Cylinders 3 & 4 not firing (Head?)

[DRMM]

Hi guys!
Been a long time since I was here! ... I still been running me white traveler, but now...

is not firing on cylinders 3 & 4! Doesn't appear to be sparkies (changed plugs points etc...)
til now 3 & 4 has always sooted up their plugs, and I have had to change them more frequently than expected...
But the other day wouldn't run properly and investigation showed 3&4 down. Started again today but only (seemingly) on 1&2 pots (very lumpy yes!) (deduced by pulling the plug leads one by one)..

So what's likely/ possibilities?
I'm wondering if it could be head gasket gone between 3&4 pots? rather than valves burnt out, or worse ?
(I have checked valve clearances and they had closed up a tad, but not that much - ranging between 8 and 15 thou. ; mostly 15 thou.)

[oliver90owner]

A compression test would be a good start. Head gasket is the likely failure - but there could be others.

[DRMM]

Thanks Oliver.
I think I'll remove the head anyway, I don't have a compression tester. Hopefully the fault will be obvious .
Cheers.

[oliver90owner]

A thumb is ofen enough if the gasket is blown. Feeding compressed air into those pots could be informative.

[DRMM]

... ah yes... thanks.

[DRMM]

...just stuck me finger down its plug holes...
and yes, there is indeed very noticeable lost compression on 3 & 4 pots compared to 1 & 2!
So is that the gasket gone then? (I can't imagine the pistons holed suddenly both together?).

[Monty-4]

Yeah, I think it would be unlikely that you burnt a valve on both cylinders simultaneously or two pistons failed at the same time. Most likely is head gasket failure.

Easy enough job on a Minor assuming there is no secondary damage. Perhaps a good excuse to get some carbon off the piston tops if it has been running a bit rich too.

[Edward1949]

Yeah, I think it would be unlikely that you burnt a valve on both cylinders simultaneously or two pistons failed at the same time. Most likely is head gasket failure.

If it's gasket failure it's obviously worth checking why; eg. head slightly distorted.

[DRMM]

... thanks guys...
update - removed head and indeed the gasket gone between pots 3 & 4 - in fact a half inch portion blasted out!
... waiting on new gasket, but it looks like it hasn't damaged the block or head surfaces where it was. Maybe a slightly faulty gasket? Anyway, I'll check the flatness of the head face with a straight edge for any warp...

Question: I assume when tightening back up the head nuts I do it in sequence bit by bit as is usual for cyl heads... But what about the rocker assembly securing nuts down the other side (I can't remember being familiar with the rocker assembly sharing its securing with the head nuts before on other vehicles (long ago now tho)) - do I do those up as I'm tightening the head nuts up, bit by bit?
Any other tips or things to notice? Like - I didn't come across any O-rings or anything, so I assume there aren't meant to be any fitted?
I've removed carbon deposits from valves and around them... there wasn't any build up on piston tops!?
What are the numbers on the top of the pistons: "U26" and "U27"? (there was the "MOWOG" name on them too).

Cheers
Dave

[philthehill]

Just tighten the 3/8" head nuts progressively as per the diagram in the wksp manual up to the required torque. The rocker shaft 5/16" nuts can be done progressively at the same time again working outwards.
The head gasket can be lightly greased if you want. It does help the gasket to spread and seal.
There are no 'O' rings fitted between the 'A' series head and block.
The numbers and MOWOG are just that and can be ignored. The MOWOG stamping means that they are factory pistons. The U26 & U27 I suspect are piston grading numbers. The numbers are usually set in a diamond.
The head studs can be removed and the stud holes in the block slightly countersunk. Doing this removes any pull up of the top of the block and allows the gasket to seat flat on top of the block.

countersunk studs.jpg (446.04 KiB) Viewed 2196 times

Centralise the head on the block as the cylinder head can rotate around the O/S centre head stud and reduce the effectiveness of the head gasket fire ring.
See:-
viewtopic.php?f=4&t=73114&p=663372&hili ... ud#p663273

[DRMM]

Thanks for those tips Phil... just about to get on with it now...

[DRMM]

... Yay! she's running again!

... There's something I'm unsure of though:
there is a kind of locating spacer (?) - sits on the first rocker pillar/post and seems to hold a strange rotating tab or whatever in place, that appears to turn otherwise. What is it? and should I have paid attention as to maintain it in the same position? Is it a kind of screw grub? it sits over what seems to be the oil hole from the head into the first rocker assembly post? (one thing occurred to me that perhaps it is a limiter on how much oil is passed up to the rockers (just a guess)...

Cheers

...Later: just looked at the manual online - seems that screw is to locate the rocker shaft. Hope it didn't turn whilst I had removed the rocker assembly! What would happen if it had and no longer located the shaft into position? How many turns would it have make undoing before it no longer located the shaft in position?
...

[philthehill]

The (locking) plate (Pt No: 2A259) is there to stop the rocker shaft locating screw/peg (Pt No: 2A258) from coming undone.
A tip - remove the part of the locking plate that the 3/8" head stud fits through. It means that you can remove the rocker shaft without effecting the head stud torque as the locking tab is then retained by the 5/16" stud/nut. BMC did a similar thing on later engines.
I doubt that the rocker shaft locking peg will have moved. It can only be tightened sufficiently to allow the locking plate to fit over the two flats of the locking peg. In reality the locking peg may not be fully tight even when correctly fitted.

[DRMM]

Thanks for the tip on the locking tab Phil. I'll do that next time I have to remove it. (I did see on the wkshop manual it shows that tab as only fitting under the one nut).

I'm still worried about something though (and I've noticed and posted about this before):
When I started the engine after re-assembly, I did so without the rocker cover fitted to check that oil was getting up to the rockers. Well, as it was before - there's no oil splashing around, even when I rev the engine!
(when I posted about this issue before, coupla years ago?, it was advised that there should be some splashing of oil taking place up there. And it wasn't then! But I've run the car now for a few thousand miles and there hasn't seemed to be any problem with the rocker assembly)... But I'm back to worrying about it again...

Thinking about it - that rocker shaft plug must locate into a hole in the shaft to locate the shaft? And presumably right opposite on the inside of the shaft there is another hole to let in oil from the oilway from the head through the front rocker post/piller? ... If that is the case, is it possible that the shaft can be located 180degrees out? ie the hole for the oilway uppermost and the grub hole downwards? with poss problem of oil flow? (probably trying to grab at straws here, but just a thought).

[philthehill]

Have you got the right No: 1 post.
Latterly when the rocker shaft is replaced the locking peg is moved to No: 2 post (BMC moved the locking peg to No: 2 post because of broken rocker shafts. The locking peg hole and the oil way reduced the strength of the rocker shaft - result failure.) . You have to drill the new No: 1 post or fit a post with the hole already through it. Just swapping the post over is no good as the oil cannot get past the undrilled No: 1 post to the rocker shaft.
Make sure that the hole in the bottom of No: 1 post is well countersunk - if not the hole in the post and head will not align restricting the flow of oil.

Rocker pillar 2.jpg (32.43 KiB) Viewed 1951 times

The hole had to relieved towards the dark shading to maximise the oil transfer between head and No: 1 rocker post.

[DRMM]

Not sure I'm understanding right, Phil?:
As my number 1 post has the locating peg in it doesn't that mean it is the right post?
To check the oilway hole in the bottom of the #1 post do I need to remove the whole rocker assembly? And if so does it mean loosening all the head nuts? Or is it okay to just loosen the one side of head nuts (rocker nuts side)?

If I do this change/mod - moving the peg/post to number 2 post position - do I then need a later shaft (as well as drilling the new #1 post) with a locating hole at number 2 post position? Or does the shaft have locating holes in all post positions?

[philthehill]

If your locating peg is in No: 1 post it is as the original factory set up and the oil oil for the rocker shaft rises up the No: 1 post and along the inside of the rocker shaft.
BMC changed the peg to No: 2 post to get over the rocker shaft breakages. When you have the locating peg in No: 2 post you use a later design rocker shaft.
The locating peg is either at No: 1 or No: 2 post. No other posts/rocker shaft positions have the locating peg facility.

http://www.minispares.com/product/Class ... o%20search

Here is the Heavy Duty shaft which shows the locating peg hole at No: 2 position.

http://www.minispares.com/product/classic/C-AEG399.aspx

To check the oilway is correct you have to dismantle the rocker/head nuts for access.

[DRMM]

Thanks for clarifying that Phil, I get it now...

Update: I've continued to run her here and there. It seems to be getting oil up to the rockers, like before though - whether its enough I don't know but she's not complaining...

Pots 3 & 4 - the spark plugs still bit oily/sooty than should be. And on initial start up there is a bit of misfire, which clears when I get going. I'm wondering if this could mean the valve stem oil seals need changing (my thinking that initially some oil has seeped down into the combustion chambers whilst standing?), especially on pots 3 & 4?

Cheers.

[DRMM]

... I put a new set of plugs in and there was no misfire. So maybe it was simply the plugs?

[oliver90owner]

I might be checking out the plug leads and caps if there is a problem with miss-firing. Also, were the plugs tightened properly. All sorts of things can contribute towards a miss-fire - it’s not always the plugs.