IMD flat top pistons.

[7iain7]

I ordered some new IMD pistons for my 1952 A10.
The old pistons are what I would call raised flat top pistons.
I'm Guessing the IMD flat tops are the correct ones?
There are a few issues with this engine that don't seem to add up. For instance, it was re-lined back to its standard and the bores still appear to be in excellent condition. However, second-hand pistons were fitted, and although both pistons are marked as 70mm, one measures 69.8mm while the other is 70mm. This is the reason behind the strip-down, as the engine is emitting excessive smoke from one of the exhausts.

Many thanks, Iain


PS these old pistons are marked 700H GPM inside.

[mikeb]

Ian - i fitted standard IMD pistons to my a10 when i sleeved it. they are lower compression - 7.25:1 from memory? hence having the flat top. that's the only compression ratio they make for A10s. if you google pistons pics on the forum you'll see the higher domes are higher compression. lower comp translates into a bit less crackle when turning the wrist but easier to kick over. and the real joy of IMDs is a modern 3 piece oil ring - mine don't burn a drop. they are a 'modern' piston - I like them.

keep in mind IMDs are quite a bit lighter that hepolites so you'll be changing the balance factor. so if you've conscientious you'll be splitting the cases and re balancing the crank. if you are not then don't worry about it, it will just vibe a bit at some rev range.
assuming your bores/sleeves are OK then check your clearances - most people settle on  4 to 5 thou - and a hone. it is weird you found a piston slightly undersized

as an aside i'm considering going back to higher compression pistons and shelving the perfectly good IMDs as I miss the crackle on the super rocket. but i'll get someone to machine out the lands so i can fit 3 piece oil rings - would be hard to go back on that one.

Edit: here's the notes i kept when going from +80 JPs back to standard IMDs. Your heploites will probably be much lighter than the +80 JPS.
JP (+080) - piston with rings: 299,  pins: 77.8 --> total 377
IMD (std - with pins, rings) --> total 310
difference = 67g. This would change balance factor by 9-10%

[chaterlea25]

Hi Iain,
Check the piston to bore clearances when you get the new pistons,
It may have been bored to suit the odd pistons

John

[sean]

I have the IMD pistons and liners ,yes they are a lower compression but bike is easier to kick over and no problems running on e10 .

[Colsbeeza]

Iain, I have fitted the IMD flat top pistons & supplied rings. The machinist honed and plateaud the bores to a #500 ( Usually about #150 for cast iron rings). He also did up the head new valves & seats etc. It was virtually run in immediately. Not a shred of smoke since. Only problem is that there is no gas leakage at all, so I can stand on the kickstarter for 5 minutes and the pistons don't move. I have had a few A10 enthusasts club members complain when trying to kickstart it. I am about 70Kg, so a heavier bloke will cope better.  Anyhow I fitted an electric start since I had a knee replacement and all is well.
Check that the pistons are level, as I had a duff small end and one of the old pistons tilted 10 thou. It took me three strip downs to spot the problem and replace the small end bushes.
Col

[sean]

Iain, I have fitted the IMD flat top pistons & supplied rings. The machinist honed and plateaud the bores to a #500 ( Usually about #150 for cast iron rings). He also did up the head new valves & seats etc. It was virtually run in immediately. Not a shred of smoke since. Only problem is that there is no gas leakage at all, so I can stand on the kickstarter for 5 minutes and the pistons don't move. I have had a few A10 enthusasts club members complain when trying to kickstart it. I am about 70Kg, so a heavier bloke will cope better.  Anyhow I fitted an electric start since I had a knee replacement and all is well.
Check that the pistons are level, as I had a duff small end and one of the old pistons tilted 10 thou. It took me three strip downs to spot the problem and replace the small end bushes.
Col

wish they would change their circlips  not the easiest one to install heavy wire .....

[groily]

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wish they would change their circlips  not the easiest one to install heavy wire .....
[/quote]

+1 for that. Very tricky little so-and-so's.