11 replies to this topic

[MikeRotherham]

Something that I wondered about for a long time and hope someone can enlighten me.

 

When an original engine, a 1275 in this case, wears out perhaps due to high mileage as opposed to catastophic failure. Wear to the cylinders and crankshaft for example.

 

When it is rebuilt will the cubic capacity always be bigger? Due to bigger pistons because the cylinder has been rebored or is it possible to maintain the same cubic capacity and power output.

Edited by MikeRotherham, 11 June 2019 - 12:21 PM.

[nicklouse]

it is possible to have the block sleeved to bring it back to the original capacity.

[johnv]

You’ll also find that by the time the block has been faced ad the head is skimmed your CR is all messed up

[Fast Ivan]

ordinarily the CC increase, so a 1275cc will become a 1293 at the next re-bore, all the way up to 1380cc

however, as Nick has said, you can sleeve bores back to the 1275cc if that's what you want or is needed

[Cooperman]

The normal thing is to re-bore in steps of 0.020". 

That gives engine capacity of:

 

+0.020" = 1293 cc

+0.040" = 1310cc

+0.060" = 1330 cc

 

Once a 1330 cc engine needs a further re-bore it is best to sleeve back to 1275 cc.

 

These are all normal standard service re-bores for a BMC A-Series 1275 engine and are not considered as modifications for insurance purposes. Pistons are all available at sensible prices (the 21253 is a good one to use with the 8.4 cc dish). Some say that the 21253 is not suitable for high CR's and/or high revs, but I have used them at up to 10.5:1 CR and 7000 rpm (not sustained revs).

 

If an engine is re-bored beyond +0.060" it is necessary to offset the two centre bores to keep sufficient block wall thickness between bores 2 & 3. That makes any future sleeving back expensive and possibly impossible, so it is not something I would ever want to do unless building an engine for out-and-out competition where block life is not an issue and saving a few tenths of a second per lap is what is wanted.

[surfblue]

To follow on from the 1275 question, if an 850 needs a reborn, what are the increased capacities and what pistons and rings are available? Saw a set of 850 +0.020" and +0.060" on Ebay recently. Realize that it's not so common or popular these days for the smallest engine but would like to keep my Mk1 standard.

[nicklouse]

To follow on from the 1275 question, if an 850 needs a reborn, what are the increased capacities and what pistons and rings are available? Saw a set of 850 +0.020" and +0.060" on Ebay recently. Realize that it's not so common or popular these days for the smallest engine but would like to keep my Mk1 standard.

how big do you want to go? AC has taken an 850 out to over 1000cc.

[MikeRotherham]

Thanks for all the info.

 

Mine isn't anywhere near needing doing but I'd always wondered what options were available.

[surfblue]

To follow on from the 1275 question, if an 850 needs a reborn, what are the increased capacities and what pistons and rings are available? Saw a set of 850 +0.020" and +0.060" on Ebay recently. Realize that it's not so common or popular these days for the smallest engine but would like to keep my Mk1 standard.

how big do you want to go? AC has taken an 850 out to over 1000cc.

Not bothered about ultimate capacity, just curious as to what capacity each overbore provides and piston availability.

[ACDodd]

850 to 1150cc.

Ac

[Spider]

 

 

To follow on from the 1275 question, if an 850 needs a reborn, what are the increased capacities and what pistons and rings are available? Saw a set of 850 +0.020" and +0.060" on Ebay recently. Realize that it's not so common or popular these days for the smallest engine but would like to keep my Mk1 standard.

how big do you want to go? AC has taken an 850 out to over 1000cc.

Not bothered about ultimate capacity, just curious as to what capacity each overbore provides and piston availability.

 

 

Cooperman touched on this, but generally +0.020" and + 0.040". JP and County have pistons for these, while not a high performance pistons, they are perfectly fine for a restoration.

[hhhh]

JP pistons can be quite heavy. I've seen their classic motorcycle pistons weigh in at up to 30% heavier than original which is a nightmare in a single.