Cylinder heads

[Alex'n'Ane]

I have read a bit and heard advice about fitting new cylinder heads, However i was just wondering what was the difference between the 12G202 12G295 and 12G940 heads, and which was the better upgrade for a 1098? Which if any would need to be skimmed, and by how much?
Thanks.

[PSL184]

202 is standard 1098 head. 295 would need skimming to maintain CR if it's not already been done (many have). 940 gives the best upgrade (seek out the big valve version) and should not need any work but just make sure the exhaust valves don't clatter the block face. Mostly I have found they don't but some have had an issue needing the valves sinking a few thou....

[bmcecosse]

The 'big valve' version of the 940 head would be too big for a 1098 (but ideal for a 1275!) - unless lots of other mods are done to the engine - and then it will just break! Stick to the standard size valve 940 head. As the heads progress - the 295 has slightly larger inlet and same size exhaust as the 202 head - but is better flowing and has better combustion chambers. The 940 has even larger inlets - and much larger exhausts. It works very well indeed - and needs NO skimming (the 295 needs 80 thou removing) - but as above - you need to measure carefully to make sure there is room for the exhaust valves. If not - they need to be sunk into the head by ~ 40 thou. Not a difficult job - and much less expensive than skimming a 295! if you can buy a 940 head that's showing a bit of valve seat recession - that's ideal for the job!

[Alex'n'Ane]

Thanks, am on the lookout for a 940 head in that case. Is the rocker assembly (the springs, rockers, and that bar across?) the same for the 940 head as the standard 202?

[bmcecosse]

No - you need the later 'sintered' type rocker assembly, although you can spend some time re-arranging the existing pressed steel rockers on the shaft so they act directly over the valves in the 940 head. the pressed steel rockers are actually the better type - but sintered are fine on a standard engine.

[chrisryder]

i'm resurrecting an oldish thread here, but considering putting a standard 940 head on a 1147 engine.

With regards the rocker assembly:

you need the later 'sintered' type rocker assembly

will any sintered rocker assembly do? i've got some 202 heads knocking around with sintered rockers on. Is the idea that the sintered ones are wide enough to reach the stem of the valve even though it's a bit out? or are the sintered ones already perfect for a 940 head? Or are we assuming that the rocker assembly having sintered rockers means that it's off a 940 head?

I've been told that you need to use 1275 push rods too, as they're slightly longer than 1098 ones. is that correct?

[bmcecosse]

Yes - the sintered rockers are wide enough to get the exhaust valves - it's what Rover did - but it's not a nice solution. Re-arranged steel rockers are a much 'nicer' solution. Why on earth would you need 1275 push rods ????????

[chrisryder]

Why on earth would you need 1275 push rods ????????

dunno! was told there was a difference. if the combustion chambers are differently spaced, then the pushrods could be at a different angle, and therefore need to be longer? didnt make much sense to me, but the guy had a 940 head on his 1098 so i assumed he had learnt from experience! maybe his followers were worn out so he couldnt get enough adjustment on the tappets.

[bmcecosse]

The 1275 rods are a LOT longer - and ONLY used in 1275 engines.

[chrisryder]

any idea why there's a difference? i cant think why... does the 1275 have a bigger distance between crankshaft centre and deck height? i wouldnt have thought it would if the stroke of a 1275 is shorter than a 1098...

[rayofleamington]

if you can buy a 940 head that's showing a bit of valve seat recession - that's ideal for the job!

If you get to see the head before buying it's possible to tell just with a quick look.
I've got one with absolutely no VSR and the valves sit into the chamber by alost a few mm. On other 940's I've seen the valves/seats have been worn (or ground) so that the top face of the valve is much closer to level with the chamber cavity.

[chrisryder]

unfortunately i've got 2 940 heads. one has a lot of recession (valves flush with combustion chambers) but that head's got bigger inlets which would be overkill on a 1098. the other head i've got is standard sizes but got unleaded seats in. so it would go well on the 1098 for valve sizes, but not a jot of recession means i may get clearance issues.

i looked in the vizard book the other day, apparently the 940 head has a smaller combustion chamber volume than a 202... guessing the 940 is shallower.

[rayofleamington]

it's not a major disaster to try and use your unleaded head...
The less common alternative is to pocket the block in the regions you could get clearance problems.
Roy might give you some advice how to do that with hand tools (although normally it's done with a milling m/c)

[chrisryder]

i have access to a miller

not too keen on milling a rather valuable 1098 midget block though! and i'd worry about the pocketing 'shrouding' the valve. also, i would be worried about the top ring on the piston passing the pocket... or do you not need to go that deep?

many moons ago, somebody showed me a topic on here with pictures and things about the work required to fit the 940 on a 1098, but despite trawling the search box, i couldn't find it!

[bmcecosse]

Yes - the 1275 block is ~ 3/8" taller than other blocks(and also slightly longer) - hence the need for longer push rods. I have hand milled pockets in the block (40 + years ago) and it's a messy business, and yes it puts the top ring v near the pocket esp as I was using a v high lift 649 cam..... , and yes it 'ruins' the block. Just use the big valve recessed head - it will be fine ! Even more power and since you have the 2" main bearing 1098 (yes?) - it will stand a bit more revving anyway.

[chrisryder]

thanks for the help. i've re-searched and found a bit more light reading on the subject! fitting the big valve head seems to make sense, as you say, trouble was i was hoping to utilise its big valved-ness on the 1275 engine! Difficult game this!

[bmcecosse]

Just buy 4 new big valves for the other head - and run a milling cutter down the valve throats..

[chrisryder]

indeed! the big valver has had some work done by the looks of things. having a standard one next to it to compare, i can see that it's had a bit taken out of the throats on there, and the guides have been turned down.

the 'standard' head i've got has been unleadified it seems, i pressume the machining required to fit bigger inlets would be ok to machine into the edge of the insert? do you have a certain size inlet that you favour? not sure how big to go! my big valve one has ~36mm inlets (1 15/32'?)

[bmcecosse]

That's the problem - nearness to the inserts. That's why later MG heads do NOT have the larger inlet valves. So you just need to use your judgement on that - or sell the unleaded head (often get good money) and buy a leaded one instead for less money. There's no point going any less than that 36mm.