Tight piston to cylinder clearance

Have you compared motors with tight and loose piston fit on a dyno ? I'd be surprised if there was a significant difference in peak power output or torque. - like light and heavy flywheels ?

Its ancient practice to build race engines with more clearance/less friction than tighter road bikes meant for long term non race use. JMS pancake type pistons have more tip tendency that wears on crown edge so he's scratching at straws to extend their race life, but so far has wasted money/time as example he's shown show no evidence of the coating being touched or even burnished in expected areas. I posted URL's that show a few pistons that did benefit from closer fitting, with significant area worn down, exactly what I found in Ms Peel coated pistons after tear down 3 furious seasons of spanking sports bikes then 2000 more miles after the OVER REV struck throttle event.

Bore coating more worth while than pistons and knurling more effective to take up clearances that burnish down to best fit and better oil retention to boot. So who ya gonna believe a non mechanic like me or seasoned experts not yet doing what I suggest. Mz Peel has .006 to .0065" tapered Bore Tech coated bores and pistons crowns, chamber, valves and ports ceramic insulation coated but not piston skirts as seen how short a life that has running in red zone routinely.

https://www.google.com/search?q=benefits+of+piston+knurling&ie=utf-8&oe=utf-8&client=firefox-b-1

Sometimes theory is different to what happens in practice. My first race bike was built with all the good theories in mind. It was horrific - just too nasty for words. Some people might think that is good, but they probably haven't had the experience of sitting up in a hospital bed. The guy who originally built the bike, rode it at Bathurst and scared himself silly at the end of ConRod straight, got to the top of the mountain and bounced off the Armco fence and broke his arm and leg. He didn't race again for 20 years, even though he had been an excellent rider.
When I got that bike, I also believed in the theories - I had a very rude awakening. It turned me into an instant dud.

If you start with the maker's specified bore clearance and the motor wears, you get a certain life before you need a rebore and new pistons. Perhaps it doesn't really matter where the starting point is unless you get piston slap which might break a piston ?

You can also use the skirt coating for standard clearances. The piston below is for a 74.5mm/2.933" bore. With a thin skirt coating the piston measures 2.929" for .004" cylinder clearance. This way you avoid metal to metal contact and should double the life of the pistons.

FWIW, I used Calguard Gear Kote & Gun Kote for 1st rebuild back in 1979. Based on their recommendations I applied Gear Kote to piston skirts, intake ports, valve springs, camshaft, gearbox internals and Gun Kote to crankcase & cylinder exteriors, engine/trans subframe, etc (both products are oven cured at 300-325 degrees F for an hour). Motor received Klymer manual (Brian Slark) cam/cr, etc "stage 1" upgrades but is a street bike. The coatings were installed mainly as perceived defensive steps rather than for power gains. All this coating stuff was then pretty "new". There was considerable contemporary +/- argument, but this period also enjoyed a number of bogus moly/TFE oil additive "miracle" products that muddied perceptions. Regarding Gear Kote, it seemed to work as advertised (nothing quantitative to judge however), and more importantly there were no catastrophic outcomes. Piston skirts still had (scuffed) coatings when pulled out.

I really like Gun Kote for exterior coatings . It forms a very hard and slick surface that protects from corrosion and transmits heat nicely. The Gun Koted parts still look great today. If you have a big enough oven would be great for frame coating, me thinks.

All these products are however very dependent on proper surface prep and application/curing protocols to be effective. For DIY processes that is often the largest variable, especially pre-treatment. Steel parts need near-white sand blasting (120 grit) to produce clean substrate and a coatable surface profile followed by neutralizing steel with a phosphate treatment (Ospho). Aluminum parts are blasted at lower pressure and treated with Alumiprep and then Alodine conversion coating prior to applying finish coats.

Lots of competitors and more product types today. Most seem to be mature, proven processes (industrial as well as auto/m-c). Necessary, as Glen points out? No, but since you are probably only considering because the motor/bike are disassembled (or about to be), not that much added work/$$ with potential upside and little likely downside (if you follow directions). And if Jim Schmidt is selling it I'd bet it works well.

FWIW(2), Calguard was sold in the 80's and I think is now based in S Africa. Original Calguard owner restarted business as KG Industries after non-compete agreement period. Moved from WI to Taylor TX a few years ago. I only know this because I recently looked them up for current rebuild to buy product.

Dave

austinnorton

I used Kalgard (not Calguard) coating on my gears and pistons when I was racing in the 1980s and you can still get it at:
https://www.kgcoatings.com/

Its a molly dry film lubricant but I don't know how it compares to other piston coatings such as Wisco uses or the graphite coating line2line offers. Kalguard is probably not as long lived as a skirt coating but would probably work well to prevent galling or seizure on break in and do it yourself would be cheaper than having it done. It didn't seem to last very long.

The graphite coating Mahle uses is probably the best or it may be comparable to the line2line graphite coating.