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A friend of mine has planned the base of is cylinder off 0,20...I reinstall the engine and the squish clearance is at 0,30...With a tight squish like that, the engine has a risk of failure or not?
 

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OK, I can only assume that it has a .020 thickness base gasket on it right now. If it does you can go up to a .025 or a .030 gasket and open the squish some. I wouldnt go anything below .035 on the squish, and if you can get it around .040 that would probably be a little better.
 

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My "STEAMING PILE OF 472cc TRINITY PERFORMANCE" was shipped with a .034!!!! I am very fast......at changing blown O-rings! New custom domes and a quality re-port to the cylinder may rid me of the V.D. my bike got from all the Trinity parts. .034 was TOO tight!
 

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QUOTE (bansheeqc @ May 23 2009, 10:52 AM) index.php?act=findpost&pid=43899A friend of mine has planned the base of is cylinder off 0,20...I reinstall the engine and the squish clearance is at 0,30...With a tight squish like that, the engine has a risk of failure or not?
We run between .35 and .45 on all of our bikes, and they are stock cylinders and head
 

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QUOTE (bansheeqc @ May 23 2009, 09:52 AM) index.php?act=findpost&pid=43899A friend of mine has planned the base of is cylinder off 0,20...I reinstall the engine and the squish clearance is at 0,30...With a tight squish like that, the engine has a risk of failure or not?


scavenging and velocity are two main factors. The cubs scavenge so good that looser clearances will have good squish velocity and burn rate.. I like around .050 on 421s with 45 to 48% band width.. on stockers I go a bit tighter .040 to .044
 

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Ok. I don't want to start anything here but I did want to get some quality input on this topic. I ran .030 squish on a stock cyl, stock crank, well ported banshee motor making 70 hp. I ran this setup for two years with no issues at all. Hard runnin duner/hillshooter. My compression was low; around 150 psi at 1000 ft. The books all say .040 is the "magic" number. I am pretty sure we can get more energy out of each engine revolution if the squish is tighter since more of the fuel and air charge is in the combustion chanber.
Has anyone run a small squish and seen the results of mechanical failure; ie. piston hitting the head?
If your motor did blow with a tight squish then was it really the result of the small squish?

Mailman
 

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QUOTE (Koolguyson @ Jun 19 2009, 08:06 PM) index.php?act=findpost&pid=47317It is very likely that a piston will hit the head with a squish that tight. Remember that rods always stretch a little under load and parts expand with heat.

I agree and also the larger bore pistons have more movement out at the edges of the exhaust and intake sides under normal operating conditions(piston Rock), if you are at .040 on the sides you may be at .020 or closer, front or rear.. no one would set a 78 bore DM under .040 or at it either ...the books dont have a "magic number" they just give "minimum clearances" and most of the info in most of these books are a bit out dated.


I have seen plenty of motors eat themselves alive with deto, from to much squish velocity
 

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QUOTE (Koolguyson @ Jun 19 2009, 08:06 PM) index.php?act=findpost&pid=47317It is very likely that a piston will hit the head with a squish that tight. Remember that rods always stretch a little under load and parts expand with heat.



I agree and also the larger bore pistons have more movement out at the edges of the exhaust and intake sides under normal operating conditions(piston Rock), if you are at .040 on the sides you may be at .020 or closer, front or rear.. no one would set a 78 bore DM under .040 or at it either ...the books dont have a "magic number" they just give "minimum clearances" and most of the info in most of these books are a bit out dated.





I have seen plenty of motors eat themselves alive with deto, from to much squish velocity




My 78mm bore Twister was sent to me (by a builder) with a 0.035" squish. I had to have the domes recut, and it ended up being 0.048" when using 1 piece of solder. It actually comes down to about 0.044" if you test both sides of the piston at once. It runs good. I have a 421 Cub that was ran with 0.038" and it ran excellent.
 
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