Longstroke Cylinder Barrel

[Swarfcut]

Fellow Sufferers   

   The newly arrived 1949 Longstroke Basket Case Motor has an undamaged cylinder barrel, the threads are all good, as is the bore. There is no wear ridge, but an obvious area of  unmarked bore above where the top ring would stop. So at some stage it has been a runner. Needless to say, there are no pistons for comparison.

 The first problem for me is a conundrum measuring the bore. I understand the Nominal Bore for this engine is 62mm.  or 2.4405"

 With a metric caliper, I get 62.3mm.  An imperial internal micrometer measures 2.454" A difference of .0135" from nominal standard bore.

 So it looks to be already bored plus 10, and with additional clearance or wear of almost 4 thou, yet there is no wear ridge. The bore is the same diameter at the skirt.

 My understanding is that the piston manufacturer simply specifies the finished bore size, and a piston running in such a bore will have the correct running clearance.

 Anyone able to shed light on this? Never seen a plus 10 piston, or a 14 thou bore clearance. Maybe I have the basics wrong?

 Swarfy.

[RDfella]

'My understanding is that the piston manufacturer simply specifies the finished bore size, and a piston running in such a bore will have the correct running clearance'.
Yes but, as you say, that's the bore size, not piston size, so it looks like your cyl is + .010" OS plus an extra 3.5 thou above normal. Was this motor used for racing? We always honed three to five thou oversize for racing. It eased friction (slightly more power) and reduced the risk of seizure.

[chaterlea25]

Hi Swarfy,
Plus 10 pistons did exist I am fairly sure, I wonder if barrels that went oversize in the factory were taken out to +010 ???
A bore gauge would give you a proper idea of the wear and shape of the bores
Not having a wear ridge is no indication, it may have been removed back in time

John

[Swarfcut]

Thanks RD and Chaterlea John.
  Looks as if finding a plus10 piston is the way forward, which I would prefer rather than going to a 20 or even 30 oversize, as these castings are rare. I have no history on this barrel, so maybe in those days a larger clearance with solid skirt pistons was the usual thing. Time to give it a good scrub and get the Mercer Gauge back in action.  Looks to be a long term project.

 Regards,

 Swarfy.

[chaterlea25]

Hi Swarfy,
+010 up from 63mm would have been the finished  bore size
the pistons usually have the clearance"built in" so would measure 63mm + 5 or 6 thou
From you initial measurements it looks like you would need +020's

John

[ironhead]

Fellow Sufferers   

   The newly arrived 1949 Longstroke Basket Case Motor has an undamaged cylinder barrel, the threads are all good, as is the bore. There is no wear ridge, but an obvious area of  unmarked bore above where the top ring would stop. So at some stage it has been a runner. Needless to say, there are no pistons for comparison.

 The first problem for me is a conundrum measuring the bore. I understand the Nominal Bore for this engine is 62mm.  or 2.4405"

 With a metric caliper, I get 62.3mm.  An imperial internal micrometer measures 2.454" A difference of .0135" from nominal standard bore.

 So it looks to be already bored plus 10, and with additional clearance or wear of almost 4 thou, yet there is no wear ridge. The bore is the same diameter at the skirt.

 My understanding is that the piston manufacturer simply specifies the finished bore size, and a piston running in such a bore will have the correct running clearance.

 Anyone able to shed light on this? Never seen a plus 10 piston, or a 14 thou bore clearance. Maybe I have the basics wrong?

 Swarfy.

G'day Swarfy.
Back in the day BSA did supply + 0,10 o/s piston assys. I have a set of brand new old stock +10s for an A10
Date packaged 1953. ( hen's teeth) Nicely bored, non sleeved barrel to suit. Came in a pile from a deceased estate & packed away since the 1970's.
There could still be some longstroke ones lurking around somewhere.

[Swarfcut]

CJ.. I think 1 mm has got added somewhere. From my understanding it has a 10 thou oversize bore, plus either wear or intentional additional clearance of 3-4 thou. So, a 10mm OS piston may rattle a bit, and plus 20 is the way to go. Thanks expressed to you, RD, and Ironhead.

Swarfy.

[duTch]

 

.... So, a 10mm OS piston may rattle a bit, and plus 20 is the way to go. Thanks expressed to you, RD, and Ironhead.

 ....eeerrrr- that one may be a bit tight   

[Swarfcut]

duTch...Should have made it a bit clearer,  I meant bored to suit a plus 20 piston. Top marks for observation.

Swarfy.