question about powermax pistons

seattle##gs

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I have a set of .040 powermax pistons for a 750, these have almost no time on them. The bore still has a crosshatch, measures right on the money but the pistons have .006 clearance. Were these pistons required to have this much clearance?
 
seattle##gs said:
I have a set of .040 powermax pistons for a 750, these have almost no time on them. The bore still has a crosshatch, measures right on the money but the pistons have .006 clearance. Were these pistons required to have this much clearance?
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Only that this was for a racing clearance.
 
jseng1 said:
Send them to line2line and have them coated - clearance problem fixed.
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What is an approx cost of coating two Norton pistons, Have you used them/ and do you run the tight specs they recommend?
 
Carl H said:
What is an approx cost of coating two Norton pistons, Have you used them/ and do you run the tight specs they recommend?
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Go with .002" (or .003) clearance to the coating and then give them an hour of easy break in - the tight spots will wear off and it won't sieze as long as you have the recommended clearance to the bare metal skirt. Use the slick plus coating. I've been doing this for my customers for years and it works great - it extends the life of the pistons.
 
You can coat them at line2line with graphite as tight as you want as long as they still slip into the bores without forcing. Tight spots will wear down without scuffing on breakin.
 
seattle##gs said:
I called LINE2LINE just now in Gardnerville NV, the price is $42 per piston.
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Post the result if you do this coating. I’m interested in this for my ‘72 combat with .040 bore and roughly .006 piston clearance.... although very good compression. My pistons have around 35,000 miles with fresh rings.
 
Coating them won't fix worn out and sloppy ring lands. .004" vertical clearance is sloppy and not worth using, .006" is smoking junk.
 
This is the fitting instruction that came in the box with Powermax pistons back in the '70s. You'll note that they don't give a clearance, just that the bore should be honed to exactly the specified bore size. In your case that would be 2.915". If you measure your bore, and it is right at that value, then you might conclude that the .006" clearance you measured was correct. That seems loose to me. When I was racing with those pistons back in the day, I fitted them at .0040" on the timing side cylinder and .0045" on the drive side, as recommended by my favorite expert, C.R. Axtell. FWIW, Powermax are the pistons he recommended back then for a 750 race engine. The pistons skirts will collapse a bit in hard use, but .006" still seems loose to me for a fresh rebuild. On the other hand, I know from experience that the engine will run just fine that way. And Norton allowed .005" of bore wear before requiring boring for new pistons, and you're no where near that, whatever the initial clearance was supposed to be.

I'm not knocking the coating Jim recommends. It's well proven. I just don't think you really need to do it in your case, but with .006" you might hear a little piston slap when cold. If that bothers you, the coating would probably prevent that.

Powermax Fitting Instructions 1200.jpg


Ken
 
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So, you made me curious. I have a set of used 750 Powermax pistons in .040" oversize that came out of my Production Racer back in the '70s. They had a fair amount of race use, but still look good, with normal wear marks on the skirt, but no signs of seizure or galling. The skirts measure 2.9086", which would be .0064" clearance. Just some data for reference.

Ken
 
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Just pulled up some more info from old race notes from 1975. This is from my Production Racer with the original Powermax pistons that came in the bike from the factory. This is after 1200 miles of racing use. I wasn't having any porblems with the engine, but back then I always tore them down to inspect at the end of the season (and a good thing too, in this case. The crankshaft was cracked on the drive side :( ). The bore was still right at 2.875" and the pistons were 2.8685" at the bottom of the skirt, giving .0065" clearance. I later used these pistons for several seasons of AHRMA vintage bike racing in a featherbed frame, and put a lot of race miles on them with the .0065" clearance. I only replaced them when the ring grooves became too worn. Over the years I also raced with a set of +.020" Powermax pistons in iron cylinders and a set of +.040" in aluminum cylinders, all with good results. I really liked the Powermax pistons.

Ken
 
Exploring the line2line piston coating option to save a cylinder barrel that was overbored by .004"-ish (when getting bored for a +.040") with a piston to bore clearance of .0085". Oops...

I have some good pistons, but if I do go in that direction, is it safe to use the standard +.040" rings with that much clearance? Gaps might be on the bigger side, but I don't have any yet.

The cylinders are for a tough to find 88 domi, so new barrels aren't really an option, and going to +.060" seems dumb if it can be avoided. I bought it this way, so there are no parties to "hold accountable" and make demands to. Just trying to build a bike. Thanks.
 
t ingermanson said:
Exploring the line2line piston coating option to save a cylinder barrel that was overbored by .004"-ish (when getting bored for a +.040") with a piston to bore clearance of .0085". Oops...

I have some good pistons, but if I do go in that direction, is it safe to use the standard +.040" rings with that much clearance? Gaps might be on the bigger side, but I don't have any yet.

The cylinders are for a tough to find 88 domi, so new barrels aren't really an option, and going to +.060" seems dumb if it can be avoided. I bought it this way, so there are no parties to "hold accountable" and make demands to. Just trying to build a bike. Thanks.
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Although I’m sure many will point out the technical reasons why you shouldn’t, IMO it’ll run just fine with the bigger ring gaps at that.

But if you're not sure, what about trying gapless rings ?
 
Fast Eddie said:
Although I’m sure many will point out the technical reasons why you shouldn’t, IMO it’ll run just fine with the bigger ring gaps at that.

But if you're not sure, what about trying gapless rings ?
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Definitely a possibility, but trying to flush out some opinions before I spend the money.
 
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