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MG MGB Technical - engine been rebored?

Hi all
I'm trying to work out if the engine on my 1970 MG BGT has been rebored or is simply badly warn.
The car has been burning a lot of oil for a while, and I decide to carry out a long needed engine rebuild.
Head off, caked but otherwise looks ok, pistons out - good clean required but otherwise also looks OK, timing chain off, etc, etc.
Having ordered the new parts, I tried a new ring in the bore..... about a 1.5 - 2mm gap... bigger then I could measure with a full set of guages.
The Pistons are also interesting. They carry a marking which reads "175-9405R".
So, if this a normal amount of ware for a 70,000 mile engine, or has someone decided to try to increase capacity?
Also, can I get oversized rings to fit?
And finally, are the pistons correct?, shouldn't that read "180-...."
Anyone, please..
Thanks,
Mike
M Kemp

Mike. The bores need to be properly measured using a hole micrometer or a dial indicator designed to measure the diameter of holes. No other way to do it. Same with the pistons. You must measure the diameter using an accurate micrometer or dial calipers.

Then, you have to compare the two readings to see how they relate to the original sizes and how they relate to each other.

Bores worn sufficiently to have the ring gap, using standard rings, would have a taper from top to bottom as bores do not wear in a perfectly cylindrical fashion. My experience is that at 100K miles, the bores have worn sufficiently that they need to be bored. But, less than .010" over standard on an engine that has been well maintained over the years. They normally clean up with either .010" or .020" over pistons with no problems.

Simply using larger rings, with standard pistons will work. But, not very well, nor very long. It is a bodge to get an engine running to sell a car, not something you would do if planning on keeping the car. Excessive piston slap will cause the excessive oil consumption you mention to reappear quickly.

Have the block and pistons measured. Take time to have the crankshaft bearing journals measured. You will be happy that you did.

Les
Les Bengtson

Mike,

Where in France are you - I will be in NW France next weekend.
Chris at Octarine Services

Mike-
If your pistons are original, then they will have an arrow on the crown that points toward the front of the engine.

I have to agree with Les on the subject of oversize rings. Simply installing oversize rings is a sleazy bodge in order to get an engine running to sell a car to an unsuspecting buyer. It definitely is not something that you would do if you intend to keep the car. Piston slap will indeed cause your excessive oil consumption to soon reappear due to rounding of the sealing edges of the oversize rings, so it really is a waste of time and money. You will have to pull the engine again, disassmble it again, and have the cylinders bored nad honed in order to accept new oversize pistons and rings, reassmble it again, install in back into the car again, etc., etc. Like I said, it really is a waste of time and money.
Steve S.

Mike

Have you tried one of the original rings in the bore to see what the gap is on them?

Dave O'Neill 2

Thanks for the comments all,
the pistons show no markings to indicate that they are oversize, and they do have a mark pointing towards the front - so I guess they are original.
I tried the new ring in the bore - at the very top and at the very bottom, as well as at various points in between - the gap remained visually constant throughout - at about 1.5mm. No visual oval effects seen at any point.
I'm thinking now that the only way to tell will be to measure the bore and the pistons - which means either buying or borrowing equipment.
M Kemp

I'm not lazy, or a spendthrift, but I can't imagine tearing down an engine and NOT taking the block and crank to an experienced machine shop for evaluation, and work, for proper sized bearings and reconditioning of the cylinder bores. I simply don't have the experience (I've only rebuilt about five engines in my life) or the equipment to make those decisions without expert assistance.

For me, the process of a rebuild is collaborative with the machine shop who works my block, crank and head. I often let them purchase the pistons and a few other parts which the engine is then machined to match, as they make a mark-up on them, and increases their 'interest' in my project as it becomes more valuable to them. Probably costs me an additional 10% on the total rebuild to do it that way, but I've always been pleased with the results, and learn a bit more each time.

warmly,
dave
Dave Braun

The replacement pistons in my car have arrows to point to the front too, so that's not necessarily a sign of original pistons.
Neil
Neil Lock

This thread was discussed between 06/05/2007 and 11/05/2007

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