock pipe , even this little job can be harder than one would think , often the new pipe will have a simple restrictor cone spot welded into the manifold but these can be a bugger to get out , use of a Dremel or suchlike is best I`ve found but if you are unsure some suppliers like Pedparts will for a small fee di it for you . Anyhow if you intend to get into some serious tuning including say porting work there are some excellent books out there on it and you can buy a couple of cheap 2nd hand cylinders to practice on before the final object , it`s amazing how a little work on the exhaust port in particular can make for significant gains even on 70cc sports kits .One last thing I find often over looked is checking that your stock crankshaft and bearings is capable of handling the power produced by the cylinder kit you are thinking of fitting , most 50/70cc sports kits work totally fine with stock cranks but when you move on to high end race kits you will need a new upgraded crank and boy oh boy that`s when the work really starts.

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