12G940 head skim for 1098

[adman]

Hi all,

First time in a very long time as rediscovered the fun of driving my (now 62 yr old) splitty.

I've got a 12G940 head sitting around from my mini days which I plan to fit to my 1098. I know it's advisable to recess the valve seats, and thinking I may as well get unleaded seats fitted in any event.

The mating face has a degree of surface rust, so thinking about a skim just to clean it up. However, I'm not sure if this is advisable or not in relation to affecting compression.

I will also be fitting a bigger carb and Maniflow LCB/large bore exhaust at the same time.

Any advice welcomed.
Thanks,
Adam.

[paul 300358]

My 12G940 had a slight skim before fitting just to remove a few marks. But after fitting the head a re bore was required as the wear in the rings and bore became apparent. Apparently it is quite common when replacing the head on an old engine. Originally the bores looked good but the wear became obvious when measured.

[Monty-4]

Whether skimming will shoot your compression ratio too high will likely depend on whether, or how many times, the head has been skimmed before! The stock thickness is 2.75" (I had one that ran fine at 2.715") and you want 0.320" clearance from exhaust valve face to block to be safe.

[philthehill]

In relation to skimming and compression:-

What increase in combustion chamber volume you will get by sinking the exhaust valves you will loose by skimming the head.

The 12G202 head (1098cc) has a combustion chamber volume of 26.1cc - the 12G940 head (1275cc) has a combustion chamber volume of 21.4. A difference of 4.7cc.

If you are concerned and I would not be as regards a light skim you could always fit a decompression plate (or second head gasket) between head and gasket. A head gasket gives approx. 3 - 4 cc per cylinder dependent upon type/make of head gasket used.

If you are going to have unleaded exhaust valve seats fitted make sure that you get the sinking of the valves done at the same time as those unleaded valve seats are hard which means that unless you have valve seat cutting/grinding stones you will not be able to do the job yourself post having the unleaded seats fitted.

[paul 300358]

A machine shop will only fit hardened seats if the valve guides are in good condition. They use the guide to centre the insert. The bypass on my 12G940 had a blanking plug fitted, I managed to undo it by welding a old wheel nut to it and then using a socket and 600mm breaker bar. Bypass stubs are available from the usual suppliers.

[philthehill]

by-pass adapter.JPG (640.24 KiB) Viewed 1299 times

Not all bypass stubs have the right thread.
The correct thread is 5/8" x 16 TPI UNS.
The correct item is available from Mini Sport Padiham Pt No: 12A2075.
The bypass stubb may be available from other suppliers but check that the male thread is correct. Some are supplied with a 5/8" UNF thread which will not fit into the head.

by-pass tap.JPG (1.64 MiB) Viewed 1299 times

[paul 300358]

The last 2 that I got from Bull Motif were both correct.

[philthehill]

very good

http://www.morrisminorspares.net/shop_item.php?ID=1759

Pt No: 10M277,

[adman]

Thanks folks, useful advice, and hadn't considered the head may have been skimmed before! I guess I need to pop the head on and try the bluetack test, see what I can see and go from there.

I shall report back. In due course... Not sure when I'll be playing with it.
Adam.

[ampwhu]

by-pass adapter.JPGNot all bypass stubs have the right thread.
The correct thread is 5/8" x 16 TPI UNS.
The correct item is available from Mini Sport Padiham Pt No: 12A2075.
The bypass stubb may be available from other suppliers but check that the male thread is correct. Some are supplied with a 5/8" UNF thread which will not fit into the head.
by-pass tap.JPG

i have this tap here in Surrey (near Chessington) . 5/8" x 16TPS. if anyone is local and needs to clean the threads up when fitted a new bypass stub, then i can help.