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What would you do to get the compression down?


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my trail bike i ran a 65 tho squish clearance on pump gas.  it ripped. ran for miles and never acted crazy.  you have to rember  "optimal tight squish" is good for short burst motors.  drag motors etc,  long drawn out trails. long heavy load like hill climbs and long low throttle lean condition rides like trails will like more clearance than the so called optimal.  plus larger bores will want more clearance also.  try running a 421 cub at 40 tho  or a big single  nope  not going to live for long at all.    i personally prefer larger than so called "optimal" squish for riders and drag bikes for many reasons.  mostly for deto issues with running low octane pump gas. and reliability concerns  the band width and dome profile seem to do a lot more than just sqush.  if your domes have a really wide band in them and a tight squish your asking for trouble.  its easy to see where a motor actually wants its band at with used domes.  at least thats how i started looking at it. 

 

the cool thing now is your going to be able to try 2 sets of domes back to back and compare them.  hopefully you spec'd a small width band less than 10mm wide..

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I pulled the head off Saturday morning to measure the domes clearance and width for David. I got a .034 clearance and 22mm width. My deck height is .006 under. So not sure why I have a .055 squish. I'm mmeasuring above the wrist pin. There is no play in the rod bearings. Hmmmmm........

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Have you tried measureing it with a piece of solder laying across the piston. Not just 1 side. I just built a new motorwith a new crank piston bearings ect and recieved a .005 difference from just sticking it threw the plug hole no matter how manytimes i did the test. I know cam has had the same experiance. Jmo so dont agree but its worth a shot. The solder threw the plug hole was giving the looser number.

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I bolted the head back on and did another test. This time I used two pieces of solder through the spark plug hole and put them on opposite sides of the piston above the wrist pin. I though this was an easier way. I did several readings this way and I get .048-.050 depending on how much pressure I put on the caliper. Before I was measuring the solder perpendicular or across it with the caliper. This time I measured the middle of the solder parallel with it. I think that this would be more accurate then across it.

I emailed David the info. I gave him tons of specs and info so the ball is now in his court. He has way more experience and I trust his judgment in cutting me a set of customs. I'm sure they will out perform off the shelf's.

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