Hi all, can any one tell me what the standard piston to head (sqiush) clearance should be. Mine is at .9mm and seems to close to me.thanks for any input, Kevin.
I have an April 1979 Technical Data sheet that specifies the "X" measurement to be 1.0mm-1.1mm and another Technical bulletin isssued on 4-26-79 that lists it as with one L-section ring @ 1.1mm-1.2mm or .044"-.048" Mike
Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Pentons-1980 KTM 175-400'S
1975 Can Am 175 TNT & 77 250 Black Widow
1979 Husqvarna OR390
1976-78 RM & 77-79 PE Suzuki's
1974 CR250M 07 CR125R 79 CR250R
Thanks Mike, looks like I need another base gasket.
Kevin.
Kevin,
The base gaskets come in various thickness. Usually, there are a few in the gasket kit.
Tony
Just so everyone is on the same page, squish clearance isn't the same thing as deck clearance. The X measurement in the Penton/KTM manual is the deck clearance, which is typically the height difference between the deck of the cylinder and the top of the outer edge of the piston ...or in the case of Penton/KTM, the top of the L-ring.
Squish clearance is the distance between piston and head in the area that's called the squish band. This can be measured by angling a piece of .062" solder an inch or so (not too far!) down through the plug hole of an assembled engine. Roll the engine through TDC, pull out the solder and measure the thickness of the now flattened area...this is your approximate squish clearance, as the head and the piston may not be exactly parallel. A good rough number for squish clearance is .050-.060" on a big air-cooled engine like your 420. Smaller engines can safely tolerate even tighter clearances because they expand and contract less, components stretch less, and their smaller bearings will typically have tighter tolerances.
I use the X height to get me in the right ballpark and follow up with a squish measurement once everything is torqued down, just to make sure nothing goofy happened along the way. Also, don't forget to retorque the head/cylinder after each of the first few heat cycles. The 1980 manual recommends a X height of 1.0-1.1mm for motocross and 1.5-1.6mm for enduro. Hope this helps a little. Kent
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