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MG MGB Technical - What's up with these pistons?

Since the previous owner didn't know anything about the car and the engine was out I decided to remove the cylinder head to check how it looked on the inside.

Said and done and it appears to have had a restoration. The pistons looks like they're for a different head than the original one anyone know more? Although they say STD but the cylinders feels and looks like they've had a re-bore or maybe just honed?

It's a 18GB engine. The pistons says STD I.D 16 and google gave me nothing.

Alexander Sorby Wigstrom

They do look like cross-flow pistons. Did you record the compression pressures before dismantling? According to the WSM standard bore is 3.16" or 80.26mm.
Paul Hunt

They look very nice but goodness knows what they are for! Probably a cross flow twin cam. Your engine may indeed have had a rebore and the pistons are standard for their usual application. I seem to remember that Ford pistons are sometimes used on over bores or to raise compression ratio, or both!
Allan Reeling

No I didn't, I expected to have to rebuild the engine since it was out and all ancillaries were disconnected/removed.

The head appears to be skimmed to (is that the correct term for when the head surface is machined?) The head says 12H
Alexander Sorby Wigstrom

Skimming is normal when an engine is re-built, again sometimes to raise CR. If you can take a picture of the combustion chambers and give us the full engine number. Also measure the bores, combustion chamber depth and the valve diameters. Those might give you an idea what has been done to the unit. But there is no carbon and if there is little or no wear ridge at the top of the bores it's a good bet it has been recently worked on.
Allan Reeling

The measurements are a bit rough since I took them with a vernier caliper and they're in millimetres, sorry.

Cylinder depth: 90mm.
Cylinder diameter 81mm.
Inlet valve base 41mm
Exhaust valve base: 33mm

Engine number: 18GB-RU-H 43492

As i said they're rough and round off upward.
The compression chamber also show sings off having work done, the bright part in the photo, my mobile reflected the light poorly.

I don't really get why you would put crossflow pistons in with a regular head, isn't the gain from the bigger piston lost due to the recess in the piston?


Alexander Sorby Wigstrom

Looks like you may have the larger, later 18v inlet valves. Depending how "off" your measurements are, larger bores. Measure the depth of the combustion chamber. They have also been worked on. A compression test will give you an idea of compression ratio, although it could be worked out mathematically. Difficult in view of the pistons, but paraffin and a buret will be quite accurate. If you just bolt it back together a compression reading in excess of 170psi will give you over 9 : 1. Settle for it if it goes well!!!
Allan Reeling

Well, I'm chuffed that I don't have to spend money on it.
I'll bolt the head back on and use it as is once the body is repaired.
Need to sort a set of carbs since it came without, any suggestions?
Alexander Sorby Wigstrom

Get a pair of SU's HS4 or HIF 4, don't go for anything else. I think I have an inlet manifold if you need one, let me know. I may have a few other bits as well when I root about.
Allan Reeling

Alright, my brother has got a load of twin SU from Volvos but I think those are HIF 6 or HS6.

Thanks for the offer, you can send me an email with what you've got to spare.

alexander.sorby.wigstrom@live.se
Alexander Sorby Wigstrom

Having just recently rebuilt a Lotus-Ford Twin-Cam engine from my 1971 Elan, I can tell you that your pistons look exactly the same as the Lotus ones.
Mike Howlett

Go for HSs rather than HIFs, they are easier to live with, at the expense of being slightly less kind to the environment. You don't say the year, but going by the engine number the cables and linkages would be more conventional as well.
Paul Hunt

Looks very like twin cam pistons as Mike says.
What is the cylinder head part number? 12H?

Peter
Peter Burgess Tuning

Cylinder head number 12H1326.
Alexander Sorby Wigstrom

The standard bore size for the Lotus Twin-Cam is 3.250 inch (82.55 mm). For the MGB the standard bore size is 3.160 inch (80.26 mm). So using Lotus pistons in an MGB block would require a +90 thou rebore. This gives a capacity of 1903 cc.

Mike Howlett

That will be fine Alexander.

Peter
Peter Burgess Tuning

Thanks for all the input guys,

I read on wikipedia that Ford Kent engines had a STD 3-3/16 in (80.9625 mm) bore which would equal the 81mm i measured?

What head gasket should I buy now, standard size one?
Alexander Sorby Wigstrom

Alexander - Using Lotus Twin Cam pistons was the standard way to get a big bore MGB back in the 80s/90s with I'm guessing overbore pistons to get to 1925cc/1950cc. The conversion used floating gudgeon pin rods from the GG series engine (I think). The later I8V rods were only suitable for press fit gudgeon pins, and the very early ones weren't strong enough. These days I presume there's enough demand to manufacture a special piston for the application. I also recall that the bores needed to be offset slightly. However, yours seems to have plenty of metal between the bores.
l

Alexander, those won't be Ford Kent pistons. They have to be from a twin-cam engine - otherwise why would they have the valve cut-outs? The Kent engine would have no need for a piston of that shape.
Mike Howlett

Quite right Mike, the X-flow have cut outs and odd shapes to accommodate valves along the centreline of the bores and combustion chamber in the piston as the head was flat (Heron head).
The first 18V engines featured horizontal split rods with bushed little ends. You can use the press fit rods for any of the circlip fit pistons and forget the circlips....don't forget the circlips if you have bushed little ends though!

Peter
Peter Burgess Tuning

This thread was discussed between 03/11/2015 and 08/11/2015

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