I'm having my *74 850* cylinders bored to .020 over and I can't find any information on what the proper piston to cylinder clearance should be. Any help?

Butch.

The final bore size is driven by the brand and type of piston. There is usually a spec sheet that shows this information included with the piston when you buy it.

Unfortunately, my new pistons came with no such spec sheet. I called the supplier to ask, but they had no clue....maybe I talked to the wrong person, but anyhow, what I got was the cheap import pistons, I don't recall the manufacturers name, but I've got the receipt around here somewhere.

Jason Curtiss wrote:The final bore size is driven by the brand and type of piston. There is usually a spec sheet that shows this information included with the piston when you buy it.

Hi , somewhere between 4 and 4. 5 thou is a good point to start with unknown pistons........my two cents.

Do you know if they are cast or forged pistons? Forged pistons require more clearnce than cast ones. Four and a half thousandths (0.0045") sounds a bit sloppy to me. I'm building an air cooled engine now and I'm using cast pistons with a clearance of around 0.001". I'll check some of my Norton literature when I get home to see if I can find some guidlines for you.

The JCC aftermarket pistons are happy with .0045

As far as I can understand for a standard 850, the clearance is 2 to 3". I don't know if this is the same for the 750?

*****the clearance is 2 to 3". ******** should have read 0.002" to 0.003" Edited by Reggie

From the technical data in the manual..

Bore = 3.030" Piston skirt should measure between 3.028" to 3.0271" which equals 0.002 to 0.0029 clearance.

When I was speaking to an engineer who does rebores recently, he explained how there can be very slight variation in piston size and therefore each bore is bored to the corresponding clearance required for that piston, with the final size required being achieved by the honing.

Butch,

Well I'm home now and unfortunately can't find any info on the Hepolite pistons I installed in my Commando engine.

However, I checked the factory shop manual for a '78KZ 1000 and the piston to cylinder clearance was specified as 0.0017" to 0.0028". This is an air cooled four cylinder engine. I still think your best bet is to get the clearance recommendation from the piston manufacturer.

Jason

When I had my 750 racer rebored to +20 with Hepolite powermax pistons ( which I believe are cast, not forged) I had them bore it to a clearance of 3.5 thou. That was on the advice of a skilled engine builder. I believe he said that 3.5 is good for a race engine but a bit loose for a road one that won't be revved as hard and will be run in a bit more gently.

The bore should measure 3.052 min, clearance should start at 0.004'' for cast pistons in a cast iron barrel, 1/2 of that if barrel is alloy,if it is a used cylinder you will probably find the clearance is more like 0.007''-0.008''.The clearance should be built into the piston and alot of after market manufactures will use the minimum factory size as their starting point, causes less warranty claims due to seizeing. A cast iron barrel doesnt expand at the same rate as an alloy barrel thats why the sizes seem sloppy, forged pistons are even worse they want to start at about 0.006''.The factory specs work out at 0.0035'' as absolute min ,0.004-0.0045 is more the average.

[quote="Reggie"]As far as I can understand for a standard 850, the clearance is 2 to 3". I don't know if this is the same for the 750?

From the technical data in the manual..

Bore = 3.030" Piston skirt should measure between 3.028" to 3.0271" which equals 0.002 to 0.0029 clearance.

Reggie that should read bore= 3.032, 1/2 athou above 77mm

Anything under 4 thou sounds awful tight in an old type air cooled bike with 77mm bore.

How about 0.0015" per inch of bore?

I wouldn't run anything less than about 4 thou in a Commando with iron barrel. Guys in our club have seized them at 3 thou, although that might be ok with an alloy barrel.

Debby

Thrasher - I think you mean 0.0015-inch per inch of bore, don't you?

The primary danger in excessive piston-to-bore clearance is piston slap. But, again, I would pursue the piston manufacturer for recommendations.

Do you still have the boxes the pistons came in? The Italian GPM pistons I just installed in my Dominator had the required finished bore size on the outside of the boxes.