Maintenance & Technical > KX500 Original

Squish & Compression & Head gasket thickness

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KXcam22:
Iridium,
  That method sounds pretty good. Sometimes you might want to factor in a thou or 2 for gasket compression. The only issue with messing with the base gasket thickness is that you are changing your port times a bit.  With the thinner base gasket it is always a good idea to use lots of good quality gasket sealer.  I had one blow a hole in the gasket and it sucked enough sand in a few minutes (time it takes to notice it) to ruin a good engine. Cam.

IridiumRed:
Actually, the base gasket that came with the gasket kit (got a full gasket set for the whole engine) was about .021

That would have given me a head to piston clearance of .046 (1.2mm approx), instead of the .055 (1.4) I went with, using the supplied head gasket

The gasket I made (using good gasket material of course) was .030, so it was actually 50% thicker.  

But still, you make good points about the sealant (which I did use, very careful to cover all the surface, let set up a bit before assembly) and also gasket compression

The gasket compression is one reason I aimed a little over the nominal 1.3mm squish height - i suspected the gaskets might crush a bit, reduce the figure :)

I do think there was some gasket compression - after the first running period (15 minutes or so of high idle, blipping the throttle, then short easy ride) i let it cool and retightened all the head/cylinder bolts, and there was a small amount more tightening on the cylinder bolts than the head bolts (head gasket was steel, should have low crush)

You're right on the

Brett:
The best and safest method to measure squish clearance is to first of all setup cylinder height to get desired port timings via changing base gasket thicknesses, bolt cylinder down on the base gaskets you are going to use to correct torque, install head with head gasket and torque down.  Then with a piece of 2mm solder bent into an "L" shape put through spark plug hole and ensure is all the way to cylinder bore and then slowly turn motor over by hand untill solder has been crushed once, then measure with vernier callipers. Ensure you measure in multiple locations in case cylinder and head faces have not been machined on the piss.  Once taken the minimum measurement adjust head gasket thickness to give desired squish clearance.  When i rebuilt my motor i had a .25mm head gasket and that gave 1.7mm squish clearance but i do not know what the base gasket thickness was or if the head or cylinder faces had been modified.  The way i see it if you are using 2 x 1mm head gaskets to make the motor tame you should have bought a 200cc 4 stroke for woods riding.

quincyman:

--- Quote from: Brett --- The way i see it if you are using 2 x 1mm head gaskets to make the motor tame you should have bought a 200cc 4 stroke for woods riding.
--- End quote ---


No 200cc 4 or 2 stroke has what my KX500 has with two 1mm head gaskets. I am monitoring this thread and if it wasn't for breaking kick starters would go back to the narrower squish. I may still go back to 1mm head gasket which is not quite as bad as the thin metal head gasket. Any ways this is a good thread. I am learning more all the time. I have a friend who suggest increasing the base gasket thinkness over doubling the head gasket. Sounds like he was closer than I am as far as performance goes.

We can't get 94 octane gas here. At least not from the pump and that is where I get my gas. 92 is the best I can get from the pump. Not sure a woods bike needs more than that. I rarely get out of third gear.

Timbowe:
Are you using a later model kicker? I managed to bust one on the 92 once. then replaced it with the later model one with thicker base. No worries at all. You must be doing something wrong to bust one of them..

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