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September 29th, 2008

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Q & A: Engine Ticking

Monday December 29th, 2008
Q & A, MGA, MGB, MGC, MG T-Type, Midget
Now I have an issue for which I could use some direct wisdom.

Here's my problem: I have a '62 MGA MkII with an intermittent, loud
ticking sound that occurs most noticeably at idle, after the car has
warmed up, and seems to disappear at driving RPMs. It's not a 100%
steady ticking, but rather "skips a beat" every now and again. To
start, valve clearances are all as should be, .015 when hot, and
compression is good: 1=180, 2=160, 3=160, 4=160. A timing light shows a
steady 20 degrees BTDC at idle.

At first it was difficult to determine the origination, but I was pretty
sure it was coming from the front of the block. After perusing the Q&A
database, I suspected it might have been a bad pushrod, but further
investigation (using a .010 feeler between the rocker and valve stem
while the car was idling) seemed to rule this out, as there was no
difference in the sound.

Next, using a mechanics stethoscope, I poked around the block and then
ended up by the water pump (having recently installed a new radiator, I
was a little worried something didn't get flushed out before I installed
it and was now living in the water pump). Then I landed on the timing
chain cover and BINGO, there it was. In retrospect, that is pretty much
exactly what it sounds like: a moving chain flapping against something.

So, while I probably seem to have tracked it down, I'm still left with a
few quandaries before I rip out the radiator. First of all, does this
seem likely, for the chain to get loose enough to contact the cover?
And would this suggest the tensioner is kaput? Should I just plan on
replacing the chain and tensioner? And though this is relatively
painless, is there anything else you think it might possibly be? I saw
somewhere else in the database some mention of piston slap, which had
similar symptoms (louder at idle, disappearing at higher RPMs), and
sounds much more dire.
You could take the fan belt off to make sure that its not the water pump or
generator, but it sounds like its the timing chain. There is no substitute
to calling the shop for help. John has his technical hour from 1-2 EST
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