proper piston to cylinder clearance

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 said:

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.

Click to expand...

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.

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.

Jason Curtiss said:

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

.

Click to expand...

Yes thanks. I'm always doing that!

Just resurecting this thread as I was speaking to Norman White today about this subject, and he said that 0.004' to 0.0045' was the optimum gap to have. I queried him with regards to the details in the factory manual (as I understood that the mathematics equate to 0.002' to 0.0029' piston to barrel clearance) and he seemed to think that they were erroneous!!!

Somebody said that's .002 for each side, or .004 total...

(I don't knowe about that, I'm just digging up a foggy memory)

Anyway, I use .0045 total on most aftermarket pistons.

I've always used the rule of thumb of .0005 per inch of dia. Forged pistons of course...

I've used .0040/.0045" for cast pistons and .005" for forged pistons in the 750 race bikes for years, and it seems to work well. When I started building 920 engines back in the '70s, I tried forged pistons from Forgedtrue, Arias, and Venolia. All recommended really tight clearances, around .0025" to .003" IIRC. They seized every time with those clearances. I worked my way out in increments to .005", and that's where they stopped seizing. I use .005/.0055" for all the big bore JE forged pistons I use now. That's in iron cylinders and alloy cylinders with iron liners. If you had alloy cylinders with Nicasil bores, you might be able to run less clearance, but I've never tried it. Air cooled Kawasakis in the same bore size range do indeed work at tighter clearances, around .002", I think, but that will not work in a Norton. I've had JE making Norton pistons for years now, and I've talked to their engineers in detail about clearances. They said they normally would recommend .002" for the 73 mm bore, but had no experience with old British Iron, and were very clear that their numbers were only a suggestion, and that I should use whatever worked for my application. The pistons I get from JE are ground for .002" clearance for the 750 and .003" clearance for the 850 and larger sizes, based on whatever bore size I give them. That's their normal practice for this type of piston. I could have them made for .005" clearance by giving them a different bore size spec, but I like them the way they are, because it allows honing worn stock cylinders out another .003". Frequently that will clean up the bore enough that the next oversize isn't required. I only sell the pistons as race parts, and the engine builders normally expect to have to hone the cylinders to fit the pistons, not to a standard bore size.

Other folks might have other experiences. These are mine, for whatever they are worth.

Ken