TC Overbore Risk

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How risky is it to overbore an LB block to + 0.020" (3.270" - 83.306mm)?
I'm at the point in my complete Series 3 S/E restoration of doing the engine. During the teardown inspection I've found that the original Type C, Grade 4, pistons have excessive wear and should be replaced. The block bores are in good condition; 3.2518" (average), no scoring and light wear, most of the cross-hatch honing is still visible. The block and head are a 'numbers matching' and have never been decked or shaved. They are at original deck height/thickness. I'm inclined to keep them that way. In replacing the pistons, I think it is good practice to rebore the block. I've found a nice set of 0.020 over pistons that will take the compression ration up to 10.1-10.4:1. This is a street engine. I've read all of the information about LAA, LA, LB blocks and their respective overbore capabilities. I appreciate the forum's real-world experience.

[promotor]

+0.020" is only 0.010" in terms of wall thickness - you'll be fine. Blocks "should" easily go to 83.5mm / 3.2874" and usually +0.040" is fine on most blocks.

You would be extremely lucky to find some +0.015" pistons (as I understand the only size Lotus sanctioned for their rebuilds, probably as they didn't want to carry service pistons beyond that and weren't thinking of offering spares too far into the future for high mileage engines) so you've not got much choice anyway!

Personally I wouldn't worry provided the block hasn't suffered from excessive corrosion or sagging/bulging of the bores.

[rgh0]

Almost all blocks will go to 83.5 mm with offset boring to centre the bores in the castings. This is based on ultrasonic testing of over 30 blocks that I have done. Despite all the myths, what block type ( 120E, 681F, 701M etc) what T number, whether L block or not makes no difference, they are all within the tolerances achievable with the casting technology used and all equal in terms of range of bore walls measured.

A bore to 83.0 mm is normally safe without a ultrasonic survey and offset boring, but if you want to maximise the life of the block it is always worthwhile doing an ultra sonic survey and offset boring to maximise the wall thickness.

cheers
Rohan

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Thanks for the replies, and so quickly.
Rohan, is there a height along the stroke in which the core shifts tend to occur?
Also, I assume that the offset bore is negligible with respect to cylinder center-line alignment with the crank. True?

[rgh0]

The core shifts are random both between bores in a block and between blocks. The bore casting does not seem to be offset at an angle, it is typically a lateral shift. The extent of the core shift is typically less than 1 mm so the offset boring to correct is not signficant with respect piston rod to crank aligment changes. You do need to check the head gasket alignment with the corrected bore to ensure the firering does not overlap the bore. I use a 85mm bore gasket with a 83.5mm bore cylinder and this is OK with all then engines I have built

cheers
Rohan