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MG Midget and Sprite Technical - 1275 engine rebuild necessary???

Car is 1969 midget with 100,000 miles

Please read the following data and comment regarding whether or not a rebore or a simple honing of cylinders with new standard sized rings is necessary:

Cylinder bore standard is 2.78"

Cylinder #1 #2 #3 #4

top 2.781 2.781 2.782 2.782

middle 2.781 2.781 2.779 2.780

bottom 2.780 2.779 2.779 2.781

Additional measurements:

Crankshaft main journals: Standard:2.0005-2.0010

Front: 2.0000 middle: 2.0010 rear: 2.0010

crankshaft rod journals: standard: 1.6252-1.6259

Cyl. #1 = 1.625

Cyl. #2 = 1.626

Cyl. #3 = 1.626

Cyl. #4 = 1.624

Camshaft journals: Standard: Front=1.6655-1.6660

actual: 1.671

standard: middle= 1.62275-1.62325

actual: 1.625

standard: rear = 1.37275-1.3735

actual: 1.373

I have never rebuilt an engine before but the bearings all look "good" meaning they all appear to be grey in color with only a slight copper color on the side of the rod bearings. I read the rods,1&3 and 2 & 4 have to be installed in an offset fashion which makes it sound to me that there should be slight bearing wear on particular sides if those bearings, which there is.

All in all, to my untrained eye the engine, and its components, seem to be in remarkable condition, so based on the data provided does it appear I can merely replace the crank and camshaft bearings with NEW standard sized bearings??? Then there is the original question of the cylinder bores???

Must go as it is 1:20AM local time and my wife will be wondering what I'm up to.



KTM Moore

On the face of it you should get away with a set of rings and bearings, a new set of thrust washers would probably be a good plan.

I'd change the oil pump while you're at it - also new nuts on the con rod bolts.

If you have kept the cam followers numbered then you can re-use them - otherwise you will need new followers too

Take the opportunity to clean and re-seat the valves (better yet get some hardened inserts put in the exhaust seats) and use top hat oil seals (the rubber rings that come with some gasket sets are pretty useless.

Fit a new clutch and release bearing before you put it back together.

Why did it come apart incidentally?


James B

...and a new timing chain... ;o)
James B


I performed a leak down test and found I had 20%,25%,22% and 55% air loss in cylinders 1-4 respectively. I could hear the air escaping the crankcase through the oil dipstick tube. Unless I'm mistaken this means I had air blowing by the rings, which means I am probably burning oil and losing compression at the same time. The spark plugs were a nice tan color when I removed them but my tailpipe is always black. I am confident the carbs. are set correctly so I don't think they are the culprit for the tailpipe. I think the culprit is oil escaping the exhaust valves, unless I am totally off track. I am planning on having my cylinder head rebuilt, using hardened exhaust seats. The main reason for the potential rebuild is too hopefully make my engine trouble free for the next 100,000 miles.


P.S. My main concern is if I use new standard rings will they make up the .001-.002 oversize bores that currently exists and give me the sealing properties of a new engine.
KTM Moore


While it will never be as good as a new engine, the difference from one or 2 thou ought to be negligible. Up to 5 thou usually gives good results, and I recall once with a BMC1300 runner we had that due to time pressure and no immediately available pistons I re-ringed it at 17 thou wear to get me through the next couple of months. Yet it was still fine 20 000 miles late when one of my sons killed the car.
Paul Walbran

I am surprised at how much higher the leakdown test figure is for #4, when the bore wear figures don't really reflect this. Maybe the difference is due to valve seal, or are there additional score marks in the cylinder wall?

If re-ringing watch out for any wear ridge at the top of the bore. Or might you consider using Cord rings?

Also if re-ringing check that the clearance between the ring and the groove is not excessive.
Paul Walbran

Pretty good shape for the mileage - congrats on good care!

Bores should be OK.

Crank may polish but should be checked by the crank guys.

I doubt that the cam journals have grown, so hopefully these are bearing measurements. Cam bearings need to be replaced.

What does the cam and lifters look like?

I'd check clearances on the oil pump before >OR after!< buying a new one. Old pumps are frequently better than the replacements.

FR Millmore

This thread was discussed between 12/05/2012 and 13/05/2012

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