12g940 head

[horrace]

so this is the best head to go for... are there any variations that wont work on a 1098? or anything to look out for?
when i find one i assume its best to have it
1) pressure tested?
2) skimmed?
3) re seat valves?
4)Check anything on cam?
and anything else?

[chrisryder]

if fitting onto a 1098 you need to make sure there's enough clearance for the exhaust valve. the distance between the valve face and the face of the head is the critical factor.

a thread on here gave a 'magic number' on ~7.8mm or more. That taking into account the 0.25 inch valve lift, the 1.3:1 ratio rockers, the tappet clearance (12 thou) and head gasket compressed thickness, plus a little bit of a factor of safety to allow for valve bounce.

[bmcecosse]

It is by far the best head, but don't get a 'big valve' version (save that for a 1275) - don't skim it, the CR will be high enough without, do seat the valves - and then measure that clearance from face of each exhaust valve to face of head. If anything less than 320 thou, you need to sink that valve into the head until it IS 320 thou...... It's worth spending some time grinding away at the gap in the exhaust valve throat - where it turns the corner - it's tiny as standard. Use a 1275 gasket and 1275 rockers (or take the time to re-align the Minor rockers so they act directly over the exhaust valve stem tips) - and you probably need to file/grind some metal off the top surface of the water pump to make sure the head can seat down without fouling the pump. Best used with a larger carb and better manifolds of course!

[horrace]

thanks what size is a big valve and a not big valve?
i have increased the carb to hs4 and put maniflow and inlets and outlets on which made a nice difference

[bmcecosse]

Std valve size is by far the most common - 33 inlets and 29 exhausts.

[horrace]

ive found someone with 2 heads... one is a 1275 and the other is a tuned 1098 bigger valves and ported (it think he said). the fella seemed to thinkthe 1275 wouldnt go on as the clearance wasnt there, i guess this is where the .325 thou comes in? both heads are leaded, now did i read somewhere you dont need to put unleaded seats in just adjust the clearance? so which one to go for?

[bmcecosse]

The 940 is by far the better head.. But what price does he want for the heads - and what valve sizes are in the '1098' head - and what casting number is it?

[horrace]

£50 each for the heads, not sure of valve sizes but much bigger apparently, ill get the casting no. whats the score with the unleaded thing?

[bmcecosse]

Don't worry about unleaded unless you plan on doing lots and lots of hard driving on M/way. Set exhaust valve gaps to 15 tou, and check/reset every 3,000 miles. Simples!!

[horrace]

that sounds great, why does the extra gap help? ill check out both heads and get some numbers

[bmcecosse]

The gap just allows for a bit more expansion of the valve stem length without the gap closing up. MG Rover moved up the gaps on all the later engines to 15 thou.

[horrace]

The 940 is by far the better head.. But what price does he want for the heads - and what valve sizes are in the '1098' head - and what casting number is it?

so i got the head and its a 2A 628 with the round inlet ports. inlet valves are 30mm and exhaust are 28 i think! it has been polished & skimmed and the inlet has been smoothed and enlarged, is this good? i paid £50m for it

[RobThomas]

That's a 950 head, isn't it?

[horrace]

I'm unsure, does it matter? someone will be along shortly no doubt to put us right

[horrace]

just in case anyone was concerned i didnt pay £50m for it, i was just £50.

[horrace]

just remeasured the valves and they are 27.5 exhaust and 31.5 inlet

[horrace]

and if anyone is interested its a 629 not a 628, all seems pretty quiet out there, is no one going to offer some insight?

[chrisryder]

i do keep seeing this thread, but i'm unable to answer.

i keep hoping and waiting for bmcecosse to come along and enlighten us all!

[Declan_Burns]

He's obviously on holidays.

[horrace]

no need to for any extra info now.... i was reseating the valves and i found 2 little cracks!! brilliant thats what happens when you buy a 2nd hand head!
i thought i may be able to get a valve seat fitted but the machine shop i went to, southern rebore services near crawley said that it was too bad and id be taking a risk in refitting it. so now this leaves me with a conunderum (?) do i just refit the old head after having it skimmed - the head gasket went after i loosened 1 bolt (not all) to put the engine steady on.
or now the head is off do i fit bigger valves and polish the head, im now put off buying another second hand head after finding the cracks and my current head has only done 1500miles and was a full recon one when i bought it so id rather use one i know is sound