15 replies to this topic

[Gilles1000]

Hi all,

 

does it make sense to build D-top pistons in my soon-to-be fast road standard pre-A+ 998? I could get 4 D-top for about £100 which look quite OK.

 

I plan to build a slightly modified 998 with a 12g295 which is original and I do not really want to skim.

 

Is it even possible? Can I just build my dished pistons and put the Cooper ones plug-and-play in my engine?

I know the bore and stroke are the same, but I have no idea concerning the axle height of both pistons and the CR which is going to be reached in doing that.

 

Regards

Gilles

[johnv]

I'm pretty sure that a pre A+ block just needs a camshaft, 'D' tops and a 12G295 head to become a 998 cooper ?

[Cooperman]

That's correct. Just cam, pistons, head, twin carbs and exhaust system will make a 998 Cooper engine. The distributor curve maybe slightly different.
A 998 Cooper will give c.55 bhp which will make the Mini not quite so slow.
Don't forget that the 998 Cooper had different gear ratios as well.

[Ethel]

If you're sure the head is unmolested. 12g295's had  big combustion chambers and the pistons took up some of that volume to get a reasonable compression ratio.

[Minigman]

If the 295 head is still standard thickness then the D tops make sense to save skimming the head too much. Or go flat top pistons and skim the head to suit but it’ll need a fair bit off it to make a good CR. Are the D tops standard bore? if so then make sure your engine still measures up to take a standard piston size.

Edited by Minigman, 11 March 2019 - 01:13 PM.

[Gilles1000]

Thanks for all the replies!

 

The head has a whole height of 2.75" which is the orignal height I reckon.

Pistons have the marking MOWOG, Front, a big 2 with for two pistons a " ^ " on the 2, R17 stamped on all of them and a further stamping which may be ME.80, could it be a +80thou repair set?. I will definitely measure their diameter to be sure.

 

My engine is as far as I know the original as it was in the car as I bought it and I do not see signs of the head having been dismounted once.

There was a nice blue smoke as well as I started it for the first and only time since I have it. So no sign of a rebuild before. I know, it does not give any clue on the health of the bores.

I do not either know if the block needs a slightly rebore even as thin as it could be.

 

Gilles

[Cooperman]

It is unlikely a D top exists at +0.080".
Engine building is a lot about measuring and calculating.
Do it right or it can get expensive.

[Gilles1000]

Yeah I agree with you, I think this is the most time and brain consuming job on a whole rebuild project. I do not want to say the body is easy to do, but the engine could be much more fine tuning.

 

Back to the pistons, I think I have heard D-tops to exist in +0.020 or even 0.040". But quite rare.

I will calculate the diameters of each and compare to these ones.

[r.tec]

Here are pix of the two 998 pistons, the 'raised D' Cooper and the Hepolite Powermax flat top out of my collection of Mini parts in case somebody has not seen them as now they are becoming quite rare:

 

Attached Files

•  Hepolite Powermax KR20950 (0)kl.jpg   139.6K   4 downloads

[Spider]

The D Crown 998 Cooper Pistons were only ever made in Std, +0.020" and 0.040".

 

The Raised D has a volume of 2.16 CC on a Standard size.

[Dusky]

Does anyone actually know WHY they made the chambers that big? A less than ideal solution for a better chamber shape?

[Spider]

Does anyone actually know WHY they made the chambers that big? A less than ideal solution for a better chamber shape?

 

Yeap, that's why - better flow !!

Have a close look at how shrouded the ordinary small bore heads are, very asthmatic !

 

A possibly smarter was to go would have been to fit flat heads and put the combustion chamber in the crown of the Piston.

[Gilles1000]

Better flow of course for the head. There is a huge imrpovement in the intake / exhaust channels and in the combustion chamber shape, but I think, generally speaking, the only pistons with an integrated combustion chamber are the pistons for Diesel engines.

 

Pistons for petrol engines are AFAIK always with raised top or a triangle shape for high compression engines (see BMW M3 E36 or two-stroke engines for example, however the shapes are for different reasons. BMW for higher CR, 2-stroke for better gas flow and less loss of fresh gas into the exhaust).

 

Usually from what I saw and studied for petrol engines the pistons are most of the time flat or slightly toped.

The real reason behind it... I cannot say... 

But I think there should be a reason for that if not State of the Art ;)

[rww]

The D Crown 998 Cooper Pistons were only ever made in Std, +0.020" and 0.040".

The Raised D has a volume of 2.16 CC on a Standard size.

[rww]

Just to clarify, the raised D profile pistons ' reduce' the combustion chamber volume by 2.16cc compared to a flat top ?