+0.020" D-top 998 Cooper Pistons (New advice needed)

[alexm]

Chaps,

I've found a chap on another forum selling some +20 thou D-top cooper pistons. They've made me think it might be nice to take my standard bore 998 cooper engine up a size (1016cc?) They appear to be Powermax Hepolite and the markings on the box say:

Ref: 18516/KR +0.020/0.508mm
Suitable for Morris
Original bore 2.5420"/64.567mm (LC25 037)

- Are oversized D-tops quite hard to come by - I don't think they're available from any of the mini spares places any more?
- Are the Powermax pistons of good repute?
- Is £150 for the set (that appear to have some rings too) a decent price?

Any advice would be great. The money should be going towards my bodywork but I think this is quite a good opportunity.

Alex

[Spider]

It does appear a Hepolite number, however a few other manufacturer's seem to also use their numbers, but I'd say being raised D crowns, they'll be the real deal. A good piston for sure but - at over the counter retail prices - I've always thought them to be very expensive and there seems little about them when compared with pistons at a lesser price to warrant that high price.

About the only thing I'd say to watch or rather be aware of is that Hepolite when to a metric ring (which those may or may not be) a while back and every other ring manufacturer for our pistons still uses imperial sizes, so you'll be locked in to using Hepolite rings, of which some types I've found to be far less than satisfactory. The other option here is to have your machine shop open the ring grooves to take the imperial rings.

[Mini4Ever]

If these are NOS, then I can only say: BUY THEM as this is fairly cheap for oversize D-tops.

You can still buy new rings for them from Minispares.

[mk1]

These are a bargain, snap them up ASAP!

[alexm]

OK, thanks very much for the advice. I've entered negotiations with the guy!

One final question is regarding the gudgeon pins for these pistons. I believe for a 998 cooper they're floating with circlips. If the piston set doesn't come with the gudgeon pins, could I use the old one's from my cooper engine or easily buy the pins/circlips from a mini spares outfit?

[Spider]

Don't re-use the old gudgons, you'll be able to get new ones no problem. Likewise NEVER re-use the circlips, these will also be available. Always fit them sharp side OUT and ears up or ears down. I like to put a dab of loctite on them and after fitting in the groove, expand them out hard with the circlip pliers just to be sure they are seated.

[alexm]

Last question I promise. Should these powermax pistons be marked somewhere (on top?) With +20 or +0.020 just to help me identify them as genuine and not standard size? I'll ask the seller but want to check with those who may have used them before.

[Spider]

They are marked on the crowns, I can't recall exactly what the marking is and it'll have the P/N too.

[alexm]

The pistons apparently only have the part number on, so the seller has said he'll try and get an accurate measurement with vernier callipers.

If the standard 998 cooper bore is 64.567mm (2.542") and a +0.020" rebore is 0.508mm, the pistons should measure at around 65mm (assuming a few though or 0.075mm) of piston clearance.

Please excuse my mixture of metric and imperial!

[Spider]

The pistons apparently only have the part number on, so the seller has said he'll try and get an accurate measurement with vernier callipers.

If the standard 998 cooper bore is 64.567mm (2.542") and a +0.020" rebore is 0.508mm, the pistons should measure at around 65mm (assuming a few though or 0.075mm) of piston clearance.

Please excuse my mixture of metric and imperial!

OI! Better drop another 20 quid in the slot there mate

Last question I promise.

OK, down to a serious answer.

No disrespect to anyone at all, but measuring pistons for the un-intiated can be fraut with risk! Piston crowns are usually round, but a smaller diameter than the skirt and the skirts are never round. Measure them on the skirt, about 0.250 to 0.500" up from the bottom of the skirt and most importantly at 90 degrees to the gudgeon pin. If there is an information sheet still in the box, it should show you where to measure them, the guys boring & honing the block will need that info. Typical running clearances will range from 0.002" to as much as 0.005" but again, this should be on the info sheet.

So, the stock bore dia is 2.542" + 0.020" = 2.562" as a nominal size (it should be bored and honed to suit the pistons, not done to a specific size), so then the pistons should be 2.557 to 2.560". I'll leave that new money conversion to you.

[alexm]

I'll stick that last question on your tab Spider!

New money conversions on your rough sizes are around my estimate of 65mm or a shade under. I think for the purposes of seeing whether these are +20 thou pistons a couple of vernier caliper measurement on any part of the piston should help, but agreed the people who eventually measure them before boring/honing need to do it properly as per your advice.

Thanks again

Alex