Bore size

[Duncan R]

Hi,

Can anyone yell me what 72.1 mm equates to in terms of oversize for the A10 cylinder? Looking at a set of s/h barrels on EBay

Thanks

Duncan

[iansoady]

1 mm is .040" (40 thou). So as standard bore is 70 mm 72.1 would be .080" over. I think that's maybe a bit much.....

[Duncan R]

Hi Ian,

Thanks, thats a shame. They have also given 2.83 inches

[beezermacc]

I recently built an engine with +0.080 pistons and it ran really well. I think the capacity is about 675cc. now! We got the pistons from SRM.

[Rocket Racer]

I have seen a blown barrel which had been taken out to +100 (72.5mm) and thoroughly abused before the thin flanges came off, running 10.5:1.

it made a good fin donor.

std is of course 70mm.

I do have a barrel that is a worn +60 that i would take to +80 on 10.5:1 (on methanol)

[Beezageezauk]

Duncan,

I seem to remember hearing something about SRM ceasing to stock the +080 pistons.  I also think that they were the only supplyers of them and that they were only available at about an 8:1 compression ratio.

However, check it out before you go too far with the ebay barrels.

Beezageezauk.

[KiwiGF]

There was recently someone in NZ selling A10 barrels that were already plus 080 and worn/rusty with it, on an auction site and the seller knew that as well but he did not mention it in the item details.....

It might be possible to fit liners to barrels at  a worn plus 080 but that would have to reduce the price I reckon

My barrel is lined and was bored to at least 73 mm to get the liners in and maybe as much as 73.5mm.

[A10Boy]

Is the Std bore size of 70mm literally 2.756 Inches so a plus 040 would be 2.796. I assume thats correct?

Thanks

[Rocket Racer]

Is the Std bore size of 70mm literally 2.756 Inches so a plus 040 would be 2.796. I assume thats correct?

Thanks

In general the oversizes are in 1/2mm, (excluding +10 which are 1/4mm), so;
+20 = 0.5mm
+40 = 1mm
through to
+100 = +2.5 mm or 72.5mm
don't ask me why but measuring bores in mm seems much more common than fractions of inches.
At the end of the day its the piston size + clearances I guess, so the bore ends up what it needs to be to work with the piston clearances required.

[A10Boy]

I have a set of barrels which measure 2.823 [under a slight wear ridge] which I believe to be + 060 plus 007. I'm trying to decide whether these will be usable or whether they will need a re bore. I know that to be A1 they will need a rebore, but the BSA book says that a rebore is required if wear exceeds 010 [top to unworn area at the skirt] which it doesnt. I dont want to buy a set of +060 pistons just to try them so does anyone have an opinion or have a good +060 piston they could could measure for me?

Obviously at + 060 I dont want to re-bore them unless I have to.

I would be internally grateful for any help.

Cheers

[iansoady]

You would have to be careful if fitting new rings as they'll hit the ridge and break - you can have the ridge removed by a reborer.

I'd have thought .007" over would be on the slack side though. Not sure what the "official" piston clearance should be but I'd be looking at around .004".

[KiwiGF]

I suspect that measuring someone elses plus 060 piston will not help you as generally a bore is made to fit the pistons for reason pistons may vary in size depending on brand and makers tolerances etc

What you probably need are new pistons at the top end of the nominal size so you can reduce your 007 clearance but that means measuring a few before buying them

For example the JP pistons I bought were nearly 002 bigger than taiwanese pistons I had to return to the supplier as the rings were no good - this suited me as I had used bores like you, honed out to a couple of thou over 70mm to get rid of a ridge and seize scratches etc.