Sloper pistons

Started by Mike Farmer, 28 January, 2021, 10:34:59

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Mike Farmer

28 January, 2021, 10:34:59
 :) :) :)

I am at the point of having my barrel "done" and am a bit undecided. Sloper is basically outside my knowledge so some advice if you would be so kind

I have two pistons of which neither is in super condition but as thats all I have its Hobsons choice.

One is High comp and the other is low comp: my question is what are the pros and cons of this situation what do I lose or gain with my choice.

I should be very interested in any possible input. Thanks

Mike 8) 8) 8)

A65ROCKET

#1
28 January, 2021, 21:14:05
How high is Hi-comp?
What are you going to be using the Sloper for, sidecars and general riding, then low-comp,
ringing at 70mph and fast touring, hi-comp

Mike Farmer

#2
03 February, 2021, 20:02:33
 :) :) :)
As I said in my original post I have two pistons one each LC and HC.

The LC piston is unusable so my choices have "significantly diminished".

It has been pointed out that there is a thought that to run with the HC pistons leads to a level of unreliability. I dont have any other info on this aspect. I am told that it would be advisory to run it with a spacer under the cylinder 2/4mm??.

I would appreciate any input/advice on any of the points raised here. Does anyone run HC. does anyone run with shims??? etc

Thanks for your input

Mike 8) 8) 8)

Group Leader

#3
03 February, 2021, 22:26:20
My little B21 had been fitted with a high compression (Triumph) piston by a PO.   You can see the results in my "Piston Broke" thread .....

https://www.bsaownersclub.co.uk/bsaoc_forum/index.php?topic=9467.0

As you will see, I managed to source a slightly lower compression, replacement piston and added an annular compression gasket made from copper and annealed before use.  The resulting compression ratio was much more respectable but still higher than the original 6.2:1.    I determined the actual CR by temporarily fixing the piston (no rings) at TDC having wrapped the piston with a layer of electrical tape to seal the piston in the bore with the top edge of the tape just covering the top ring slot.   The head and copper gasket were then clamped in place on the barrel and the whole assembly was mounted in/on a Workmate with the spark plug hole uppermost.   The volume of oil necessary to fill the combustion chamber to the point where the fluid got to where the end of spark plug thread would normally be was recorded in order to then calculate the CR knowing the swept volume of the cylinder.

Following a little nip-up and remedial action during running in I'm hopeful that it's settled down now having done something like 250 miles on it.   Itching to get out on it again but of  course that's off of the menu at the moment!

Alan

Mike Farmer

#4
04 February, 2021, 10:05:18
 :) :) :)

Hi Alan

Thanks for that. As you will appreciate I'm pretty much clutching at straws, so your input is very valuable. As I said I do have an LC piston but it is poorly and we are not at all sure if it is machinable.

Mike 8) 8) 8)

Mike Farmer

#5
05 February, 2021, 12:58:16
 :) :) :)

General Q. were sloper pistons originally cast oval and if so  A--by how much and B--Assuming that they would eventually wear round would having a piston machined round be a serious problem.

Confused

Mike 8) 8) 8)

Mike Farmer

#7
06 February, 2021, 11:06:42
 :) :) :) :)
Hi Thanks for that. I knew some pistons were forged oval and the general reasoning. But not in that detail so thats good. I know there are other methods used to combat the stresses present in the combustion chamber. For instance the piston goes from stationary to max speed to stationary every time it moves up and down the barrel. Totally unprovable of course.
I also know that a lot of pistons have other methods of heat/pressure control---split sides to allow expansion--it goes on.

Does anyone have knowledge of any company offering a ""re-ovaling""service by which I mean a machine shop that will/can resize pistons so that they keep their shape? Does such even exist?

In truth my pistons are so worn as to not be measureable for ovality. (at least by me)

So I ponder---what I really need is someone to call me and say that they have a brand new sloper piston added bonus Free to Collector. And I believe in santa claus--the check is in the post---and virgin birth.

Mike 8) 8) 8) 8)

Mike Farmer

#8
08 February, 2021, 14:42:56
 :) :) :)

Just for further info there is a company in Germany Klaus Wahl engineering, who will rework vintage pistons. Obviously the piston has to be in a good enough state for the job to be completed. About £100 inc VAT rings and Gudgeon Pin.

They will produce a pattern piston (to fit bore) 8) 8) for about £275 inc etc

I'll keep you informed

Mike 8) 8) 8) 8)

Gadorey

#9
08 February, 2021, 16:09:58
Glad to hear that he could help.

Mike Farmer

#10
12 February, 2021, 20:01:03
 :) :) :)

I have two or three books on the Sloper 500cc OHV earlyy 30's. One says the piston is 80mm the other says 82mm.

So which is it.

Mike 8) 8) 8) 8)

Mike Farmer

#11
18 February, 2021, 20:12:41
 :) :) :)

Sent my piston to Wahl Engineering in Germany today. They are saying lead time 9 to 11 weeks. I'll let you know

Mike 8) 8) 8)

Y13 Alan G

#12
19 February, 2021, 14:11:24
Hi Mike, 500cc  BSA Sloper used an 80mm piston. The 557cc SV Sloper used an 85MM piston.

Mike Farmer

#13
20 February, 2021, 11:08:15
 :) :) :)

Thanks for that. My mind is now at rest that I have sent the correct piston to be reprofiled.

Mike 8) 8) 8)

Mike Farmer

#14
03 March, 2021, 17:39:03
 :) :) :) :)

I have just been offered a brand new Sloper piston at a very good price. I'm sure in my mind that there were two or maybe three different gudgeon pin diameters. 500cc OHV

I can't fine the detail in my books so would someone kindly advise/enlighten me 

Mike 8) 8) 8)
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