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MG Midget and Sprite Technical - Assessing how tired an engine is

I am thinking of doing a bit of refreshing of my engine. But the question is how much? So to start with it is down to some diagnosis of the amount of wear.
The engine is a 1330 which has done 70,000 since its last major rebuild. It is running well, revving freely and starts "on the button"
However it is burning a bit of oil - blows some blue smoke when hot, and not just on the over-run or after idling.

First stage assessment is spark plugs. - 1 and 4 are fine, but 2 and 3 are a bit sooty. Dry black soot, not too wet and oily.

Compression tests:
Dry: 192/160/174/190
Wet: 220/188/200/220

The dry readings are pretty well exactly the same as on the last 2 test, it hasn't got any worse. (I routinely test and record compression every 6 months)
The wet readings are higher than they should be - I think I was over enthusiastic with the amount of oil I added! They are normally 10 to 20 psi lower than that.

Will probably take the head off tomorrow to check gasket condition between 2 and 3 and take a look at the bores. I think if it was blowing between 2 and 3 it would have got a lot worse in the last 6,000, but test as at April 2011 is almost identical.

Here are the plugs as removed. They are NGK BP5ES.
#1 is on the right, so the blackest one corresponds to the lowest compression. Normally with adjacent cylinders reading low I would suspect the HG, especially with the middle pair. But as it hasn't got any worse I am thinking maybe worn rings.


#1 looks like it's still glowing with heat :)
Nigel Atkins

LOL, yes it does a bit. It was cool enough to take out though! - Don't worry Nigel, it will effectively get a service treat of a set of new plugs!

as Lawrence would tell you if he were here, you're being wasteful just clean them up and they'll be good for another 70,000 :)
Nigel Atkins

Nigel, you think you are joking, but those HAVE done 70K ! Time for a change, but being me, they will be kept as spares!

Actually I cracked the insulation on one of the original set and one of these is an odd one out. Can you spot it?

tut, tut, tut, you go to the trouble of having a 1330 engine and then you stifle it performance by having plugs past their prime - they can look ok but be past their best, I'll see if I can find a veeery long article on this :)

I was looking for the plug that looked the least decrepid and thought of three then noticed the seal looked crudded so looked at the set again . . . and then noticed three was a different type

I'm lucky to see the monitor screen let alone any details, just remember if you ever see me driving on the road :)
Nigel Atkins

nr 3

I'd see if you can just get away with a hone and new set of rings.
check the bearings and replace if needed
Onno K

Yes nr 3, but why?

Onno, I was thinking it may need new rings, or possibly a set of those Cords rings, depending on what the bores look like.

Yes I will probably put new shells in. The crank was ground, line bored and balanced, together with a lightened flywheel. Its had regular 3000 mile oil changes and hopefully any wear will be on the shells rather than the journals.

because the others are pre-1980 design :)
Nigel Atkins

No 3 is a different shape and less rusty.

Aren't BP5ES a bit on the hot side? I thought that a 6 was a standard replacement and a 7 for a 'hotter' tune?! I used to run 8 on the racer.
Dave O'Neill2

fit new plugs (full set, correct spec) and take a fresh look in a couple of weeks...
David Smith

nr 3 is a BP 6ES the others are 5s.

I fitted grade 5 when it first went on a rolling road. They don't explain the varietion in compression!

This thread was discussed between 29/01/2012 and 30/01/2012

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