Pistons - dished or domed?

[mole42]

My car has a recon engine that was fitted in 1985, it's in the correct range (20000) for the year.

When I had the heads off just recently, I noticed that the pistons were dished. I have seen mention of domed pistons being used.

Has mine got the wrong pistons? what effect would that have?

Richard

[nib]

Hi Richard
Mk2 (from 20000 series) normally had domed pistons(my old jan 73 build did anyway) but as yours is 72 there's no telling as it is earlier still in the MK1/MK2 crossover.
The MK1(dished) was listed at 145bhp and the MK2 was 146bhp(domed) so I don't think your missing out on an extra 50bhp.
Mark

[tonyh]

There were three types of pistons fitted, federal Stags had a fairly deep dished piston, MK1 cars had a flat top, very small dish, MK2 had a dome top piston.

In the early 90’s dome top pistons were no longer produced as the early piston can be used on both MK1 and MK2 engines.

The MK1 heads have a slightly smaller combustion chamber than the MK2 heads, dome top pistons should not be used with MK1 heads as it raises the compression ratio and can cause problems for a normal road car, MK1 flat top pistons can be used with both MK1 and MK2 heads, using them with the MK2 heads drops the compression ratio very slightly, this helps with low octane unleaded fuel, it also drops BHP by about 1/2 % , this is unnoticeable on a normal road car.

The casting number on MK1 heads end in 72 and 73, casting numbers for MK2 heads are 10 and 11

Tony.

[tractorjames]

I always thought mk1 power was somewhere in the 120's hp?

James

[mole42]

Thanks Tony, it looks as though I have a Mk1 engine, the heads are 72 & 73 (but the cooling system is Mk2, 20psi.) Typical MK1½ car!

Engine was replaced in 1985, recon unit costing £750 (those were the days!) - new engine number is LF21162HE

James, you are thinking of the 127BHP USA spec engines, the Mk1 UK spec were 1BHP lower than the Mk2.

Richard

[nib]

I'm doing my David McCallum impersonation,lol.

[tractorjames]

Thanks Tony, it looks as though I have a Mk1 engine, the heads are 72 & 73 (but the cooling system is Mk2, 20psi.) Typical MK1½ car!

Engine was replaced in 1985, recon unit costing £750 (those were the days!) - new engine number is LF21162HE

James, you are thinking of the 127BHP USA spec engines, the Mk1 UK spec were 1BHP lower than the Mk2.

Richard

Thanks for clearing that up, knew there was something involving that kind of power.

[HersnotHis]

Thanks Tony, it looks as though I have a Mk1 engine, the heads are 72 & 73 (but the cooling system is Mk2, 20psi.) Typical MK1½ car!

Engine was replaced in 1985, recon unit costing £750 (those were the days!) - new engine number is LF21162HE

James, you are thinking of the 127BHP USA spec engines, the Mk1 UK spec were 1BHP lower than the Mk2.

Richard

But then surely if you have deeply dished pistons your engine is effectively the lower compression 127BHP US spec engine??

H

[GDPR]

So you feel that Tony Hart is wrong?

The lower BHP was due to ignition and carburation differences for the US emissions requirement

[HersnotHis]

So you feel that Tony Hart is wrong?

The lower BHP was due to ignition and carburation differences for the US emissions requirement

In part may be, but Federal Stags also had lower compression ratios which I assume is as a result of using dished pistons which in turn I would anticipate was also a significant factor in the over 12% power reduction?

H

Nemo Me Impune Lacessit

Ad utrumque paratus

[tractorjames]

Surely if UK domed pistons with MK2 heads is 146hp and UK flat top/dish whatever you want to call them pistons with MK1 heads (smaller combustion chamber makes up for some of the compression lost for losing the dome) is 1hp less, them using flat top pistons with MK2 heads is going to produce considerably less compression and maybe power than a standard MK1 setup, partly down to the lesser compression (less than normal MK1 spec) and partly down to the now incorrect advance curve in the dizzy?

Man there's a lot of mores and lesses in there

James

[Staggard]

So you feel that Tony Hart is wrong?

The lower BHP was due to ignition and carburation differences for the US emissions requirement

I am surprised you are arguing with the professional Russ now get back on that naughty step

[mole42]

But then surely if you have deeply dished pistons your engine is effectively the lower compression 127BHP US spec engine??

H

In post #3 the Mk1 Pistons are described as having a shallow dish - as mine are. I also have Mk1 heads, so the engine should be basically Mk1 spec although it has Mk2 cooling and god knows what ignition.
As I said earlier, the engine number is LF21162HE so that puts it firmly into the Mk2 range, although as it's a recon, who knows what parts they used, in the days when it was essentially a distress purchase!

Richard

[DJT]

In post #3 the Mk1 Pistons are described as having a shallow dish - as mine are. I also have Mk1 heads, so the engine should be basically Mk1 spec although it has Mk2 cooling and god knows what ignition.
As I said earlier, the engine number is LF21162HE so that puts it firmly into the Mk2 range, although as it's a recon, who knows what parts they used, in the days when it was essentially a distress purchase!

Richard

Richard,
The 'Mk2' (20psi) sealed cooling system was introduced quite early in production. Sometime in 1971 when the Mk1 was still in production. James Taylor quotes LD10747, but I think it was earlier as my first Stag (LD10456) had this system.

[mole42]

Thanks Dave. It's becoming quite an adventure, seeing which spec parts were used in this car! It's commission number is LD20165, built in November 1972, but it has a very early (shim-adjusted) differential, it has a night-dimming relay, there is no hazard warning switch or seat belt warning lights.

Richard