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- Oct 28, 2014
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Using JCC or Emgo pistons in an 850 MK3 I have been told that .003 is sufficient but it makes me very nervous to go that tight. Anybody with real life experience running .003 clearance on fresh pistons?
For reference, the factory spec for original piston fit allows .0037" - .0047" for the 750 and .0020" - .0029" for the 850, and says wear up to .005" is acceptable. By that spec, .003" would be fine, as long as the JCC or EMGO pistons have the same design (cam and ovality, mostly) as the factory pistons. It really depends on the piston design.
I've never run less than .004" with a cast piston or .005" with a forged piston, and generally a bit more, but that's mostly on race bikes, where we generally run them a little looser.
The last engine I built with cast pistons was in a 750 race bike, and I used .004" on the right side and .0045" on the left side. That was what I used to use with the factory AE and Powermax pistons, as recommended by Axtell, and it worked very well for road racing. The pistons were aftermarket. My memory says they were from Taiwan, but I don't recall the brand.
That doesn't answer your question, but maybe it will be of some help. I'm sure someone who has actually used the current JCC or EMGO pistons will chip in with some relevant experience.
Ken
First time I've ever heard of different tolerances right to left. What's the reasoning?The last engine I built with cast pistons was in a 750 race bike, and I used .004" on the right side and .0045" on the left side. That was what I used to use with the factory AE and Powermax pistons, as recommended by Axtell, and it worked very well for road racing. The pistons were aftermarket. My memory says they were from Taiwan, but I don't recall the brand.
Ken, what’s the logic behind the slightly larger 850 pistons having a smaller clearance?
I would have thought that if you replaced cast pistons with forged ones without greatly changing the bore size, you could run with less clearance . Cast pistons are more likely to change metallurgically in use and are usually thicker in the skirts than forged ones. Does the thinner skirt on a forged piston cause more distortion when subjected to heat ? Personally, I never worry about bore clearance as long as there is enough. The rings do the work of keeping compression up, not the bore clearance. And who ever heard piston slap in a long stroke motor such as a Commando engine ? A forged piston copes with piston slap better than a cast one anyway.Beats me. I'm just quoting the specs out of the factory service manuals. My experience with forged pistons in race bikes is that the larger bore definitely requires a bit more clearance.
Lower compression with consequently lower heat on the 850? Shooting in the dark here...Ken, what’s the logic behind the slightly larger 850 pistons having a smaller clearance?
I would have thought that if you replaced cast pistons with forged ones without greatly changing the bore size, you could run with less clearance . Cast pistons are more likely to change metallurgically in use and are usually thicker in the skirts than forged ones. Does the thinner skirt on a forged piston cause more distortion when subjected to heat ? Personally, I never worry about bore clearance as long as there is enough. The rings do the work of keeping compression up, not the bore clearance. And who ever heard piston slap in a long stroke motor such as a Commando engine ? A forged piston copes with piston slap better than a cast one anyway.
Using JCC or Emgo pistons in an 850 MK3 I have been told that .003 is sufficient but it makes me very nervous to go that tight. Anybody with real life experience running .003 clearance on fresh pistons?
Buzzard, that's the word I should have used, and if one over paid just a bit for postage you'd get a couple of Canadian coins back with your package.I remember getting parts and perusing the catalog from Nicholson's. But I'm an old buzzard.