12g1316

[Bigrob]

Evening all


I've just got my hands on a complete 12g1316 for a relatively ok price (I didn't pay £400 for it).


Now am I right in thinking it's basically a US spec 12g940?


It has 'smog holes' (whatever these are) in the head.

How would I go about plugging these?


Cheers all

[philthehill]

The head was used on American market 'A' Series engine in an attempt to combat engine pollution.
The smog holes are where air was pumped into the cylinders to combat that engine pollution.
The air pump was mounted on the O/S of the head and was driven from the crankshaft.
The holes can be blocked without loss of power to the engine. You may have to drill & tap to fit the plugs. You may even get away with just screwing plugs into the existing threads.
Here is a picture of the setup:-

MG midget exhaust air injection.jpg (63.89 KiB) Viewed 937 times

[Bigrob]

Beauty thank you Phil.


Would this not have the same effect as having a leaner mixture?

I'd be happy to drill and tap it if I can't get the correct thread.

Would brass or steel be sufficient with a bit of loctite or equivalent?

[kevin s]

Where do the drilings go? All the systems I've seen inject air into the exhaust to burn off hydrocarbons, they look like they go onto the cylinder though? Or do they go right accross the head.

[philthehill]

The air passages go across the head and into the exhaust tract of each cylinder.
Here is a 12G1316 head for sale on 'e' bay and it clearly shows the air pipes exiting into the exhaust port
https://www.ebay.co.uk/itm/MG-MIDGET-EN ... 2608093342
It does make the exhaust mixture lean.
Steel plugs would be best.

[Bigrob]

In that case if I just wind a bolt in and 'trim' it at either end that should be fine?

[philthehill]

Just screw the bolt in from the outer face of the head. It does not have to go all the way through. You do not have to block the hole fully.
Any small protrusion into the exhaust tract will be of no consequence.

[Bigrob]

Ideal thanks Phil.

Assuming any porting etc will be overkill on a 1098 block?

[philthehill]

The porting of a 940 head whatever variant is more than adequate for a 1098cc engine.
Make sure that you have adequate clearance between valve and the top of the block. May require the valves to be pocketed or as some have done take a light skim off the face of the valve.
Use the search facility or details of fitting a 940 head to a small bore block. Plenty of information on here.

[Bigrob]

Yeah I've read loads about them as I intended to do the same with a 998 metro I owned years ago before someone went up the back of it.

Might make up a new exhaust as I imagine the standard is a little restrictive, but the larger bore aftermarket system is too big.

[Bigrob]

Partly for future reference, but I think the smog hole thread is 7/16 20UNF.

I've measure the head, and I've got some sinking to do!


The head depth is 2.66 inch, which means it's had 90Thou skimmed I think.


I measured an exhaust valve, as I'm not cleaning the head til I've actually done the restoration, and it sits at approximately 260thou below deck.


Is a 60thou sink a bit much?