Stag Mk1 & MK2 Engine - cylinder head / piston combinations....

[Ferds]

Okay I've searched for a definitive answer on this without any luck so here are the questions...

1) Is it correct that domed pistons were only used in Mk2 engines?

2) You cannot use a Mk1 head on an engine with domed pistons - due to valve clearance issues?

3) You should always have a matched pair of heads - a pair of Mk1 or Mk2s otherwise the engine will not run properly (inbalance)?

Thanks
Ferds

[Guest]

I think you will see the answers to all this on the flying farmer threads about head mods.
There is a fairly large difference in chamber volume between the early and late heads, which is what they made up when making the domed type pistons.

The valves are inclined off the vertical in these heads, but the valves sit in exactly the same height relative to the deck, no matter if it's Mk1 or Mk2.
(They have to be otherwise the cam/chain/tappet clearances would have to be changed and all sorts of other datums).

Because the valves are inclined off the vertical, and the rods are very short, there is very little lift of the valve at TDC (because the camshaft is very mild timing), the clearance problem doesn't arise at TDC.
However on the TR7 and later dolomites they fitted a substantially larger inlet valve, but with the larger bore they could get the compression ratio back up easily without clearance issues.

When you have short rods & short stroke combined, the dwell time of the piston at TDC is affected.
This means as the engine is OHC, the valve accelerations are relatively high, so the valves can be opened relatively fast.
This means the valve to piston clearance problem invariably arrives before and after TDC.
That is why there are often digs in the crown of pistons after over -revving allowed them to meet each other.

I suspect because the exhaust valve is so much smaller, it can't hit the piston, but the inlet valve probably has clearance issues at around TDC +10 or +15 especially if the heads have been skimmed a lot.
Skimming the heads on a OHC engine affects the valve timing also.

Being as valves are designed to rotate, I suppose the valves don't get bent after repeatedly kissing piston crowns, because they are constantly being turned and another part hits the crown next time around (!)

On the Stag it has a much smaller bore, as it was originally designed around 2.5L and the dolomite cousin around 1700cc.
That is what makes it hard to get a decent CR on the V8.

This means, the answer to your question:- can you fit Mk1 heads on a Mk2 block is YES of course....it gives much higher CR.
Probably the ultimate "cheap tweak".

However, I don't yet have the flow figures in, but when I do I should be able to demonstrate whether or not the modification to the chamber from Mk1 to Mk2 that Triumph did (to relieve the valves) was actually worth anything.
I suspect it makes a fat lot of difference in view of the already extremely poor breathing of both the inlet manifold system and the really dreadful inlet ports.

You don't want to be mixing head on an engine, for the simple reason one bank would have a much higher CR than the other.
On a V8, cos it's not inherently balanced, it would cause some really nasty secondary crank vibration and uneven loadings on the main bearings. (that is to say even worse than it originally had!)

[flying farmer]

As above, but if you have a mismatched pair of heads as in one mk1 and 1 mk2 it is possible to open up the chamber of the mk1 heads to equalise the compression ratio using the mk2 heads as a pattern.

The real problem would be the ignition timing couldn't be set for optimum efficiency with greatly different compression ratios from mismatched heads, though whether you could notice much difference in real world driving conditions is debateable.

The dome of the Mk2 pistons misses the area where the valves open so this is not an issue.

The factory only fitted domed pistons to the Mk2 engines, but since most engines will have been rebuilt more than once anything could have been fitted. The Mk1 engine in my 246bhp Stag was fitted with +40 thou domed pistons when I got it, but it had also been fitted with mk2 heads
Neil

[Lingen]

Re the reply by down_the_plug_hole, I think the comment in paragraph five may be incorrect. The cam followers (tappets) are made to rotate by being very slightly off centre beneath the cam lobes. This helps to equalise wear in use. But the valves themselves cannot possibly rotate as their stems are firmly wedged in place to the valve spring assembly by the cotters.
Mike.

[tonyh]

Valves do rotate, https://www.youtube.com/watch?v=_REQ1PUM0rY

[Guest]

Sorry, yes, Tony is absolutely correct.
Rotation of springs and valves is a long established fact.

eg. My STD OEM, Jaguar 24V collets (cotters) and caps are specially made and designed to have a specified groove shape and working clearance, so that there is no resistance at all to rotation,- particularly the exhausts which have a triple grooved collet.

In fact it's extremely suprising to check out in your hands, push the collets hard into the caps, with the valve clamped hard between them.
Voila, practically zero resistance!

Older style valves and collets are not quite so good in this respect, but we weren't talking about the rotation of cam followers (both pushrod -OHV and OHC designs rotate).
+
I have it on good authority from 2, major piston ring manufacturers, - SIM and Goetze, that piston rings ALSO rotate - usually several times a minute, making it pretty much pointless fitting rings with gaps staggered.
In reality it makes no difference whatsoever (!)

[Lingen]

I stand corrected, and am amazed at the video of it happening. I couldn't visualise how the high friction between the head and valve spring seat could be overcome, but it seems that at very high revs, that some sort of harmonic oscillation occurs which briefly overcomes the normal expansion of the spring, (shortens it) allowing it to be turned. Worrying!
I will keep below 7000 rpm to avoid the situation.
Mike.


I

[Guest]

Lots of things have resonant frequencies, coil springs, drive shafts, crankshaft torsional stresses, suspensions, and even wine glasses.
A singer can smash a wine glass if she picks the right note.

If we couldn't get resonances to happen in simple things like crystals and coils of wire there wouldn't be any radio or television.
In fact, anything subject to sinusoidal oscillations, given enough energy will demonstrate uncontrolled movement unless damped in some way or other.
Loudspeakers do it, by having stiff cone surrounds, lots of stuffing/damping and avoiding frequencies which make things go out of linear movement range.

The trick is usually how to increase the level of excitation without losing control or breaking something.
That's what engine tuning is all about,- same as in electronics or nuclear physics for that matter.

[richardthestag]

I stand corrected, and am amazed at the video of it happening. I couldn't visualise how the high friction between the head and valve spring seat could be overcome, but it seems that at very high revs, that some sort of harmonic oscillation occurs which briefly overcomes the normal expansion of the spring, (shortens it) allowing it to be turned. Worrying!
I will keep below 7000 rpm to avoid the situation.
Mike.


I

I see what you did there

[Guest]

I did once prove a pushrod engine with exactly the same length of valve as the Stag could run reliably with a single valve spring at 8000rpm without bouncing.
Peak power was 6800rpm, and the engine (a road car) made exactly 100bhp/L from 1300cc.
(I will leave you to calculate torque).
Valve lift was under 0.400", and it was 100% reliable.

In this case, there is good reason to imagine,especially as it's OHC, (ideally using a slightly lighter cam follower), that the Stag head will do exactly the same revs perfectly reliably.

[Ferds]

All!

Thank you for the very informative replies. In the 30+ years of owning and working on classic cars (17 of which are my Stag) I've not got into this level of detail regarding combustion chambers and the difference between Mk1 and Mk2 engines...

I have a spare Mk2 engine which I will tear down (hopefully) with a view to building one good engine out of the two... however if I had a Mk1 RH head I could very well be tempted to build a higher compresssion engine... perhaps I'll do this next!

Best Regards
Ferds