12g940 head modifying for n/a use
Turbo Mini Forums

I have in my possession a 12g940 from an a series engine, and a 12g940 from a late carb engine with unleaded inserts.
First question is, with a 276, 1.5 roller tips, around 10:1CR and unsure of bore size yet (have a few options) what would be optimal valve sizes for a home ported head, a basic remove all casting marks, port match the gasket and a mild tickle inside the ports and possibly chambers. Second question is what are the largest valves i can safely fit into the existing inserts in the unleaded head? Last question is what valve springs to use with the cam and roller tips, would 160lb doubles be the ones to go for?
I also have a stack of valves from unleaded 998 heads which I've just removed.
Cheers.

I'm running 35.7x29 in mine with a 276, the valve seats get recut for bigger valves anyway so new inserts would be fitted regardless?

I'm not really sure what the crack is with inserts, don't know whether you can cut them bigger, where to get inserts from if needed as minispares only list std sizes i think, just a grey area for me that hopefully someone will be able to clear up!

I've got 30mm valves in my unleaded inserts.

If something is worth doing, it's worth doing half of.

I guess yours is 33 x 29mm. MG metro heads were 36 x 29mm which seems to be a fairly sensible upgrade (for NA at least). That would mean larger inlets, but it's only the exhausts that need hardened seats to exposure to the exhaust gasses. If you only increased the exhausts size to within the edge of the hardened seat, I don't think it would need to be changed, just cut larger.

Have you looked at prices to get the seats cut bigger, or had you got a cunning plan to do that DIY ? I'm Interested to know as I'm doing a head at the moment and I've always done as much as I can with the ports and chambers, but never looked at changing the valves.

Edited by PhilR on 24th Oct, 2014.

Well im not sure of the cost, but the guy who does all my machine work is very reasonable on cost (crank regrind and 4 hones cost £90) But if you wanted to just do a small size increase, there are some tools you can buy to do the job: http://m.ebay.co.uk/sch/i.html?_nkw=valve+...lve+seat+cutter

Edited by Carlzilla on 24th Oct, 2014.

A good basis of a fast road home brew head would be to use the later unleaded casting, use the standard 29.4mm exhaust size but enlarge the inlet valve to the early NA MG Metro size (35.7mm)
You will need to do some chamber work, possibly to achieve your desired compression ratio, but also to de shroud the valves. You're going to need a decent grinder as the main restriction in the port is the guide boss, simply tidying the port will give little benefit unless you remove this. You can fit a 31mm ex in the standard insert but I wouldn't advise it as it leaves little in the seat. A 30mm valve can be fitted though.
The cheap cutters on eBay will be next to useless on the hardened inserts, you'll need something better.

Phil.

WWW.TURBO-MINI.COM

what Phil sais.

port matching to a gasket is pointless. even mathcing the manifold to the head makes for very little gain.

don't bother cutting seats yourself . you have to spend a good deal on cash to get anything decent. Just get a decent seat cut by a good reconditionig place.. try to see in somebody in the neighbourhood has a Newen Contour , that can cut any angle and seat configuration you can think of including the downcut. Before you cut the seat you should clean up the bowl witout enlarging it.

I reckon that that way you get the max gain for the least amount of work


get the valves back cut , this gives a lot of gain for very little cash.

Thin the guide bosses ( leave the guides in and replace afterward before you cut the steats) you dont need to remove them fully.


Or get somebody to do it for you

Edited by Sir Yun on 25th Oct, 2014.

That sir, is not rust, it is the progressive mass reduction system

http://aseriesmodifications.wordpress.com/

First of all, The 276 does not like 1.5 rockers apparently.
Second of all, on a 276 canned engine a properly ported head with a 33,3mm inlet valve will be sufficient. You don't need to enlarge to 35,6. I guess no one would like to not use a 35,6 as obviously bigger is better, but trust me, it isn't. You want torque with that engine.