Hi All,
I'm hoping someone can help me ID a set of small bore pistons.

They are ACL & stamped with 20397 on top along with + 1.524 FTF.
Internally there is casting numbers in it. These are 203978 and 1128A
They measure 66.01mm on the bottom skirt 90deg to the pin which unless I'm mistaken makes them +060".

I've searched and searched but can't find relevant info on the net.
What I wish to know is if they are for 998 or 1098?

If anyone can ID them or give me a dimension I can measure to check it would be very much appreciated. Thanks, cheers All.

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68 Cooper, Clubby Van, 2Mokes

1. Measure the height from the crown to the centre of the pin.
Give the dimension here and we can check it. (I just don't have a piston handy at the moment to measure).

2. Look at the underside of the piston, is the pin exactly in the middle, or offset to one side? 998 piston pins are central, 1098 ones are offset.

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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.

no, the measurement you've got there isn't the oversize. There would be a stamping on the crown of the piston along with the other numbers saying '060'

measure from the centre of the pin to the top of the piston crown - that's what is needed to tell if they are 998 or 1098

stock bore is 64.6mm so that makes yours 1.5mm bigger, = + .060" (allowing for measurement accuracy and wear).

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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.

The pin looks pretty central by eye from the underside. Is this easily noticable?

The measurement from centre of pin to top of crown is 30.20mm

There are no other stampings on the top aside from the ones I already listed.

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68 Cooper, Clubby Van, 2Mokes

I'd say 998 (without going under the house and measuring a 998 one).
1098 are shorter than that.
Is there a radius cut on the bottom (looking on the pin axis)?
If not ie flat, they are 998.

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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.

"Is there a radius cut on the bottom (looking on the pin axis)?
If not ie flat, they are 998."

If I understand correctly - looking at the pin, below this it is cut out or scolloped approx 4mm. 90deg to the pin comes down lower than this.

Seems a little contridictory, maybe I've given wrong measurement.
top of crown to top of pin hole is approx 22.3mm
+ half of pin OD (15.82mm approx) = 7.90

so centre of pin to top of piston is 30.20

thought I'd break it down in case I was giving wrong measurement.

Thanks very much Doc and Simon.

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68 Cooper, Clubby Van, 2Mokes

I gave you a bum steer.
998 one is about 34mm pin height. (I measured with ruler)
So, yours are 1098.

998s ones (mine) also have a radius on bottom but not as deep.

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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.

Thanks Doc,
I was thinking 1098 from info you gave me. A closer inspection of the bottom it does look to be slightly offset. Its difficult to see but measuring it although a bit difficult to get in there it does seem to be to one side.

So it's only about 4mm difference between 998 & 1098"s. Good to know.

Thanks again!

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68 Cooper, Clubby Van, 2Mokes

Yep, stroke of a 998 is 76.2mm and a 1098 is 83.7mm.
The throw of the crank difference is 1/2 that, ie the pin height difference is (should be) 3.75mm. Rods are same for both.

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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.