I have a 1970 MK3 with a supposed 1275cc engine I am trying to restore. I ordered new pistons but the new ones are not the same size. The original pistons have a 71.6mm (2.82in) diameter, the new pistons are 70.8mm (2.79in). Which are the correct size piston for a 1275cc engine? The block is being refurbished so not currently available to check cylinder bore to confirm engine size. I can provide pics of pistons and con rods if needed. Thanks in advance for any insight provided.

 

As far as I know there is no 'correct' piston size for the A-series engine . . there are multiple different versions of that engine, with a variety of piston sizes. Since your original pistons are 71.6mm, why not just replace them with 71.6mm pistons

 

I ordered pistons, pins, bearings, and con rods based on the engine size I was told. Are there any identifying marks on the block to confirm the engine series? Best to confirm, I might have been told the wrong displacement.

 

who'd ya buy from?

I've always had excellent phone support from Seven Enterprises and Mini Mania ....

they may not be the least expensive, but sometimes the extra $ pays off ....

 

Who ever is doing the refurbishment on your block can tell you what size you need. Usually have the machine shop check the bores to see if they need to go up one size first, then order the pistons. Have known a few guys that order ahead of time only to find out they ordered a sized too small!

 

Potential answer is found in the 'workshop manual' and the general data section

assuming the engine is correct to a 1970 the workshop manual lists the bore for a 1275 as 2.780 inch 70.6 mm (note that the bore for the Cooper and S motors of 970, 1071 and 1275 all had this bore. The stroke differed, giving the displacements. The later 1275GT had this same bore ...) {btw the 998 bore is 64.588mm ; 997 is 62.43mm; 848 is 62.94

This might suggest you have an engine that has oversized pistons due to earlier rebuild?
2.78 + .040 oversize = 2.82 inches ...



I'd like to know what you find out ...

 

I agree, the machine shop should tell you which pistons you need, the engine my have been overbored before like the capn says......

If the engine plate is still present, post a pic of it and we can decode what engine it has, but you still need to measure the bores to know which pistons you need.

 

Thanks for all the input. I bought the car and imported from Mexico City with the engine and most of the car already disassembled. The pistons were ordered from a local shop in Mexico who was supposed to be a MINI specialist. Some of my research says pistons should have a full skirt on the piston, the pistons I got do not have full skirts and the "originals" do. Is this correct? The block is due back from the shop next week and I will verify the cylinder bore then. The crankshaft was a total loss so unable to verify stroke. Any recommendations to source the crankshaft?

 

https://www.7ent.com/products/cranks...ed-aeg315.html

Crankshaft, 1275/Cooper S, Used (AEG315)

$1,995.00

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https://www.minimania.com/Piston_Information

https://www.minimania.com/Piston_Fact_File

 

Thanks, I don't know why I haven't seen this info. I've been all over MiniMania for my '06MCS.

 

An update for those interested.
The machine shop refurbishing the block noticed some issues with the cylinder placement. As it turns out, the cylinders have been bored over to achieve a larger displacement. Apparently there is an offset for the two middle cylinders that has to be factored into the new cylinder bore center, this was not done and the two middle cylinders are now too close and do not have proper crankshaft alignment. The misalignment is very small but catastrophic for my current setup and the only option to correct it is a custom crankshaft. So, I'm scrapping the entire current engine setup.
Thanks for all the opinions and advice.

 

MiniWorld "summer" issue has an article of choosing a 1275 replacement block .... discussed differences in the A vs A+ and certain fitment issues ...... and goes on to discuss modifications.

"1275cc Mini and Metro engines: buying, rebuilding and tuning"

they usually have back issues available thru their web site ..... www.miniworld.co.uk

 

all according if the engine is std un touched 70.6mm or has had a rebore in the past looks like its had 1st stage rebore from your measurements

 

I know a guy who is selling an A+ block that has been bored .040 over with pistons and new cam bearings. If you're interested I will contact him. He is in the Phoenix area.