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MG MGB Technical - Information on a 1950 engine

I have a 1950cc engine which was built about 6/8 years ago. I’ve stripper the top end off for the first time and have a few questions which someone might be able to shed some light on.

I’ve measured everything very accurately (including all the volume measurements above the piston) and have calculated a compression ratio of 9.8:1. This is ok for my use, however I found that the piston crown to deck height at TDC is nearly 2.5mm (0.098”), this seems a bit high as values of 0.04 – 0.012” are often quoted, but I’ve also heard that big bore engines do sometimes have the piston set down a bit. If I decked the block to give 0.04” it would up the CR to 11.2:1, which would be too high for my use. Are there any implications of the piston being set down this much, or does it not really matter provided the CR is right……….any thoughts please?

The second thing I’ve noticed is that the pistons appear to have two valve recesses in the top (semi-circular indentations) either side of the piston and the middle section is slightly raised (by 0.3mm). Are these Lotus Twin cam pistons? (they look like some other pictures of TC pistons I’ve found, and P Burgess’ book mentions that TC pistons used to be used in big bore conversions).

Obviously the valve recess isn’t required in the B application, any thoughts on how much disruption it makes to the air flow, the recesses also lowers the compression ration slightly. I’m just wondering whether its worth the expense and trouble of fitting flat top 1950 pistons.

Any thoughts on the above would be appreciated.

Robert.
Robert

Robert, I think the best answers for your questions might well come from Chris at Octarine Services. Check his website, he's built some pretty stout engines for his customers and would know the details you're interested in. http://www.octarine-services.co.uk/
Bill Young

Robert,

the pistons seem to be Lotus TC ones.
The block can be decked but you should considre upon the usage of the car. If it is for sprint or racing, have it decked, if it is for normal road use, keep it as it is and enjoy the bit more torque the 1950 engene can deliver. Allways remember whether it is easy to have the right fuel awailable for the CR that would be reached in this engine.

Taking the power to the max. for a B-Series engine will become an expensive exercise as there can be a lot of money spend for cam and valvegear, carbs, manyfolds, cylinder head preparation ... and you allways have to think what you want to use the car for and if 'ends does meet'.

Ralph


Ralph

Extract of an article of Steve S, Mgb american guru
“Most 1950cc kit use +.040 oversize 83.57mm domed Lotus TC pistons to produce an additional 8.2% (9 cubic inches) of displacement more than stock. These pistons use a standard thickness set of rings that lack the flexibility to compensate for flexure of the cylinder wall. They also have tops that are approximately .090” closer to their wrist (gudgeon) pins than standard MGB pistons, thus it is necessary to mill the deck of the block .100” in order to achieve a reasonable compression ratio of 9/1 with the 39cc combustion chamber of the heads used on the 18V engine. This will place the deck of the block very close to the cooling jacket, the consequent loss of rigidity resulting in a risk of cracking in some blocks. Because this reduction of the thickness of the deck of the block will 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 without incurring the risk of cracking and/or distording the deck when the cylinder head is torqued. In addition, the use of these piston also require the use of either the horizontally split connecting rods of the 18GG, 18GH,18GJ, and 18GK engines that have bushed small ends to accommodate the use of floating pistons. As an alternative, either of the later connecting rods of the 18V engines that have balance pads or no balance pads 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 greater reciprocating mass of the larger pistons.
Unfortunately, the domes of the TC Lotus pistons interfere with both airflow and combustion characteristics. If the bore is increased radically, then the squish area also increase and flame propagation becomes a problem, especially if domed pistons are used. Let’s face it : A domed piston design and the Weslake kidney-shaped combustion chamber design are not exactly in harmony with each other. Domed pistons present enough problem in a hemispherical combustion chamber, but in a Weslake kidney-shaped combustion chamber, they are bad news.”
I fitted in my mgbgt 72, sixteen years ago, a 1950 Aldon engine, he work very well always, but he waste oil always.
Regards
mw michel

Michel,

My engine is also an Aldon 1950cc. It was in a friends car and I've never actually driven it. What did you think of the engine, do you know how much BHP it produced.

Robert
Robert

Robert,
Sorry for my vocabulary, it's interesting this encounter, the engine has 90000kms about, he work very well, powerful, flexible, he has HR270 camshaft, SU HS4 carbs with KN air filter, Maniflow LCB and exhaust system, one box, Aldon dizzy with Luménition Optronic. He consume much oil, it’s the piston fault. I don’t know the power, I think 95/100 to the rear wheels.
Michel
mw michel

Michel
How do you know the pistons are the basis for the oil consumption?

If the engine was put together without valve stem seals or the seals are badly worn, you can burn a lot of oil fast in my experience.

Barry
Barry Parkinson

Hi Barry,
By the past I read an article, I don't remember where, concerning a bad conception of rings Lotus piston, principally scraper rings.The valve stem don't have seals, the bronze valve guide was tired, strangely the spark plugs are beautiful. This engine work magnificently,I will don't change it.
Regards.
Michel
michel

Sounds like you need seals on your valve guides to me
Barry
Barry Parkinson

Barry,

you know that every block can be bored out to build a 1950 engine. obviously nearly every block...

There are some castings that tend to become week when bored out to this dimension and the cylinder walls can tend to flexing then. The result is poor sealing of the rings and a high oil consumption too.

If the blocks are bored offset, it should work with most, but to be on the right side, liners should be used for this size of engine.

Ralph
Ralph

I built one a few years ago using Lotus TC pistons. The gain in HP was not worth the extra expense of doing it. To boot, the block had to be decked by 0.100" in order to get reasonable compression ratio so now the block is ruined for anything but the use of Lotus pistons. There are other ways to build horsepower that do not require as much work as a 1950
Gerry

very confusing lads, too darn many maybes on second hand info. Vem
vem myers

Best advice is not to go cheap by using the Lotus pistons. Buy properly made flat top pistons with the correct compression height for the MGB engine.

Ralph is correct, most MGB blocks can take the 1950 over bore, although there have been some instances of core shift that leave parts of the cylinder walls too thin.

I have used this technique for race engines, but I honestly think that the money you would spend on the big bore might be better spent on a really well prepared head for sporting street use.
Bill Spohn

This thread was discussed between 25/05/2007 and 26/07/2012

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