12g206 head thickness

[BaritoneUke]

Got hold of one these (fairly scarce) heads, looks in reasonable fettle, somebody's put in slightly bigger inlet valves but I understand this was common BITD, I've put a vern. calliper on it and I'm getting a thickness of (mm)
68.84
68.80
68.80
68.52
68.70
69.22

average 68.81 (2.71)
depth of rocker oil-way 65.70 (2.59)

This look normalish?
cheers

[philthehill]

Normalish......I like it

The head should be a depth of 2.75" (69.85mm) unskimmed.
Therefore I would suggest that the head has been skimmed.
The 12G206 combustion chamber at 28cc is quite large and needs to be skimmed from standard by around 0.060" if the head is to be fitted to a 948cc or 998cc 'A' Series engine.
The standard valves are 1 7/8" inlet and 1" exhaust.
There should be at least 0.040" between the head to block face and the bottom of the vertical oil riser to the rocker shaft.

[BaritoneUke]

Thanks for that, looks promising. Yes I thought skimmed too. It's had a bit of life somewhere already. Going to clean it up proper now as it's collected some fine grit in it's travels. I was planning to put it on a 1098 IF that engine come through a thorough examination...but it could go on 948...I've one to exchage for a good recon. short block. Any qualms about using it on a 1098?
Cheers
Richard

[philthehill]

No problems with using a 12G206 head on a 1098cc 'A' Series engine.
The head was also used on the 998cc Cooper but that engine had raised 'D' crown pistons which reduced the combustion chamber capacity.

[BaritoneUke]

[liammonty]

You'd likely need to skim it a little more beyond what it's already had (or at least measure the combustion chamber sizes as they are now) even if fitting it to a 1098, as the chamber sizes are larger than the standard 1098 12G202 head ones. Measure it properly before you fit it to make sure you get it right as it's easier to do before you fit it

[philthehill]

Forget about doing anything else to the 12G206 head until you have measured the thickness of the head below the vertical oil riser. If you do not have at least 0.040" then everything you do will be a waste of time.

[BaritoneUke]

You'd likely need to skim it a little more beyond what it's already had (or at least measure the combustion chamber sizes as they are now) even if fitting it to a 1098, as the chamber sizes are larger than the standard 1098 12G202 head ones. Measure it properly before you fit it to make sure you get it right as it's easier to do before you fit it

Depending on the measurements I suppose flat-tops are possibility, or even dome-tops but I think it's been scalped so would need CR reducing gasket which many engineers frown apon
Currently awaiting cylinder head vol. measure - just got a 5ml syringe with 0.2cc gradations so that should do. Then vol. above top ring at tdc. Then crushed gasket thickness which I think I'll prob. estimate from what I can find on the web, prob. only significant if you're into challenging CR territory.

[liammonty]

There's a lot of useful info on that in David Vizard's book 'Tuning the A-Series, if you have access to it, regarding determining CR.

Use a sheet of glass or perspex over the cylinder head chambers too for an accurate measurement

[BaritoneUke]

There's a lot of useful info on that in David Vizard's book 'Tuning the A-Series, if you have access to it, regarding determining CR.

Yes, essential reading once you start on some proper fettling. I got the 3rd edition from an online bookshop.