Date: Thu, 18 Jun 1998 22:09:06 -0700
From: Robert Smith
Oh wise ones,
What's the conventional wisdom on positioning piston ring gaps relative to the gudgeon/wrist pin and each other?
I remember a thread some time ago that concluded, I think, with the consensus that the two compression rings should have the gaps diametrically opposed with the oil control ring gap somewhere else.
I also remember, I think, that there were those who thought the gaps should be at 45 degrees to the wrist pin, 90 degrees, or zero degrees.
Anyone have an opinion? I'm just about to pop the barrels back on my Interceptor.
TIA.
Robert Smith - Freelance Writer
Date: Fri, 19 Jun 1998 13:11:38 -0700
From: Eric William Lamberts
On Thu, 18 Jun 1998, Robert Smith wrote:
> Oh wise ones,
>
> What's the conventional wisdom on positioning piston ring gaps relative
to
> the gudgeon/wrist pin and each other?
[SNIP]
It generally is discussed in the shop manual. For Commandos the 3 gaps are positioned at 120 degrees from each other.
Eric Lamberts
Date: Mon, 24 Aug 1998 11:50:58 -0600
From: William Larson
On Mon 24 Aug, Holden, Frank R wrote:
> Can someone tell me what the piston skirt clearance for a Norton
750 with
> 9:1 Hepolites should be? I was told it should be 0.004" and
that sounds
> like a bit much but I haven't done this sort of thing with air-cooled
> engines. (There is no reference to it in my Haynes book)
>
> Also, where should it be measured?
Don't know about 750's, but my new 10:1 Venolia forged pistons for my 850 came with the following recommendations:
Top of skirt clearance: .008" - .010"
Bottom of skirt clearance: .005" - .007"
As for opening up my rod small end .001" while still attached to the crankcase, I mentioned the suggestion of using an expansion reamer to our company machinist. He expressed much concern about the possibility(probability) of tool chatter and a resultant out of spec I.D., as many on the list also have. So he set me up (his recommendation) with a 25,000rpm die grinder (hand held grinder) and 1" wide, fine abrasive cloth wrapped loosely around a split 1/4" dia. steel dowel. I have to tell you it worked beautifully, and slowly. It gave a super smooth finish all the way around the bores, and the pins are now a very close sliding fit. Thanks for all of the recommendations! It's great to have all types of ideas to consider, but in the end we have to make our own choices based on the perceived risk, and of course cost.
Bill Larson '74 Norton Roadster
Date: 08/24/98 11:08:02 AM
From: Philip Pick
On Mon 24 Aug, Holden, Frank R wrote:
> Can someone tell me what the piston skirt clearance for a Norton
750 with
> 9:1 Hepolites should be? I was told it should be 0.004" and
that sounds
> like a bit much but I haven't done this sort of thing with air-cooled
> engines. (There is no reference to it in my Haynes book)
>
> Also, where should it be measured?
Can't help exactly, but the following may be of use.
A Hepolite book from 1984 shows the BORE size of all Norton 750, whatever the varient, to be 2.8746 inch. Oversize are just that plus the oversize.
regards
--
Philip Pick
Date: Mon, 24 Aug 1998 11:38:54 -0700
From: John Ebert
On Mon 24 Aug, Holden, Frank R wrote:
>According to JB Nicholson in "Moder Motorcycle Mechanics" it is
>750 Commando solid skirt .0045
750 combat .0045
>750 Commando T slot .0035
750 combat .0035
Hope this helps
John In Phoenix
Date: Mon, 24 Aug 1998 17:41:15 -0700
From: Vernon Fueston
On Mon 24 Aug, Holden, Frank R wrote:
>Can someone tell me what the piston skirt clearance for a Norton 750
with
>9:1 Hepolites should be? I was told it should be 0.004" and
that sounds
>like a bit much but I haven't done this sort of thing with air-cooled
>engines. (There is no reference to it in my Haynes book)
>
>Also, where should it be measured?
On the box the Hepolites come in there is printed the standard bore. Theoretically you add the oversize to it and get the new bore size. Unfortunatley the pistons vary a bit in size, i.e. all .020" pistons do not measure the same. If you give it a little break in time .004" will work. If you are headed straight to the track give it .0045. Measure the piston at the bottom of the skirt at 90 degrees to the wrist pin. Pistons are tapered and cam ground so as to fit properly when hot.
Vernon Fueston
"It's hard to make things fool-proof
when fools are so ingenious."
Date: Wed, 02 Sep 1998 21:36:38 GMT
From: John Woodgate
At 08:57 AM 8/24/98 -0700, Holden, Frank R wrote:
>Can someone tell me what the piston skirt clearance for a Norton 750
with
>9:1 Hepolites should be? I was told it should be 0.004" and
that sounds
>like a bit much ...
Frank,
Your source is correct. A little while back when fitting new +.020 pistons to my 850, I discovered some real discrepancies in comparing the Norton specs and the specs that came with the new Hepolites. I decided the problem was that someone (not me) had difficulty doing inch-to-metric conversions!
Also, it seemed that no matter which theoretical dimensions I used, either the ring gap or the skirt clearance was going to be wrong. In the end, I went with finishing the bore to end up with 0,004" clearance, and left the ring gaps slightly full. I can report some several thousand miles down the road that this engine is very mechanically quiet, and does not use oil nor smoke.
Odd, now that I come to think of it, we had a similar problem last week with a friend's BSA A65: skirt clearances were OK on the nearly-new pistons, but the gaps of the new rings (by GPM this time), when measured in the unworn part of the bore, were too "full". Not sure, but that may tell us something about how well finished new rings are .. I for one will never again guage ring wear by measuring gap increase from nominal specs.
One more thought to pass on about fitting new pistons. A while back on BI, there was a long thread about new wisdom of fitting new pistons dry, to speed the bedding-in process. Always a believer in avoiding dry-starts, I nearly decided to try the "dry" method this time .... nearly, until I recalled that while you might want the rings to scuff down quickly, you sure don't want the piston skirts to run dry. Thus, I tore the engine down before starting it, and went for a compromize: dry rings, lubed skirts. Worked well!
Hope this helps,
... Gregg