The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: RichardL on 17.03. 2016 18:26

Title: Piston knurling
Post by: RichardL on 17.03. 2016 18:26
As I suspected, my inquiry about an orphan A7 ring  has opened a can of worms. I really want to avoid going up to .O60. For one thing, that sends the barrels to their limit, for another thing, I have a perfectly good set of .040 pistons. My machine shop has what I think is a specialized knurling tool for upsizing pistons. If I can gain 3 to 3.5 thou I think it might work OK. It is a lot cheaper to hone the bores and knurl the pistons than to bore to .060 and buy new pistons. If it worked I'd still have another bore available. I'm figuring, if I get a fit that won't slap horribly, what's the harm?

Does anyone have a knurling disaster or success story?

Richard L.
Title: Re: Piston knurling
Post by: duTch on 17.03. 2016 21:28
 I may have had something similar done a while ago to expand the pistons, but don't remember the outcome. A process called 'shot-peening' comes to mind;

  https://en.wikipedia.org/wiki/Shot_peening (https://en.wikipedia.org/wiki/Shot_peening)
Title: Re: Piston knurling
Post by: muskrat on 18.03. 2016 07:48
G'day Richard.
If it's only 4 thou the knurl wouldn't be that harsh.
As I said in the other topic "Knurling pistons was quite common back in the day. OK for a quick fix to last a few thousand miles. The knurl will wear faster due to it's smaller surface area but will hold more oil and not wear the bores as quick. The main thing I'd want to rectify is any bore taper. Hone the lower part of the bore to match the top."
duTch, peening was another method. The piston skirts end up looking like a golf ball.
Cheers
Title: Re: Piston knurling
Post by: duTch on 18.03. 2016 09:30

 
Quote
The piston skirts end up looking like a golf ball.
Cheers

 Batter up ! Pistin skirts or not *smile*
Title: Re: Piston knurling
Post by: BSA_54A10 on 19.03. 2016 12:32
When we were young and bike parts were epensive the goldie piston went out for expanding after every meeting.
Knurling was another method as was center punching.
All of them are only short term and require fitting oversized rings ( deeper radial depth).
So while the piston may not slap you can get into the situation where thers is insufficant overlap between the rings and the ring grove to maintain  pressure.
The same net result as having too much ring clearence.
Title: Re: Piston knurling
Post by: RichardL on 19.03. 2016 13:15
All very interesting stuff.

Trevor, Muskrat or anyone,

Do you know if the knurling is done top to bottom on the piston or just below the scraper groove? Trevor's comment seems to imply top to bottom. I was thinking that maybe below the scraper groove might be all that's need to avoid slap.

Richard L.



Title: Re: Piston knurling
Post by: muskrat on 19.03. 2016 19:35
G'day Richard.
From below the oil ring to the bottom of the skirt.
http://www.pbase.com/stealthfti/image/53355694
Cheers
Title: Re: Piston knurling
Post by: RichardL on 19.03. 2016 23:48
Very ueful picture and description. Thanks Muskrat.
Title: Re: Piston knurling
Post by: BSA_54A10 on 20.03. 2016 10:03
All very interesting stuff.

Trevor, Muskrat or anyone,

Do you know if the knurling is done top to bottom on the piston or just below the scraper groove? Trevor's comment seems to imply top to bottom. I was thinking that maybe below the scraper groove might be all that's need to avoid slap.

Richard L.

As Musky stated only the skirt.
Problem is the bore will also be worn
So the rings will sit further out from the piston groove
If there is not enough conact between the ring and the groove the ring will not be able to make an effective seal thus you fit rings with a bigger radial depth
Title: Re: Piston knurling
Post by: RichardL on 20.03. 2016 14:32
Trevor,

I did not read you correctly the first time regarding the overlap. Now that I do, I can say to a certainty that, unless you know of a good supply, I won't be finding such oversized rings. I'm also thinking that a few thousandths shortage in this area isn't going to matter much. As might be obvious, I am totally OK with a less-than-perfect top-end for this build as long as the bike runs well. Assuming I go the knurling route, it will last as long as it lasts and the top-end can be rebuilt to the 60-over spec next time.

Richard L.
Title: Re: Piston knurling
Post by: shuswapkev on 24.03. 2016 13:00
I used to do the knurling..whether it worked or not...as far as I know...nobody ever came back to says it did or didn't
most likely new pistons and rebore would have broken the budget...

 cylinders can always be resleeved