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MG MGB Technical - bent pushrods, coincidence?

I recently installed a modified engine side cover from British Automotive ( Its all one piece instead of two seperate plates. It seemed to work great -no more leaks. The design is such that there is actually enough pressure agaist the block to seat the gaskets. However, after a serious loss of power, I thought I burned a valve due to the low octane gas here (91 is the highest available in NM). After a compression test, no compression in the #3 cylinder. Upon inspection one of the pushrods was bent and another showed signs of rubbing the side of the head. It finally occured to me that the original design had a vent tube, and the new one dosen't. Is this the reason for the failure?


i do not think that the side cover from Dough Jackson does any harm to the valve train. I also had problems with a(tubular) pushrod on one of the roadsters, there it dit not bend but the head sheered off at the rocker adjusting screw. It also was on no. 3 cyl. and i took off the cylinder head and checked the guides.

The head had been overhault by a machine shop a few month before with bronce guides and new valves but they reamd the gides to wide open and fitted the rubber seals to the stems of the valves. parts of these o-rings semmed to had come apart and worked down the guide of the inlet valve, blocking it that much that damage to the pushrod was the result.

I had new valves and guides fitted by an other shop and had no further problems within the past three years since then. My engine was not fittet with the single piece lifter gallery cover but with the normal pressed steel ones.

You should check the valves and the guides for wear and black deposits on the stem and inside the guide, i think and new items should be fitted if there is any doubt. With the head removed, also check the lifters for wear but do not intercharge them, they must return to the bore and loop they came from or new ones must be fitted.

Hope this helps.

Keep us posted, please.


Firstly octane has no efefct on valve seat recession, that is down to whether you use an anti-wear additive or not and the useage the engine gets. Having said that recession in practice is virtually unknown.

You say 'recently', but don't give any mileage. I'd be wondering if I had seated all the components of the valve-train correctly before setting the clearances. Possibly something *wasn't* seated properly, which meant the gap opened up greatly while running, and the push-rod partially came out but wedged somehow.

But there is another question-mark over what you have posted. You say that there was no compression on No.3 cylinder. Even if both valves were permanently closed I'd expect to see *some* compression, albeit very low. With no compression at all that implies a badly burned valve or holed piston.

I'd also be concerned about no vent on the cover, and why Doug provides them like this, although it is nothing to do with the bent rods. With no vent on the cover there is no crankcase ventilation (unless provided via another route), which can lead to a build-up of 'mayonnaise' and internal corrosion.
Paul Hunt 2

The unvented side covers will have no effect on the pushrods. Bent pushrods are almost always the end result of over-reving the engine. In addition, (sorry to be the one to tell you, but) you should be made aware of the fact that a bent pushrod usually means that the adjacent tappet has stopped rotating, leading to a ruined tappet and a worn camshaft lobe. I agree with Paul about the compression issue. Its either a burnt valve or a holed piston.
Steve S.

Coincidence? Hmmm. You did remove the intake manifold to do the install, right? Did you leave enough of a vacuum leak that your mixture might have gone lean in a moment of extreme glory, when you were overrevving the motor and bending pushrods whilst simultaneously setting afire various bits in the combustion chamber? Nay, a lean mixture might have caused valves to burn and possibly seize, and the pushrods would bend trying to push them?

Either case requires exuberant driving on your part.

I also have a friend who bent two pushrods on his '69 when the lubrication system failed. The pushrods were the least of his worries - he had toasted the engine :(

You'll be removing tappets and looking at the cam and such. Look at the parts as you remove them to deal with this. If it's oil starvation, the signs will be evident on many of the things you remove from the engine.

I agree that the vent -should- be there, as removing blow-by and such is important for long-term engine longevity, but it can't cause a sudden failure like that.

Blow-by is less of an issue as for correct crankcase ventilation there must be *two* vents - one suction vent and one filtered and restricted vent letting fresh air in. On non-emissions engines this latter is the oil filler cap, on emissions engines the restriction is in the port on the back of the rocker cover and the filter is in the charcoal canister, the filler cap being non-vented. Even with the suction vent blocked any pressurisation of the crankcase will relieve itself via the fresh air intake route, albeit not as rapidly as with correct ventilation.
Paul Hunt 2

With a blocked or non-existent suction vent, the engine may well relieve itself of it's oil through the rear crankshaft seal. It happened to me when I took off the closed circuit breather and replaced it with a little K&N filter. It happened to someone else I knew with a midget. I phoned MGOC at the time and they told me it was a common problem - when I asked why they sold them (the little filters) in that case, I was told "because people want to buy them".

A closed-circuit system may well blow seals due to crankcase pressurisation, but the MGB *should* have an open-circuit system with separate inlet and outlets.
Paul Hunt 2

This thread was discussed between 12/11/2007 and 19/11/2007

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