I recently had a similar experience with a TD motor and it turned out to be a
broken lockwasher on the cinch bolt on the piston end of the connecting rod.
With the washer missing, it developed a knock that gave the impression of
originating in the top end or possibly the push rods. Investigation showed
nothing wrong there so the oil pan revealed the problem with washer parts in
the pan.
Just another possiblilty!
Tim Clifford
'51 TD
-----Original Message-----
From: Dodd, Kelvin [SMTP:doddk@mossmotors.com]
Sent: Tuesday, January 18, 2000 2:59 PM
To: Randy
Cc: mgs@autox.team.net
Subject: RE: Engine Noise
Number 3 connecting rod is no longer happily spinning on the crankshaft.
DO NOT RUN THE ENGINE!!!!!
A common failure for the 5 Main engine is lack of oil to the #3 rod
journal. The dry bearing seizes on the crankshaft and is damaged. The
engine then exhibits a knocking that sounds like it is coming from the
rocker area.
The problem is that if the engine continues to run, the rod hammers
the crank. Once the rod bearing starts to deform the rod can start to hit
the camshaft. After a short while the rod bolts let go and on the next go
round the static rod gets hit by the crank journal pushing it through the
camshaft and out the side of the block.
Ok, that's the bad news. If you are really lucky, the crank and rod
are not damaged. But chances are the car was driven far enough to cause
overheating and severe damage to the crank and rod.
There are two ways to check this out. Neither of them easy. Pull
the head and check to see if any of the pistons rotate. Or lift the engine
up far enough to enable the pan to be dropped. Look for a discolored rod
journal. This is typically #3 but I have seen #2 fail for the same reasons.
Given normal maintenance the 5 main is pretty bulletproof. If it
runs low on oil or doesn't get regular oil changes the #2, or #3 rod bearing
is where it often fails. This weak spot can be helped by cross drilling the
crankshaft to supply oil from the adjoining main cap.
Some stupid stuff to check before pulling the engine to bits. It
sounds like you have already checked the valve gear. A really tight or
burnt exhaust valve can cause the piston in one of the carbs to bounce up
and down, making a knock as it hits the top of the dashpot.
That's about it. Good luck.
Kelvin.
> -----Original Message-----
> From: Randy [mailto:rap@palmnet.net]
> Sent: Tuesday, January 18, 2000 2:13 PM
> To: mgs@autox.team.net
> Subject: Engine Noise
>
>
> Hi,
> I recently purchased a 71 MGB with engine noise. The previous
> owner said
> he was test driving after installing a new fuel pump and the engine
> started to make noise. He drive it a few miles to his fathers
> house and
> parked it, putting it up for sale.
>
> The car starts easily and the noise sounds like a very loud valve tap.
> Any ideas of what would cause a sound such as this? I have not torn an
> engine apart in a long time. I got as far as removing the
> valve cover in
> hopes of seeing something our of place. That didn't work.
>
> Thank you for any ideas.
> Randy
>
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