Can I use a big bore piston/cylinder on a stock crank?

Will'smotobikes19

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I weighed both the stock piston with the piston pin and rings and the big bore windowed piston and they are both 3.7 oz. My question is can I use the big bore cylinder and piston without using the crank that I bought. The crank needs trued by a machine shop since I was a noob at engine building and hammered it too much probably. I bought a pk80 style engine with 8mm mount studs and was hoping to just install the big bore cylinder and piston on that. The only thing is that engine is silver while the big bore cylinder is black. Maybe I could take the black covers and put it on the silver one. If both pistons weigh 3.7 oz there should be no problem right? I heard it's the weight that can unbalance the crank or whatever. I know it would have more power with the stroker crank but I don't really want to take it to a machine shop right now. I might use it in the future on a stock 66cc engine.
 


crassius

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no idea what you have, but the name 'stroker crank' seems to imply a diff stroke length - this would usually be done by changing the position of crank journal - I doubt your old crank has that
 

FNTPuck

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Is the piston crown and pin height identical?

If the piston is designed for a longer crank throw, it would be shorter(or have higher pin) leaving it farther below the deck and killing your compression. On the flip side, if the piston is taller or has a lower pin height, it will stick up too far and likely hit the spark plug or chamber.

I would lean towards running it with the proper crank, but without personal experience this is just a suggestion.
 

Will'smotobikes19

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I compared a stock 66cc piston and the big bore piston and they look about the same height from hole to crown. It has a 40mm stroke ZL rod I calculated it and displacement should be 72cc instead of 74cc with the 40mm rod. The intake side is windowed and the bottom of the skirt is ground down slightly. Both weigh the same though so it should work fine. Haven't put the cylinder on yet could I adjust for the longer crank throw? it has 2 base gaskets my guess is because it has a 40mm stroke. The black engine I got had a ZAE 50 rod which I'm pretty sure is a 38mm stroke and that engine only had one base gasket.
 
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gary55

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Put it together with the head off and turn it over. Look at where the piston is at TDC. then make a decision. Or show us a pic. There are more variables than your listing, not to mention their's no such thing as a 40 mm rod. 85mm or 89mm. The ZL is 89mm rod with a long stroke (40mm) crank. I won't go into how the short rod was the long stroke engine before the a80's and zedas. Just put it together and eliminate the guess work.
 

Will'smotobikes19

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Oh ok stroke is different than total rod length I see now. You know what I meant but ya I was really hoping to use the 41mm stroker crank but I made it out of true by hammering it. (didn't cool or heat crank half or crankshaft) noob mistake. Quick question how many mm is the 38mm stroke rod? is it the 85mm?
 

Will'smotobikes19

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Bottom pic is where bdc should be the cylinder wasn't against the case though. Definitely a problem here. I don't know if adding base gaskets would help or what.
 

gary55

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Oh ok stroke is different than total rod length I see now. You know what I meant but ya I was really hoping to use the 41mm stroker crank but I made it out of true by hammering it. (didn't cool or heat crank half or crankshaft) noob mistake. Quick question how many mm is the 38mm stroke rod? is it the 85mm?
The GT5 used 38mm stroke B piston long rod. The 1st gen pk80's used 40mm stroke short rod A piston. You have the 2nd gen pk80 which is really an A80. That baby is really sticking out there. It looks to me your going to need a short rod bottom end or possibly a adapter plate at the base with a gasket on each side. That looks like the big bore piston has a little less meat between the ring and the crown and at around 4mm to high that is a lot of base gaskets.
 
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