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Post by captincvmn on Aug 8, 2021 9:22:00 GMT -5
So lately it seems there has been a smattering on board of ruined cylinder kits. Some are salvageable and some are not.
My line of inquiry is this: how interchangeable are 70cc kit pistons? Is the pin to piston top measurement the same on all the Minarelli pistons for a standard stroke crank set? The outside piston diameters most likely vary somewhat but marginally. Is the limiting factor the placement of the roll pin to stop ring rotation around the piston in relation to the cylinder ports?
My inquiring mind wants to know.
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Post by oldgeek on Aug 8, 2021 11:41:06 GMT -5
I dont believe there is any "standard" for any measurement, it just depends on the kit design. Everything you mentioned, overall diameter, pin to piston top distance and ring locating pins have to be considered. Add to that the piston skirt length. Too long and you can trim it, too short and it will not cover the exhaust port at TDC as I have found out a couple times.
Certain mismatches can be worked out by adding base spacers or gaskets, head spacers or step cut heads that extend into the cylinder, even taking a small amount off of the piston crown can get you where you need to be. Measuring durations may be necessary unless it is a straight up easy swap.
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Post by 90GTVert on Aug 8, 2021 11:50:16 GMT -5
Compression height varies. That's the pin to crown measurement (center of pin bore to edge of crown). Shape of the crown can vary as well, though most seem to be similar enough to not be a deal breaker. Small differences in compression height can probably even be dealt with simply by changing gasket thickness.
You could also have pistons that have different skirt lengths. If buying replacements for other like-style engines, then it probably won't be a big issue, but if you go looking for any say 47mm piston kit then it could. You'd need to be sure skirts are long enough to cover ports and short enough not to make contact with the crank.
The bore is critical. "70cc" is a round term. There are different bores that fall into that category (46, 47, 47.6, 48), so you have to know the actual bore diameter. Clearances may not be ideal swapping one 47mm piston for another, but they usually seem close enough.
The locator pin could be an issue because of port layout.
The easy method is to get a direct replacement piston. If that's not an option or you're looking to save money, then make sure you check this stuff. Even if ordering a direct replacement, it's not a bad idea to see if there are multiple versions (like A, B, C, D) with slight oversizing. Take precise measurements and see what you need to get back to original clearance specs. It's really helpful to measure a brand new kit so you know what you've got day 1 and what the manufacturer intended for clearance, but most of us slap them right on.
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Post by 190mech on Aug 8, 2021 16:34:46 GMT -5
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Post by ThaiGyro on Aug 10, 2021 16:09:23 GMT -5
Haha! Inside that rabbit hole is a mole a vole and a rat...Here is another thing to ponder:
Where do you measure from? And to? My example is that I never use a false center to center when spacing/sizing two or more holes. I measure "edge to edge" on the same plane. Maybe the human eye cannot detect it well...but measuring from vapor space to something else can induce issues.
Brent said it well above. Most of us just put them in and go. In racing 2T days, we never took that plan. We measured EVERYTHING! Pain in the arse!
Me thinks the actual pin is rarely the issue, (it is hardened, after all) but the hole it is stuffed into has loads of room to be off. Component spec matching is not a street thing! (But is recommended)
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