sam Posted September 3, 2004 Report Share Posted September 3, 2004 My friend is about to place a new cylinder on his 97 321 gasgas, we are both a little nervous about the amount of thickness of the base gasket. There is techniques buy using soder to determine this, any tips on this subject would be appreciated. sam Quote Link to comment Share on other sites More sharing options...
charliechitlins Posted September 3, 2004 Report Share Posted September 3, 2004 You'll probably be safe if you measure the old gasket and use the same thickness. If you want to be safe, you can also measure from wristpin-to-piston crown to make sure the new piston isn't any taller. OR...you could put it all together with a layer of silly putty on top of the piston, crank the engine, take the head off, slice the putty with a razor blade, peel half of it off, and look at the height of the remaining half to see your clearance. The super thorough way to do this is to then throw out the gaskets and use fresh ones. I usually just snug the nuts/bolts, and figure in .003" for gasket squish. I do not know, however, how much squish to figure for o-ring head gaskets. Quote Link to comment Share on other sites More sharing options...
jtt Posted September 4, 2004 Report Share Posted September 4, 2004 Sam, you can take a soft peice of solder and bend it to a 90 degree bend or so. The key is to have it touch the cylinder wall above the wrist pin. Ideally you use two peices at each end of the wrist pin. Just give the motor a quick kick over with the solder bits in there and carefully remove them back out the plug hole. The solder will crush to the clearance available. You can now measure reasonably accurately what your squish clearance is and calculate your gasket from there. I always hated the putty solution, like Charlie mentioned, because it is so easy to deform the putty and get a bad reading...plus it requires removing the head each time....I'm much too lazy Quote Link to comment Share on other sites More sharing options...
sam Posted September 5, 2004 Author Report Share Posted September 5, 2004 I do not mean to be so simple minded, but could you explain that even a little slower, JTT! I put this soder in the the plug hole to the left and then to the right touching the cylinder walls, as Im facing forward on the machine, towards the exhaust port. Is my objective to have how much clearance from the top of the stroke to the cylinder head? Between the top of the piston and the head. I'm basically making sure I do not make contact between the piston and the cylinder head, correct? simple sam. Quote Link to comment Share on other sites More sharing options...
charliechitlins Posted September 5, 2004 Report Share Posted September 5, 2004 I think you've got it. The idea is to make sure the piston misses the head by a comfortable margin' and doing it on both sides (left and right) would allow for checking if there are any differences in the shape of the combustion chamber that you would need to take into account. JTT's way should work just as well as mine. I just get a little nervous about putting anything in a plug hole and cranking; but, judging by his previous posts, I'd trust his advice. I would imagine that if you use thin solder, it won't even get squished. When I need to measure this clearance, it's usually because I'm putting together a very long stroke, air-cooled engine (Harley), and the rod not only grows a bit when it gets hot, but it also stretches a bit due to the heavy piston and very high piston speed. And I HATE doing engines over for free. Quote Link to comment Share on other sites More sharing options...
sam Posted September 6, 2004 Author Report Share Posted September 6, 2004 Thanks, fellow trials enthusiast! sam Quote Link to comment Share on other sites More sharing options...
jtt Posted September 7, 2004 Report Share Posted September 7, 2004 Sorry Sam, I've been away for the weekend. Yes, you have it correct. You take the solder, making sure it is VERY soft solder (not silver or such) and bend it into an "L" shape with the lower horzontal part of the "L" just long enough to reach the cylinder wall from the plug hole. Be sure you have it against the cylinder wall though, as the closest clearance will be there. Feed the second piece in the same way only in the opposite direction. So you will have a piece touching the cylinder on both the left and right side while sitting on the bike. Now give the kickstarter a turn. Remove the solder and use a micrometer or good vernier caliper to measure the clearance you have. This is referred to as "squish" (from the squish band). This measurement is important as it is one of the determining factors or the flow of gasses within the combustion chamber (combined with squish angle...but that's yet another lesson ) On a big bore like a 321 I would suggest you don't go any thinner than perhaps 1.4 mm. Hope that helps. Quote Link to comment Share on other sites More sharing options...
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