Stumped

[Peetee]

I've mentioned this before but since then I have eliminated any fault in the ignition/charging circuit by fitting an alternator - so here goes.

Since removing the cylinder head and replacing it with a 12G940 head the car has been running rough. It is low on power and reluctant to rev and to listen to, it sounds like the exhaust is blowing or one of the cylinders is not firing but both have been checked and are ok. Compression is poor; testing dry No's 1 to 4 are 100, 80, 70, 110.
With oil squirted down the bores No's 1 to 4 are 110, 90, 100, 145. "This suggests worn rings" I hear you say. however immediately before the head was removed the car ran faultlessly from Hertfordshire to Southampton. I can't say if the engine has been replaced or not so it may well be a low compression unit. I don't know if the compression readings would suggest this(?)
The valve clearences have been checked and set.
Carb has been topped up with oil and the needle moves well.
There is no excess noise from the engine valve train.

So, have I missed anything obvious?

[SR]

could it be something to do with valves hitting block with that head, pete,see my thread "valve clearances" mate,steve

[rayofleamington]

you will get some improvement by adding oil - even if the rings are ok, so it doesn't look especially like a problem with the rings.
Did you reseat the valves (grinding paste + grinding stick + sore hands after 6 hours)? If not then did you check for valve leakage before fitting it?
If you did reseat the valves, then did you check for valve clearance on the block - if not, then it may not be good news (bent valves). The 12G940 only works on a small block A-series if the valves are nicely sunk (or if you do it the profesh way by machining clearance pockets on the block)

[Cam]

Yeah, it does sound like a suspect head. Also are you sure the head is not cracked or damaged?

I know it's a waste of a gasket but it might be an idea to try the old head on (or a known good one) and take a compression reading on that. Then inspect the 12G940 VERY carefully........

[Peetee]

yeah could be a cracked head I suppose but wouldn't that show poor compression on one cylinder in comparison with the rest?
Forgot to mention a few things.
There is no sign at all of any steam or oil contamination in te exhaust.
the plugs have been checked; No1 is sooty, No2 to 4 are pale browny white.

[chickenjohn]

I would just put the old head back on! and carefully check the 12G one. Not that I know anything about non standard engines, its just what I'd do

[turbominor]

Hi Petee

are you fitting the 12g940 on to a 1098? If you are what head gasket did you use?

[bmcecosse]

Your compressions are ALL terrible ! Should be 160/180 on all bores. If you have not pocketed the block then you have almost certainly bent the exhaust valves where they have banged the block. I explained on another thread just recently that you cannot do this without pockets - or deeply seating the valves in to the head - by at least 40 thou. Don't run the engine - take the head off and inspect !!

[Peetee]

I understand the potential problems of fitting a 12g940 head but before I did this I posed the question on this panel and more than one person replied to suggest that it was quite possible to run this combination without problems. Now that tells me that valve/block contact is not a certainty. Given that I don't know the history of the parts used I conceed that if the parts have been skimmed I run a greater risk of bent valves but surely, should this have happened, compression should be practically non-existant.

Remember, this could be a low-compression engine. can someone give me a rough estimate of the readings for this unit?
Given that the carb has been off the engine for about 3 months then it could be that the fueling is shot but in the mean time I will try to get the cylinder head off and take a look.
Turbominor, right now my brain is a mangled mess but I did follow your advice on gasket choice when this choice of head was discussed a few weeks ago.
One other thought, I have used a fibre gasket. I believe some people have found problems with these.

[chickenjohn]

Use a copper faced gasket- its the one all the Minor parts suppliers sell as part of the head gasket set.

[turbominor]

Hi Peetee

As long as you used a 12g940 head gasket the new head should be ok on a 1098

There is a lot of talk both ways on the subject of pocketing the block. I have done it a few times with a standard cam and rockets with no problems even when thrashed for many miles.. Ray has fitted 12g940 to a 1098 with no problems

Only thing you can do is wip the head of and have a look.. If your head has been heavily skimmed or you have a non standard cam you could have bumped a valve on the block but i cannot see it on a standard engine.

I test fitted one with no head gasket and the valve clearances set to 0 and it turned over with no contact..

Only problem i can see is a poor seal between the cylinders or a cracked head..

It should be easy to see a bent valve if you take of the rocker box and turn the engine over you will see the valves opening and closing if you have a bent one it will be stuck as they tend to look like this......

[bmcecosse]

Turbo - i explained the maths on another thread - there is 30/40 thou interference !! It can only work if the valves are recessed by 40 thou - otherwise the exhaust valves WILL hit the block.

[rayofleamington]

there is 30/40 thou interference !!

The interference is different on every head - it can be much more than that it it was skimmed!
Other heads (probably the well used ones that have had the valves lapped a few times) are already in clearance. The only way to know is to measure it. Your numbers regarding valve lift are certainly useful to anyone who needs to check their head to see if it further work is needed.

[Peetee]

Ok right, hopefully I shall find some time today to get the head off and take a look. If the valves are OK I will check the gasket and bores.
If I need new rings how do I establish if they are standard or oversize. does the piston info stamped in the crown hold this information?

[bmcecosse]

I just measured a used non-skimmed head from my 'stock'. Obviously if the head is skimmed then the interference is even more - similarly if a higher lift cam (544/649) is used - or the daft high ratio rockers !!
Pockets are not hard to do - worth it just to be sure nothing will hit.
You need to measure the bore size - but if the rings are damaged chances are the pistons will be too - no point putting new rings in worn pistons.

[Peetee]

The head is now off and I've noticed no damage to the valves. However on the inlet valve of no 1 cylinder the coking on the underside is missing on the segment closest to cylinder no 2. This suggests to me that the valve is extremely close to the block wall when open, if not just touching!!
The other valves appear to be intact and unmarked.
It does have a 1300 gasket.
Also the pistons do not have comparable markings No2 has M2007 and P20. the other three have M2007 and P19.
What effect does putting a 12G940 head have on compression? It won't have the same volume in the combustion chamber a a 12G202 will it? I know you have to skim a 12G295 to restore comparable compression but what about a 12g940?

[Alec]

Hello Pete,

Owen Burton list the 12G940 head at 21.4cc and the 12G295 at 28.3cc, and the 12G202 at 26.1cc.
I can't help you with those piston markings, you might need to contact the manufacturer or their agent to decipher those.

Alec

[bmcecosse]

Yes the 12g940 head should raise the compression very nicely - it's one of the big advantages of using that head ! It's the exhaust valves that can hit the block - you will need to take them out the head to check them for straightness. It may only be slight - and there may be enough 'play' in the guide to allow the head to 'appear' to have seated. If you dribble some paraffin into the combustion chamber (head upside down obviously) the valves must hold it - not allow it to run away.

[rayofleamington]

If you dribble some paraffin into the combustion chamber (head upside down obviously) the valves must hold it - not allow it to run away.

I've usually done it the other way - pour some petrol or parrafin into the ports and see if it gushes / runs / trickles / weeps / seeps past the valve.
(Worst case it will just gush past the valve - best case you just see some wetness 'appear' around the valve - when the valve is ground-in you should get nothing)

[bmcecosse]

Either way works !!