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**flying farmer** · 20 April 2023, 20:56

I read somewhere years ago that Mk1 heads with Mk2 domed pistons would give a 10.5 :1 CR.

I have no idea if this is actually correct.

I have measured the piston bowl on some original dished pistons at 5.3cc, and on some +20 County pistons at 4 cc according to some stuff I wrote on the inside cover of my Haynes manual years ago.

Some +20 Mk2 domed pistons measured no effective bowl at all so the domed pistons make the combustion chamber about 4-5 cc smaller than the dished pistons.

Mk 2 heads have slightly bigger chambers than the MK1, and the static compression is 9.2 for the mk2 against 8.8 for the MK1, so I would guess that an unskimmed Mk1 head would have a chamber about 2cc smaller than a MK2.
Without going to the trouble of working it all out, I would have thought it unlikely that a Mk1 head would lift the compression as high as the 10.5 I have read about, that figure may have involved some degree of head skimming.
One problem with the MK1 heads is valve shrouding. The gap between the valve head and the wall of the combustion chamber is too small for good flow on both the inlet and exhaust, the MK2 is much better in this respect.
If the chambers are reshaped around the valves similar to the Mk2 head then there is very little difference between the two heads

Neil

**trunt** · 20 April 2023, 21:17

Mk1 heads do have smaller chambers, ISTR they are a slightly different shape for the “squish pistons” to work, I’m sure Neil knows more about that! As an aside the chamber difference caused the mk2 Federal to drop compression ratio from 8,0 to 7.75.

My mk1 heads cc’d out at 34.5cc but they had been skimmed, though not enough to cause any clearance issues, however with the skim and the +20 overbore I still had to use thick gaskets to keep the CR down to 9.2. (County pistons)

I often wonder if many with overbores and skimmed heads are pushing the CR without realising it, especially with a mk2.

Neils 4cc for county piston ddish is around what I measured..

**Wheelz** · 21 April 2023, 11:38

I have been told that you can't do it, but so far then, no mention here of any concern about contact between piston and head nor between piston and valves, assuming of course that measures are taken to compensate for skimmed heads.

My experience is that my engine ran OK with this combination.

**DJT** · 21 April 2023, 12:26

The Mk1 engine in my fist Stag was rebuilt for me using flat top pistons. The RH head was sent away for an exchange head from a well-known supplier in Lincoln who sent back a Mk2 head. This was fitted and it was some time before I became aware of the mis-match. The engine was run in this configuration for 14 years (45,000 miles) before I acquired and fitted a matched pair of Mk1 heads.

**flying farmer** · 21 April 2023, 19:35

Originally posted by trunt View Post

Mk1 heads do have smaller chambers, ISTR they are a slightly different shape for the “squish pistons” to work, I’m sure Neil knows more about that! As an aside the chamber difference caused the mk2 Federal to drop compression ratio from 8,0 to 7.75.

My mk1 heads cc’d out at 34.5cc but they had been skimmed, though not enough to cause any clearance issues, however with the skim and the +20 overbore I still had to use thick gaskets to keep the CR down to 9.2. (County pistons)

I often wonder if many with overbores and skimmed heads are pushing the CR without realising it, especially with a mk2.

Neils 4cc for county piston ddish is around what I measured..

I have found some notes refering to Mk2 heads as having a 36.5cc combustion chamber so it might be close to the numbers I guessed at.

I know I measured my MK2 engine as having a CR of 10.1 :1 when I built it about 12 years ago. That engine has standard cams and runs fine on regular unleaded. My higher performance engines use fast road cams and a 10.5:1 CR and only need regular unleaded, but its possible to run more static compression with long duration cams as they are not so efficient at low rpm.

Standard 40 thou thickness head gaskets add about 6cc to the combustion chamber, so the extra thickness head gaskets at 60 thou add an extra 3cc which is probably the difference between Mk1 and Mk2 heads
Neil

**jbuckl** · 23 April 2023, 20:28

Have to do compression ratio checks regularly in 3D cad. For work….. until recently moving to electric powertrain

If you need a ratio check doing, I do have 3d cad for mk 2 heads & pistons…. At std size. Easy to adjust for rebores & skims.
Dont currently have mk1 pistons or heads in 3D though.
can anyone help with those? Sketch or cad ?

its relatively simple to check once the cad is available.

remember :- a rebore increases swept volume, as does a thicker head gasket.
a head skim usually reduces clearance volume.

Compression ratio is s.v. + c.v. / c.v.

mk2 pistons do seem to clear if used with mk1 heads, generally speaking.

would be interesting to check various combinations for clearance’s & compression ratios.

just need the mk1 3D combustion chamber & mk1 piston crown 3D data….. then all is possible to check.

however, 3D cad is not a panacea…. Real world measurements are also necessary…. But most cannot be bothered these days it seems.

In my younger days, actual measuring volumes was a very regular practice though.

only saying as real world combustion volume measurements seem to have been forgotten about these days.