12 replies to this topic

[xwhatsit]

Hihi,

Bought an 850 1962 Mk.I and blew the head gasket and boiled it on the way home. Pulled the head off (cor, what a pleasure to work on compared to fiddly little alloy air-cooled Honda motorbikes); looks like the head is stuffed. The `ridge' which intrudes into the combustion chamber between the valves is eroded and rusted on No.3 cylinder. Almost looks like I can see a crack too. While it was sitting upside down, I poured kerosene into the combustion chambers and let it sit; the only cylinder that lasted more than half an hour was No.1 cylinder. Of course that could just be poor valve seats but I won't hold my breath.

So, new cylinder head it is. Over here on Trademe (New Zealand's home-grown eBay) there's quite a few cheap cylinder heads kicking around; unfortunately most seem to be 12g940s. One 850 head but I haven't seen any photos and the man won't answer any questions about it so far.

I've had a good search but when every search turns up 60 pages of 99% unrelated results I figured I'd simply ask

I hear contradictory things. I may/may not have to pocket my block to fit a 12g940 head. Nearly everything I've read is about 998s; with a stock 850 cam, does anybody know if I'll still need to pocket the block?

There's also 1098 and 998 heads available. An easier swap onto my 850 than a 12g940? I take it a Morris Minor head is a very bad idea

I'm not looking for any more power or anything, just want to get my new purchase back up and running.

Edited by xwhatsit, 19 September 2009 - 12:37 PM.

[Cooper-202S]

A 998 head would be the best swap, a 1098 head marked 12g202 would do but will lower the compression a bit.
Fitting a 12g940 from a large bore will mean pocketing the block which requires a full strip and rebuild, I doubt you will ever find a 12G295 head in you neck of the woods but sticking with a small bore head would be easiest.

[xwhatsit]

A 998 head would be the best swap, a 1098 head marked 12g202 would do but will lower the compression a bit.
Fitting a 12g940 from a large bore will mean pocketing the block which requires a full strip and rebuild, I doubt you will ever find a 12G295 head in you neck of the woods but sticking with a small bore head would be easiest.

Wonderful, thanks. Didn't realise the 1098 head would lower compression.

Ideally the 850 head would still be best, no?

If I do fit an alternative head, do I track down rockers to go with it (if they don't come with the head), or can I use my 850 rockers and pushrods?

Sorry if that's a very silly question, I'm used to overhead cams and camchains.

[Ethel]

998 head is exactly the same as 850 & 948.

Chamber volumes:
998 - 24.5cc
1098 - 26.1cc (12G202)
1275 - 21.4cc (12G940)
12G295 -28.3cc

All close enough to get the right CR with a skim or grinder.

Not sure about using a 940, it would be exactly the same as with a 998 but even worse of a match at low revs.

[Cooper-202S]

If I do fit an alternative head, do I track down rockers to go with it (if they don't come with the head), or can I use my 850 rockers and pushrods?

Your 850 push rods and rockers should transfer over to any of the small bore heads ie
cam4810
12g202
12g295
sorry not sure of the 998 casting number off the top of my head.
watch out for heads that have had a lot skimmed off them in previous refurbishments.
If all you can get is a 12g202 head then its not that big a difference on the compression ratio as i would think you 850 was built for 2 star petrol anyway so will be similar C/R

[xwhatsit]

Many thanks Ethel and Cooper-202S -- looks like I might be picking up a 998 head for $NZ25 (?!), so for that money it could be whatever chamber volume I like and I just spend the rest of the money on machining

It did strike even a numpty like me that 1275 ports and valve sizes wouldn't be much help on a 850cc making 34hp. Particularly rammed that point home to me this morning when I picked up the carb and realised that it has approximately the same throat size as that on my single-cylinder Honda CB250RS, yet it's servicing 850cc and four cylinders! Nothing like strangling your intake and exhaust to get lovely low-speed behaviour.

[Ethel]

I know nothing about bikes but a 4 cylinder 850 is only filling one 212cc cylinder at a time.

[xwhatsit]

Okie dokie, bought two heads. One is on its way (the 848cc head), the other one I've picked up. The seller listed it as a 1000cc head -- unfortunately I've got it home and it's stamped `2G202'. So looks like I'll have to have it skimmed. Did the sums according to the figures Ethel posted earlier and it'll drop my compression ratio from 8.3:1 to 7.9:1, which isn't too bad I suppose, but given there's not a lot of horsepower to lose in the first place I might try and sort it.

Of course will see what the 850 head is like first, if it's in as good nick as the 1100 head I'll use it instead. The 1100 will have bigger ports too, which isn't great, is it?

One weird thing, my 850 engine has only two studs protruding from the top of the head. The rocker cover has two bolts which screw all the way in. This 1100 head has four studs, and looks like the rocker cover is supposed to use two nuts instead. I love cast iron (as compared to aluminium), the two outer studs just wound right out with a pair of vicegrips -- wouldn't happen on an aluminium head!

Edited by xwhatsit, 27 September 2009 - 04:42 AM.

[Cooper-202S]

"Of course will see what the 850 head is like first, if it's in as good nick as the 1100 head I'll use it instead. The 1100 will have bigger ports too, which isn't great, is it?"

If by 'Ports' you are refering to the section between the valve seat and the manifold connection, then bigger ports are better for gas flow.

If you end up using the 202 it will stand an 0.080" /2mm skim without causing to many issues with the push rods and rockers.

Glad to see you have made some progress.

[xwhatsit]

"Of course will see what the 850 head is like first, if it's in as good nick as the 1100 head I'll use it instead. The 1100 will have bigger ports too, which isn't great, is it?"

If by 'Ports' you are refering to the section between the valve seat and the manifold connection, then bigger ports are better for gas flow.

Yeah, I see what you're saying, but I figure with the small low-compression 212cc cylinders it's probably better to have a restricted intake and exhaust so I keep that lovely tractable bottom end (clutches must last forever in Minis). A free-flowing intake and exhaust will make it breathe better at higher revs but I'm going to guess it'll suffer in the bottom and mid-range.

If you end up using the 202 it will stand an 0.080" /2mm skim without causing to many issues with the push rods and rockers.

Glad to see you have made some progress.

Cheers for that; I might figure out if I can take a little less off to get back to that 24.5cc volume rather than the 202's 26.1cc. I'll guess it'll almost be 2mm anyway though, giant combustion chambers! Dished pistons too, no wonder compression ratio so low!

Thanks!

[Cooper-202S]

I have a unskimmed 202 on the bench at the minute, if you can give me a couple of days and can let me know the piston number I can work out how much you would need to skim it.
I that will help?

[madaboutcherry]

go for the 12g202, it has larger valves than the 998 and it was fitted to the early 997 cooper.

[xwhatsit]

I have a unskimmed 202 on the bench at the minute, if you can give me a couple of days and can let me know the piston number I can work out how much you would need to skim it.
I that will help?

Cheers mate, that's very kind of you. The 850 head should be here soon though so I'll check that one out first. Thinking about it though, I don't know if the 12g202 has been skimmed before or not, so I may just take it to the cylinder head shop and say `make it 24.5cc!'. Especially if the seller really *was* selling parts off a 1000cc engine, it's possible it's been modified.

go for the 12g202, it has larger valves than the 998 and it was fitted to the early 997 cooper.

Yeah but those engines have a fair few extra cubic centimetres and I'll assume the 997 was running fairly high compression?

At some later stage I might go crazy and put domed pistons and a lumpy cam but I've ridden too many outrageously over-tuned, over-ported motorcycles (a 12.5:1 compression 500cc Honda single with a silly camshaft fitted to an NSR250 chassis sticks in mind), and for my first car I'd prefer to keep it mellow and driveable.