Head swap (940 on 1098)

[tomti]

Hello all,

Some time ago, I got hold of a 12G940 head from a 1275 engine with a view to putting it (with a suitably larger carb) on my 1098 engine to help it breathe better and hopefully improve hill climbing and general all-round nippiness. My thought was that, as I do a lot of motorway driving and my standard head doesn't have hardened valve seats, at some point in the future I'd be faced with a need to refurb the existing head anyway and therefore could do the machining to the 940 head and swap it in at that point in preference to recovering the 'proper' one.

Recently, I've found that the valve clearances on my engine seem to need adjusting quite regularly (I've only had the car for about a year but it's done a lot of hard miles so I've been keeping an eye on them) in order to prevent a kind of rattly roar under acceleration. My question to you all is, do you think this means the anticipated moment is upon me? My guess is that it does, but I'd appreciate the advice of more experienced owners before leaping to conclusions.

Thanks,

-Tom

[bmcecosse]

Depends! You should set the exhaust valve gaps to 15 thou - and then check them 1000 miles later. Have they closed up significantly?? problem with fitting 'unleaded' seats to a 940 head is that it will then put the exhaust valves too close to the block when open. I use a 940 head - and I had the exhaust valves sunk into the head by 40 thou (45 degree countersink in a pillar drill). I took 10 minutes in a lunch break. That was years ago - and no significant change in the valve gaps. You MUST have 320 thou clearance top face of exhaust valve to surface of head.

[tomti]

I set them to 15 thou - as a result of your posts elsewhere on this forum, as it happens! Inlets at 12 thou. I, stupidly, didn't actually measure the amount they'd closed up, but I couldn't get my feeler gauge in any more at the original setting after around 600 miles. It was the sound of the engine that prompted me to check, as the last time I set the gaps was in a (successful) attempt to remove the 'rattly roar' sound that I mentioned earlier.

I'm not in a position to do any machining work myself - neither the skills or the equipment/space - do you think a machine shop would be happy to sink the seats as you suggest as part of any facing/'unleadedening' work? I assume the amount of sinking needs to be calibrated to the clearance between the valve faces and the face of the head itself - to prevent them striking the edge of the bore?

Thanks,

-Tom

(edited for clarity)

[rayofleamington]

I assume the amount of sinking needs to be calibrated to the clearance between the valve faces and the face of the head itself - to prevent them striking the edge of the bore?

Yes

[tomti]

Great - thank you - glad I've got the hang of that, at least! So, is it common to go to a machinist and specify such things or is this an out-of-the ordinary job that would be met with raised eyebrows? I've never had such work done before, so am not sure what counts as standard...

-Tom

[bmcecosse]

It's a bit out of the ordinary - the first thing is to measure what you have on the head - many don't need any machining at all. I wouldn't (and haven't) worry about 'unleaded'. It will be absolutely fine for many a mile - and at worst, all that happens is that the valve sinks a bit more and the gap closes up. As long as you catch the gap before it completely closes - no harm done. I now use 18 thou on the exhausts - just to be sure of a gap when working hard.

[tomti]

I see. So, assuming the head looks OK in terms of clearances, state of the current valves/guides etc., then you would suggest just sticking it on and seeing how I go?

-Tom

[bmcecosse]

DO grind in the valves at least! And you MUST have that 320 thou clearance on the exhaust valves.

[John.n]

would it be advised to do anything with the inlets? (except lap them in)
Regards,
John

[bmcecosse]

Not really - nice and clean of course. There is no clearance problem with the inlets.