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MG Midget and Sprite Technical - No compression - baffled!


I was driving my car the other day and as I took a "spirited" swing around a roundabout, having a fight with the steering wheel, there was a fart and a pop and the car started to misfire very badly. I was bouncing off the rev limiter at the time so knew it was all my fault. Doh! :o(
The car limped for the 2 miles home on two cylinders...

Anyway, upon further investigation I stuck a compression tester in each plug hole in turn. In pots 3 & 4 I got around 160psi but in both 1 & 2 I got nothing, zilch, nada...
A thumb over the plug hole also produced next to nil pressure.
The plugs were all OK in colour and appearance, nice light brown colour and dry. Each fairly similar. No oil or wet bits on any of them.
I took off the rocker cover and checked the valves were opening and shutting fully, they were OK.
I shone a light into both plugholes and can see the top of the pistons are nice and even so no big holes in the crowns.

So where is my compression going? Is this just a blown head gasket or is there something else obvious I'm overlooking? I'm pretty sure it is the gasket but just starting to question my judgment.
Bob T

Head gasket would be a main cuplrit, i.e. it's leaking between the cylinders... but I must confess that, when keeping up with a 911 on a country road, my wife's spit lost the crown to one of its pistons... it still drove but the power was down. An unequal contest became no contest!

Can you see the pistons rise/fall OK?

A
Anthony

Have you checked that those 2 piston crowns are actually going up and down. I have heard of a couple of cases where the top of the piston on a Triumph engine had come adrift but apparently hadn't effected the engine running noticeably.

Even with a blown gasket I would have expected some compression to register.
David Billington


hmm yeh get the point of a crown detaching but two at the same time?

Seem to rise and fall ok
Bob T

Pistoons could have kissed a valve in those two cylinders. "Rev limiter" in most of these cars tends to mean the point at which the valve springs are being asked to close the valves in such a small amount of time that it isn't happening (valve float). It doesn't take much to bend it enough to keep it from sealing. Have you pulled the valve cover and checked the clearances on those four valves? If it is bent, you should have a larger clearance on the valves in question. If you are lucky, you could simply pull the head, lap in a couple of new valves and be on your way...
David "like there is a chance of getting lucky" Lieb
David Lieb

Bob,

Maybe unlikely that 2 would fail at once but I had a dynamo that failed one day and when I opened it up both brush leads had broken. I do like Davids idea about the valves. Sounds like the head may have to come off to check thoroughly.
David Billington

No - almost certainly nothing more than a blown HG between cylinders 2 and 3. No compression as when one piston is ascending its neighbour is descending and all of the compression is simply lost to the one next door.

I would be surprised if it is anything more than that. Only problem could be erosion of the head/block faces with hot gasses going through there if you drove it another 2 miles like this. But you may be lucky!

Guy
Guy Weller

I'm thinking gasket also. For two cylinders to go at once, it would almost have to be the gasket blowing between the two.

Keep us posted --

-:G:-
Gryf Ketcherside

I'm with Guy!

Either way, its head off for a look!

Nick

Thanks for the suggestions chaps. Much appreciated.

Ok so my assumption of HGF is probably the most likely, although valves and pistons need a good inspection. So its head off time then.

Does anyone know where I can get a real good quality head gasket? I've had them from both M*ss and R*mm*r Br*s. Slight differences but neither were particularly good. Is a copper one worth considering?
Bob T

Guy,
You might want to go back and re-read the original post. The problem was in cylinders 1 and 2, not 2 and 3. If it was HGF, it would have been most likely 2 and 3. Since he was pushing valve-float rpms, the probability is still valves. Oh, wait a minute, this is a 1500, isn't it. All bets are off then. It is time to pull the head no matter what.

Bob,
You will probably have to got to a Triumph site to get a valid opinion on head gaskets ;-)
David "head up my gasket again" Lieb
David Lieb

So, is now a good time to advocate ringing the block as an alternative?

Oh, and you will be wanting a set of ARP studs ;-)


See you tonight?
Toby Anscombe

David,
Yes, careless of me to type 2 & 3 instead of 1 & 2. Of course the two middle pistons move in sync and my theory would not work had it been those two. But with 1 and 2 the pistons are 180 degrees opposite on the crank and pressure loss between one cylinder very easily discharges to the other with HGF.
Incidentally, the same applies to both A series and 1500. The only difference is that the gasket thickness between 1 & 2 and 3 & 4 is less on the 1500 and that is where they are prone to failure.
Guy Weller

Guy,
Not nearly so careless as I was in not realizing that Bob=1500! (And that David's knowledge of 1500 ends with the fact that they have 8-port heads. After that the jealousy takes over.)
David "I can count to 5" Lieb
David Lieb

>>> Of course the two middle pistons move in sync and my theory would not work had it been those two. <<<

Actually it would still work, since when 2 is on its power stroke, 3 is on its intake stroke, and vice versa. Those two cylinders would talk to each other very effectively through a gasket gap!

-:G:-
Gryf Ketcherside

Good point Gryf, and on the compression stroke of one, the other would have its exhaust valve conveniently open!

Still think that is the problem. HGF between adjacent cylinders!
Guy Weller


Unlucky Bob, at least it went pop when you were 'enjoying' it, rather than at tickover in the local BP garage car park in front of sneering yobs drinking cider...(don't ask).


robnrrugby

Guy,
If I were to be able to pick and choose the problem, that is the one I would want, too.
David "I hope that noise is the diff and not the Datsun 5-speed transmission" Lieb
David Lieb

Will keep y'all updated.
Head off very soon.
Bob T

Oh and BTW:

<So, is now a good time to advocate ringing the block as an alternative?>

They're built like that from factory Toby, but it clearly doesn't help much!

No pub for me tonight sadly. Still on solitary confinement and haven't earnt any time off for good behaviour.
Bob T

Check your valve opening & closing in reference to the marks on the damper. It is possible at high revs on a worn timing chain to have it jump a tooth without breaking causing the valves to be out of time. It would be a shame to tear off the head, replace the gasket, bolt it all together only to discover it has to come apart again.
mike

I hate to be a lone voice, And I will state right now, that I dont know alot about the 1500....BUT...I have a problem with it being a headgasket failure, something else major would had to happen for that type of catistrofic HGF....your always going to have time after a gaskets goes, (burning oil and water).....was there a slimy green goo in the oil or radiator.....

1. my guess is crank flex / block flex...thus breaking the compression rings.

2. a warped head

3. streached a valve stem/chiped valve head

4 clyinder head bolts worked loose

does it have a no lead head kit installed...if so maybe it losed the hardened vavle seat.

prop
Prop

Surely Bob has had enough time to pull that ead by now! Come on, Bob, the suspense is killing us.
David "I hope for Bob's sake it is just HGF" Lieb
David Lieb

I will wade in now and suggest that HGF seems a bit unlikely, as the rev limit/armful of opposite lock and a four wheel drift around ASDA roundabout suggests somthing mechanical gave up..happens to all the great racing drivers you know! Still, it's all largely irrelevant as the head has to come off regardless, for what it's worth, my money would go on chipped/bent valve caused by valve bounce from the high revs, leaving a valve open when the piston introduced itself to the valve!! Should be a relatively easy fix assuming not terminal piston damage.
Robin Cohen

Chain jumped a tooth or two...

My gut feeling when Bob opened the thread

:-(

And boy will I be glad it was the HGF instead

C'mon Bob

do tell
Bill

Bob did say that he could see no damage to the piston crowns through the plug hole, and that he had checked the valves were all going up and down OK which would suggest no bent valve stems. If the chain had jumped enough for valves to be open on the compression stroke for both 1 & 2, wouldnt it also effect compression on 3 & 4 ?

C'mon Bob!

Git on w'it

Guy Weller

OK OK but I've been busy!

Hopefully will get the head off soon but its a PITA to remove and label all the injection stuff first so not relishing the idea...

The more I think about it, the more I'm convinced it is purely the head gasket.

On further thunking, some may remember that I had a problem with my torque wrench a while back. It was only giving half the torque it should when checked against another. Kindly, Nick (Cherry Scoop) loaned me his wrench and I tightened everything up including the head nuts. However, this was only after it had covered several hundred miles with the wrong settings. Had the gasket already been damaged before I got the correct torque on it and this is the symptom?

Maybe Saturday chaps. Missing my toy :o(

BTW robnrrugby,
Do tell the story of the yob encounter.
You know you need to get it off your chest.
;o)
Bob T

didnt you have a problem with this engine 3 years ago with the rebuild shop putting in the wrong size rings, to small as I re-call....or they overbored it to much....something like that.

If so, how did you fix that situation, I dont remember?

prop
Prop

You have a good memory Prop! It was about 18mnths ago when I had to sort it.

However, it was fixed with a rebore, new pistons and a reground crank so should be sound.
No guarantees tho, is there?
Bob T

you should be okay in that department then, being that you re-bored, new pistons and rings.....I was thinking maybe there might have been a little extra play in there

18 months...time sure flies


maybe its a sign from the MG gods, that its time to go the "zetech" ...lol

bummer, If weather holds Im going to try and hook up side pipes of my car this weekend.


prop
Prop

I dont know the answer, but I wonder if having the engine re-bored...40 to .60 over and using a standard size head gasket, would be a problem.....meaning I wonder if the HG became to small, and was subject to all the forces acting on the inside lip of the gasket.....I dont know, its just a thought in the back of my mind


prop
Prop

Got that head off yet Bob? ;-)

Guy
Guy Weller

Nah no chance.
Been working away for past week.

Gonna try again Sunday.
Bob T


I got my head off today!

Pic attached...



Bob T

and another...




Bob T


Cheapie head gaskets huh?

I got hold of a Payen gasket from the local engineering workshop. When I compare it to the one that came off it looks so much more of a quality product. Hopefully I should have more luck with this one.

Didn't put it back together today. The car is outside the garage at the moment and it was p!ssing down. I'll sort it another day soon.

I probably wont even check the valves, I don't suspect them at all now after seeing the state the gasket was in.
Bob T

Bob,

I'm surprised a failure that size lost you all the compression. Hopefully you have inspected the bores and found no damage as I knew a guy that had a pinto gasket go like that and it scored both bores. While you have the head off it might be worthwhile checking the valves to make sure no damage has been done by the fire ring bits getting caught by a valve.
David Billington

Glad that was "ALL" bob

How are the bores, any scoring as David wonders?

Bill

Ahhhh,

Alittle RTV sealer and that gasket will be good as new...LOL.

I dont know that I have ever seen one blow like that...If you dont mind, who was the manufature, so the rest of us can avoid.

I certianly agree with at least checking valves to make sure they didnt treat the remains of that HG like chewing gum.


prop
Prop

Prop,

RTV?, what happened I though all you in the US used JB weld for this sort of thing.
David Billington

I had a gasket go just like that, but between cylinders 3 & 4 when I had a 1500. Which is why I thought this was the exent of the problem all along.

Mine went when I was touring/ camping in Scotland about 10 years ago. I got a replacement gasket sent up from Glasgow and fitted it at the campsite (actually, a field behind a garage where the owner gave me free use of his workshop facilities). Having fitted it I got less than 100yards up the road before it went again! I then got a second gasket - a Payen one this time - and this one worked without any problems.

The quality 1500 gaskets have a fire ring around each bore which is offset towards the lower side, and this locates in a slight groove around the top of the bore. Cheap gaskets don't have this, and are prone to failure.

Guy
Guy Weller

As Prop said,
Who were the cheap gasket suppliers, so that they can be avoided?

Dave.
Penwithian

Dave, since cheap copy ones are just as likely to be in unlabelled packaging, then the only sure way is to make sure you go for a good known brand, such as Payen, rather than attempt to avoid a whole range of indeterminate copies.
Guy Weller

Hey David,

You have to follow the logical order.... RTV sealer, duck tape, THEN the JB weld, when this fails, then you do it correctly..lol.


prop
Prop

Not an uncommon failuer to big-bore B series engine owners!

While I like the Payen gasket better, I've seen plenty of those gaskets used with good results, even on big bore (73.6mm) engines, so I wouldn't be too quick to damn the gasket.

Check the block and head faces for damage, they can get burnt there.

Guy - The 1500 recessed gasket is a differnt concept, if you tried using a simliar one without the recessed block you'd have problems.


Paul Walbran

T'other way around, Paul. My 1500 block did have the recessed lands but the first gasket that got sent out to me didn't have the thickened ring to match.
Guy Weller

That would definitely be a problem! It is most likely an earlier spec gasket (same in all other respects) crept into the system by mistake, but duds are never out of the question.

The problem I referred to was fitting the thickened fire ring to the non-recessed block of the earlier Triumphs. Either the fire ring doesn't compress much and reduces compression on the rest of the gasket, or it does compress ... inwards into thebore space creating a different set of problems.

Like you, I've learned to check it's the right one!!
Paul Walbran

Reading through the thread again...

The valves wont have been bouncing when it went. The springs should be good for at least 6500 rpm and my ECU rev limiter is set to soft cut at 6000 (it retards the ign and richens the mixture to lose power) so it should have been ok apart from the odd little spike in rpm.

That gasket would definitely have lost me all compression. It created a gap about 8mm wide and about 2-3mm thick. Gaping hole in compression terms. I'm hoping it won't have damaged the faces but I need to have a proper clean up and whip the valves out to check the seats and have a clean up as advised.

Thanks for the help so far chaps, I'll keep y'all posted.
Bob T

Gad!

I remember the late 70s when my GF had a Triumph 1500; I managed to pick up on one of the pistons (always gave it the same welly as I would with Aseries engines, but had one piston pick up (1500) and later a piston crown separate in her Spit (1298)).

Even by giving BL the engine number, it still took 3 goes for them to give me the right gasket for the 1500... doh!

When it was all back together, there was still a enough water in the sump to ensure steam from the rocker cover when hot!

Would not like to go through this again.

A
Anthony

Thanks for the pics, Bob! After hypothesizing a blown gasket, I was curious to see if I was right. I don't expect that sort of failure in my car; I don't cane it, as I can't afford the repairs if something goes! ;-) But I do like to be sensitive to what my engine's doing up there, so I pay attention to any posts about 1500s.

By the way, I've always thought that looked like a puny little bit of gasket between 1-2 and 3-4. If it was going to blow anywhere, it would certainly be there! You can see why the proper gasket config is critical.

-:G:-
Gryf Ketcherside

This thread was discussed between 03/03/2008 and 17/03/2008

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