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MG Midget and Sprite Technical - Changing oil rings only

Hi All,
Have had some issues with an engine I rebuilt a few years ago. Was rebored and fitted with new pistons. Head was also rebuilt with new guides, valves and seats..

Anyhow the car goes really well, smooth and fast (has a swift tune cam)..

Only trouble is that it burns oil prodigiously..

And I mean prodigiously... takes 200 miles on the motorway to got from full to empty!

Now during reassembly I fitted the oil rings incorrectly (three peice).. I overlapped the crinkle spacer rather than fitting it butted at the ends (only excuse is that the guys who did the rebuild lost the instructions that came with the pistons)..

Needless to say fitted incorrectly I had amazing smoke screens out of the exhaust..
I pulled the pistons and fitted the rings correctly.. way less smoke..

My questions is can I get away with only changing the oil rings? My compression is good (approx 130 psi across all 4 bores).

Cheers
Tim
T Dafforn

Probably, but occasionally for one reason or another it doesn't. But is it worth the risk of repeating the work just to save a small amount of money?
And can you get just the oil rings?
If it were me, I'd give the bores a scuff and go with a full set of rings.
When I wasa student and had no money and a lifetime ahead I might have thought differently. But I learnt in the end ...
Paul Walbran

Ok. How easy is it to hone the cylinders? How do you prevent the swarf from getting everywhere?
What sort of hone is best?
(The engine is now out of the car)
Cheers
Tim
T Dafforn

With the engine out of the car honing is going to be easy. You can either take it to a professional engine builder or do it yourself fairly easily. I bought a honing tool for not a lot of money and you just use it in an electric drill. The finish won’t be as good with a home job but it is possible, I did one cylinder about 10 years ago and it is still going strong. It would be good to measure the bore first to make sure you don’t go too far but you can do do that with a new ring if you haven’t got a micrometer. Just fit a new compression ring near the top of the bore and check the gap, as long as it’s well within spec you will be ok to hone it a little. Just keep checking as you go.

As Paul points out I’d just get a full set of rings, hone as above (or get it done professionally) and then I’d use a running in oil for the first few hundred miles to make sure the rings bed in nicely.
John Payne

Tim, I just stuffed a clean rag in from the bottom end, carefully arranged, and pulled it out upwards. I used oil on the stones, so the swarf didn't fly around.
Nick and Cherry Scoop

And if you spray a light coating of grease onto the rag - the sort that comes in spray cans for greasing door hinges etc. - then any dust generated sticks to the cloth.
GuyW

OK.. sounds like a plan..
The other issue I was concerned about was my oil pressure at idle..
After a good long runs on the motorway at 4000 rpm the idle oil pressure drops to 10 psi. It is 20-30 psi after normal "round town" driving" This seems a little low to me given the crank was reground very recently and new shells fitted.
Now the engine is out I am going to check the journals..
I have only looked at the two centre big ends so far and overall the bearings look good (dull grey like when I fitted them). However both have also got a single scratch running around them. I am guessing that this is some foreign material. At first glance the crank looks OK but will need to check more. Could this be what is giving the low oil pressure?
Cheers
Tim
T Dafforn

Tim,

I doubt that the single scar you see in the bearing is the cause for low oil pressure. I would suggest that you get some plastigauge and check the clearance of all of the main and rod bearings. Checking the front and rear mains will require more disassembly, but that will solve the problem of cleaning up the block after glaze breaking or honing.

Charley
C R Huff

Have looked more carefully (or rather felt) and I can feel indentations in the big end journals.. Looks like I'll need a regrind!
Tim
T Dafforn

Oh dear. What started as a "change the oil rings" now looks like a full monty rebuild. That's old vehicles for you I guess.
Graeme Williams

This thread was discussed between 14/10/2017 and 21/10/2017

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