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MG MGB Technical - MGB 1950 Pistons

I have just stripped a block that was sold as a 1950.
The pistons are flat top and stamped +40 7L and have 2 valve cut outs.
Does anyone know if this is correct.
Thanks to all
Jay

Lucky Jay

Yes they are O.K. Lotus pistons +.040. Very commonly used for 1950cc
jim soutar

Many thanks Jim.
When i bought the block the paper work was vague . Now stripped it look very good with huge valves and the correct pistons.
Can wait to get it in the car !!

Lucky Jay

Jay

Are you taking the pistons out? I note in your picture that there appears to be fine particles in the outer edge of the piston.
Bruce Cunha

Morning Bruce,The pistons where cleaned and any particles thoroughly removed before putting back together.The bores where wiped through and checked
Lucky Jay

Hi all forgive me for jumping on this thread, I too have aquired an MGB block with JP pistons 0484A +40. This is an unrun short engine and I note these pistons have their crown 60thou below the block face. I also have a new set of Accralites.
Question - is it best to deck the block and retain the JP pistons or supplement the accralites?
Many thanks in advance for any responses.

Mark
Mark Dollimore

Can"t comment on your Accralites but best by far to deck the block to .025/.030. which ever pistons you choose to give much better squish, then work on the head to adjust compression ratio. Jim
jim soutar

Thanks Jim
Are the JP pistons considered to be good?
Mark
Mark Dollimore

Certainly good enough for reasonably quick road engines. (i.e. 125/130bhp) Though I would recommend forged pistons for competition use Jim
jim soutar

Any concerns or stories on bore flex at 1950? Is this urban myth or a result of the few done on core shifted casts? Just wondering out loud. Cheers, VEM

PS- Jim, got any new news on BHP on your SC car?
vem myers

Hi Vem I have built quite a number of 1950cc engines in various state of tune for customers. Using upto stage two cams and big valve heads variously producing between approx 90 and 130 bhp. Most of them were just bored in the block and a few with new liners. NEVER had one blow up and oil consumption IS not a problem . I can only assume that there must be some core shifted block about, but I have not had any problems. Must stress that all these engines are in road cars and are not used for competition.

No further development on the SC other than fitting a 123tune system. Pehaps its due another session Peters dyno.

Jim
jim soutar

The Lotus pistons were a cheap-assed way of getting a suitable slug to fill the oversize holes on the 1950 engines, but IMHO their use was short sighted.

It requires you to deck the block by around .120" to get back your compression (the pistons have a much lower compression height than an MG piston), which can compromise deck rigidity. Much better to buy a custom piston with the correct deck height.

There were a ton of surplus Hepolite Lotus pistons floating around and some bright light figured out that they could sell them for MG use despite the shortcomings, so we still see them popping up.

I agree with Jim that there is rarely any problem boring to 1950, although going much more especially in later blocks, which seem to suffer from casting irregularities can be dangerous.

Mark, I suggest that you stick your engine, assembled, in a stand, rotated with the plug hole up, and the valves closed, and use a Burette to run light oil in the plug hole so you can calculate what compression ratio you actually have. I usually run much lower deck heights than .060, and because the Lotus pistons have valve pockets and, IIRC, a small cut away around the periphery, you can't treat it like a flat top piston for purposes of calculation.
Bill Spohn

Thank you all for the responses.
The attached image shows my block. These pistons appear to be the 64 - 71 lotus pistons without any dome or cut outs.
Please do not worry about the apparant mess in the bores the block is due to be removed and stripped clean and I'll berefitting with the forged accralites.

All the very best
Mark

Mark Dollimore

The Lotus Twincam pistons that are sometimes used for 1950cc conversions do not have offset wrist (gudgeon) pins as the Original Equipment Hepolite pistons do, so the bores must be offset in order to compensate for this difference. They use a standard thickness set of rings that lack the flexibility to compensate for flexure of the cylinder wall. Because of their difference in Compression Height (1.5138” vs. 1.6417”), they have crowns that are approximately .1279” (3.248mm) closer to their wrist (gudgeon) pins than standard MGB pistons, thus it is necessary to end mill the deck of the engine block .1279” (3.248mm) in order to achieve a reasonable Geometric Compression Ratio (GCR) of 9:1 with the 39cc combustion chamber of the cylinder heads used on the 18V engine. This will place the deck of the engine block very close to the top of the coolant jacket, the consequent loss of rigidity resulting in a risk of cracking in some engine blocks. Because this reduction of the thickness of the deck of the engine block will also consequently decrease the number of threads available for the cylinder head studs, the depth of the threads will need to be carefully examined prior to redecking in order to determine that they will still be able to offer sufficient grip to the threads of the cylinder head studs without incurring the risk of cracking and / or distorting the deck of the engine block when the cylinder head is torqued. In addition, the use of these pistons also require the use of the horizontally-split connecting rods of the 18GG, 18GH, 18GJ, and 18GK engines (BMC Part # 12H 2445) that have bushed small ends in order to accommodate the use of floating pistons. As an alternative, either of the later connecting rods of the 18V engines that have small balance pads (BMC Part # 12H 3596) or no balance pads (BMC Part # CAM 1588) can have their small ends suitably modified in order for the wrist (gudgeon) pins and their bushings to fit properly. These later, lighter connecting rods would also help to compensate for the dynamic effects of the greater reciprocating mass of the larger pistons.

Mark-
The Accralite pistons have the same compression height and offset wristpins as the Original Equipment Hepolite pistons, so if your engine block has been decked for Lotus Twincam pistons, the crowns of the Accralite pistons will project above the deck of the block.
Stephen Strange

This thread was discussed between 30/06/2012 and 08/07/2012

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