piston to cylinder clearance (2017)

[seattle##gs]

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?

 

[lcrken]

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

 

[htown16]

I had a +20 rebore done and gave my JCC/Emgo pistons to my machinist to measure and hone cylinders to a .0045 clearance. Hot weather sometimes here in SE Texas. Maybe I gave up a little cylinder life but at the rate I ride the bike, I'm sure I'll never have to touch the cylinders again.

 

[Danno]

The +.060 Emgo pistons in the Titanic were clearanced to .004

 

[Fast Eddie]

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

Ken, what’s the logic behind the slightly larger 850 pistons having a smaller clearance?

 

[acadian]

I'm setting up for a rebore with new +.020 pistons, and I will be having an anti friction coating applied to the piston skirts. Curious if anyone has experience with this coating? The service provider says that the coating adds 1/2 thou "when burnished", so I'm assuming that means I have to add that 1/2 to the bore specs?

 

[grandpaul]

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.

First time I've ever heard of different tolerances right to left. What's the reasoning?

 

[batrider]

If the minimum spec is 0.0037" clearance per Ken's info then 0.0030" is too tight.

 

[lcrken]

Ken, what’s the logic behind the slightly larger 850 pistons having a smaller clearance?

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.

 

[lcrken]

Just for clarity, these are the numbers from the specification sections of the factory service manuals.

750 bore - 2.875 in. (no tolerances stated)
750 piston diameter at bottom of skirt - 2.8713/2.8703 in.
The difference (clearance) is .0037/.0047 in.

850 bore - 3.030 in. (no tolerance stated)
850 piston diameter at bottom of skirt - 3.028/3.0271 in.
The difference (clearance) is .002/.0029 in.

These are the specs in all the factory service/workshop manuals I've seen from 1970 through 1975.

It has always seemed odd to me that they call for less clearance with the 850 than the 750, and I have never tried fitting 850 pistons that tight. I'm sure we'd all like to know the reasoning behind the factory specs.

Ken

 

[lcrken]

Just to add to the confusion, the 750 Atlas manual specifies the bore in two different places as both 2.874" and 2.8758"/2.8750", but does not give any piston dimensions or clearance specs.

Ken

 

[acotrel]

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.

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.

 

[Nater_Potater]

Ken, what’s the logic behind the slightly larger 850 pistons having a smaller clearance?

Lower compression with consequently lower heat on the 850? Shooting in the dark here...

 

[Fast Eddie]

Yes, the CR is lower, which would lower the heat being generated, all other things being equal. But they’re not equal, there’s an extra 80 ish cc being burned in the same cylinder head design, which is surely gonna generate more heat?

Maybe the raised crown on the 750 piston is thicker? Leading to great expansion?

I’m shooting in the dark here too Nater !

 

[lcrken]

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.

The principle reason the larger clearance is needed for forged pistons is because the alloys used have a higher thermal expansion rate than those used for cast pistons.

Mechanical design differences between forged and cast pistons might also play a role, but I was told by one of the design engineers at JE that the principal reason is the expansion rate.

I guess I need to add the caveat that there are exceptions, depending on the alloys chosen and the mechanical design of the piston, but none of that applies to the commonly available pistons for our Nortons.

Ken

 

[seattle##gs]

This is an 850.....I know .0045 works so I will stick with it. I'm doing this for someone else so I'll play it safe.

 

[Biscuit]

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?

In my opinion running any less than .0045" is dicing with danger. That is the clearance used in the shop I worked at in the late 90's. When overhauling my own MK3 engine I decided to try a tighter clearance, .0035". It's true the factory manual's math seems to recommend .002" - .003" so thinking the shop's clearance of .0045" was just too conservative I opted for tighter. 1000 + trouble free well running miles brought me into late October when the air starts getting cooler. A spirited late fall run up a particular weather changing pass (RT 112, the Kancamaugus Highway in New Hampshire) brought disaster to an otherwise great ride. Warm at the bottom, perhaps 65°F, towards the summit the temp dropped to about 35°F. This is the steepest part too meaning 3rd gear, lots of throttle to maintain 4500 + RPM. At one point, all went quiet as the engine seized without warning. After a while on the side of the road the engine free'd up and I got it started but the damage was done. Subsequent teardown reveled two badly scored pistons and cylinders. Working this hard the engine is making pretty good heat and what I believed happened is all was fine until hitting the cold air. I believed that caused the barrels to cool fairly rapidly and shrink at the same time the pistons were running hot and expanding so .0035" was no longer enough. Though the pistons were now junk, fortunately the bore cleaned up with just a .001" hone giving me the .0045" clearance that works every time. I learned my lesson.

You asked for real life experience, and that's mine. Ordinarily I go by the book but something ain't right about a factory recommendation of only .003" clearance. There is a book written by Bernie Nicholson called Modern Motorcycle Mechanics (my 7th edition is dated 1974). You may never have heard of it or him though I'm sure some of the older guys know of him and the book. He had one of if not the largest shop in Canada opening in the '40's in Saskatoon, Saskatchewan. For what it's worth he recommends .0045" for both 750 and 850 Nortons with solid skirt pistons.

 

[Danno]

I remember getting parts and perusing the catalog from Nicholson's. But I'm an old buzzard.

 

[Biscuit]

I remember getting parts and perusing the catalog from Nicholson's. But I'm an old buzzard.

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.

 

[Danno]

They had everything in stock for Britbikes back when American dealers were going belly-up and selling off their leftovers for cheap.