MG T-ABC

Hello all,
New guy here......
After many years of Brit car ownership, I purchased TC5464 in February. It has a sleeved and bored XPAG 1500 which runs well albeit with some miles on it. The car came with the matching numbers engine that I’ve now torn down for inspection. I’ve found it bored .100 over, crank has been ground to .20 under on the mains and .30 under on the big ends. Number two big end is scored. All others visually look OK. Visually the pistons look ok. They’re old Jahns (Now known as J&E). I dropped the block, pistons and rods, and crank off at the machine shop for cleaning and measuring as I don’t own the appropriate measuring tools. There is no discernible ridge in the cylinder walls and I’m hoping if The bores are within tolerance I can get away with a hone.
Once I get the call from the shop I’ll have decisions to make and would like to have some idea ahead of time. Im prepared to sleeve if necessary. If so, what would be an appropriate size? Standard? Overbore? Also the crank. The big ends are .30 under and is .40 too close to the edge considering the .100 over bores and their increased displacement and compression? I’ve heard of welding journals, but am not familiar with the process. The good news is that the head is full thickness and the valves aren’t leaking. I’ve not brought that to the shop yet.
Cheers all,
Rob Zucca

My personal limits are +100 bore and 30 under on the journals. I've heard of people going 120, even over 120 a few times, but the cylinder walls are pretty thin at that point and it's a crap shoot if you will break through into the water jacket or not. If the block needs to be bored again then I would probably go the minimum amount to make it straight, and then get custom sleeves made to take it back to 80. Or if I was feeling brave, maybe bore a little bigger and go back to 100.

On the crank, 40 is probably ok but I'd be worrying constantly, especially on a beefed-up engine. For that reason I think I would either replace the crank or not get so aggressive on the build. Peace of mind is a valuable thing. It would be an easy decision if cranks weren't so expensive though.

Thanks Steve. One or the other would be a much easier decision. Both gets expensive. Not sure what you mean by “aggressive on the build”, unless you mean that the journals and bores are at their limit.
I’m still going to have to choose a cam as well. Yet another decision.
Rob

By aggressive I meant power-wise. In other words, if I was to use a 40-under crank, I would feel better using it on a standard engine rather than a powerful one.

Thanks Steve. I think we’re on the same page there. I don’t intend to supercharge, go with a hot cam, or raise compression any more than it already is, but the combination of the large bore and undersize crank concern me. The goal is to have the most robust and reliable engine budget will allow.
Cheers,
Rob

The larger bore will raise the compression and therefore put a little more stress on the engine, but I wouldn't have thought that a 40-under crank would make much difference to anything in terms of reliability. The centre of rotation of the con rods will remain the same as the extra grinding on the crank is exactly compensated for by the extra thickness of the bearings. I assume that the weak point on the crank will remain the bottom of the counter weights rather than bearing surfaces. I could be wrong, but I don't think that cranks tend to break in half. The important thing is to get it crack tested before spending money on a regrind.

Thanks (other) Steve. The shop will crack test as well as the fellow who actually does the grinding.
Rob

I know it doesn't actually help much but as far as the calculation for a re-grind goes; a 1mm (near as matters 40 thou) re-grind on a 50mm shaft reduces the actual strength by 5.88% - this is using a calculation we use in the power industry. Just thought I'd throw it into the mix.
Cheers
Tim

Thanks Tim.......actually having a number is helpful. Not being a math wiz, just curious, is the calculation linear in that the almost six percent would be cut in half on a journal ground to .20 under?
Cheers,
Rob

Hi Rob - not linear but very close. A 0.5mm grind gives a figure of 2.97%

Thanks Tim.....a nice yardstick to be aware of......at this point it’s a bit moot as I bit the bullet.....or bit the billet So to speak and bought a new crank. The wallet will heal and I’ll be able to sleep better..
Cheers,
Rob