Hi everyone I have pistons with letters and numbers on them. I have looked at 305 pages of previous chat and used the search over the past 2 days I have asked a mini parts supplier and a MG parts supplier the same question without any help both telling me to see an engine rebuilder all I want to know is how big of an oversize my pistons are .I do need to buy pistons and things but looks like I wont be buying from them . I have two sets of pistons in two motors one is a 76 998 and one is a pre 76 1100 from a morris 1100 sedan so im told and both have the same set of letters and numbers on them looking down on top of piston
they are lD6 T15 at the top and +1.016 and an arrow
they have 3 compression rings and two oil rings and seperater in bottom groove looks like 20397A is inside the skirt the pistons have clips holding the pin in
the piston measures 2.5665in 65.14mm and the hole they came from measures 2.5815in 65 57
the questions are what over size are they at and do we need to bore the block to the next size up
this would be greatly appreciated
ps don't know where this is going or how long it will take to come back new at this

in the How-To forum on this site you will find a Workshop Mini Manual. Its for a 67 but the 998 engine (and the 1100 block) is for all intents identical.

viewtopic.php?f=25&t=84427

So to identify the oversize, and therefore the next bore size. Your bore from your measurements is 2.5665, and from General Data page 12, the standard bore size is 2.543. Subtracting your measurement (being bigger) from the other gives 0.023 inches difference, that approximates a 20 "thou" of an inch overbore (More precisely, 23 thou). So the next bore you would consider would be 40 thou over bore.

The two pistons, although identical in bore sizes are not the same between a 998 and 1100 engine. The skirt on the 1100 is a little shorter to clear the longer throw crankshaft, so are different part numbers.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

damn, I would've said different to that Mick, I reckon the existing pistons are 0.040" oversize.

standard bore for a 998/1098 is 64.58mm. Add 1.016mm to that and you get 65.596 which is 2.582"

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did I tell you that I won a trophy?

I am working off the imperial measurement Simon, have I made a mistake just working with that? I don't like to swap back and forth imperial to metric.

Even working with the metric measurements I get 21 thou....

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

1.016 marking means they are +.040" oversize now.
This ties in with your approx bore measurement, the original bore 64.6mm and is now 65.57. near enough to 1mm = +.040" difference.
Bore sizes are the measurement, standard sizes these days are std, +.020, +.040, and +.060. You can go bigger......
Pistons are smaller than the bore and are never measured at the top. They are much smaller there to allow for expansion.

Next oversize available to you is +.060" , will give 1152cc.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

How did I stuff that up...

The maths from the measurements seem good, Is there two engines and two bores? A part number from one and a measurement from the other?

Thanks Doc and Simon..

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

I think it's more that you've gotta go off the bore measurement not the piston measurement - IE a .040" piston oversize fits into a .040" oversize hole with whatever clearance

rj's bore is spot on .040" and the piston has 1.016mm stamped on the top, which put two and two together for me

lol, 2 replies while typing...

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did I tell you that I won a trophy?

so first correct answer gets a prize, right??

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did I tell you that I won a trophy?

Ha. Thanks Simon. You get a sticker next time you drop in.

Other than that I'll crawl back into my box

Mick

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

it's usually me crawling back into my box after getting things painfully wrong... but at least when someone points out that I'm wrong they actually give the correct answer to the question instead of ignoring it. You posted your original reply while I was typing mine, which I then adjusted slightly ("damn") but very carefully checked my numbers

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did I tell you that I won a trophy?

Peer review!

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

thanks guys

can I put a new set of flat tops in these bores ? just asking

Well you will probably find the bores are worn oval & tapered, , the usual practice is buy a set of oversize pistons, then get the engine shop to bore and hone to suit. Putting new pistons into old bores doesn't work.
Your choice to buy, flat or dished.

Price to bore & hone 4 cyls is $100-200 total, depending where you go.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

ok cool thanks a lot im restoring someone else's nightmare so will end up with 76 clubby 1100 eng 295 head a cam with bigger lumps on it [sorry about the technical terms]on a set of 12in contessa's thanks again for your help