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MG MGA - DPO strikes again!
|This is about my brother-in-laws MGB which he bought last year, it was top end price for a 77 roadster and it does look pretty good. It has had a nice chrome bumper conversion and lots of extras fitted including remote electric door locks, what look like RV8 leather seats, minilite alloy wheels etc etc|
Last week he was caught in a motorway closure for over 4 hours and the engine became very hot.
It has an electric fan but it didnt seem to cope with the situation and eventually the car began to misfire.
We checked the usuall culprits but we couldnt fix it and so I then did a compression test.
Cylinders 1 2 and 4 showed about 165 psi but number 3 was well down and its spark plug was oiled up.
So we decided to take off the head and we found a burnt out exhaust valve on number 3 with all the other valve seats also showing signs of erosion due to detonation.
Unfortunately the head has a deep crack beneath number 3's valve seat and so it looks as if we will have to replace it.
The real surprise was in the valve gear, when we took the valves out we found that there was only the outer spring, the inner valve springs and the valve guide shrouds were all missing!
Im amazed that the springs have not jumped out as they are just so flexible that you can almost compress the valve and remove the cotters by hand.
Didnt the DPO realise that there were some extra bits left over when he had finished rebuilding the head?
I just cant think of a sensible reason for not fitting them.
My 18V only has one spring per valve. Looking in my Haynes manual this is pretty standard for the later engines.
|Beginning of 18V engine production changed to single valve springs, single row timing chain, short bucket tappets and longer pushrods.|
|Well there was perhaps a very good reason for not fitting the inner springs if they never fitted from new!|
Maybe Im not too hot on the MGB motor then, the MGB motor fitted in my MGA is a 3-bearing and it does have double valve springs fitted and I assumed that all 5-bearing engines were the same.
However, the valve spring cups fitted just dont seem to locate the spring very positively at all and you can move them around on the spring even when it is all assembled.
I will have a close look at the new head when it arrives.
Thanks for putting me right
|All MGA and some early production MGB (To engine 18G4348) had oil deflectors around the valve guide, sandwiched between the inner valve spring and the spring cap. When the oil deflector was deleted, the pilot diameter on the spring cap got larger to properly locate the inner spring.|
If you remove the oil deflector but keep the earlier style spring cap, the inner valve spring will be free to move radially under the spring cap, perhaps enough to rub on the ID of the outer spring. If you use an umbrella seal on the valve guide (one reason for removing the oil deflector), the inner spring might move enough to rub and possibly damage the valve seal.
For the earlier engines, if you remove the oil deflector you need to change the spring cap to the later style with larger inner spring pilot diameter. Find picture and more details and dimensions here: http://mgaguru.com/mgtech/faulty/ft008.htm
|Thanks Barney, that could well explain it.|
I have had a closer look at the valve gear on the head.
I looks to me like only the number 3 exhaust valve has the early design spring cap fitted which explains it being so loose. The other caps appear to have a smaller internal diameter to match the valve spring.
Other factors that may well have contributed towards the valves burning are that the valve seats were the original non-hardened type but being run on unleaded fuel.
Also, his car had the quietest valve rocker noise that have ever known on a B-Series, you just couldnt hear them at all and I always suspected that there was little or no valve clearance.
This thread was discussed between 04/07/2013 and 05/07/2013
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