MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Pistons - 3 Bearing Choices

I am currently rebuilding a 3 bearing 1800 engine and I need a new set of pistons. These will need to be +60 after the rebore, although that should not affect my question.

My question is what should I use. I have heard that the 3 ring 18V pistons are the one to go for. I can't see any reason why I can't use these in a 3 bearing engine. As far as I know, the gudgeon pins are all the same size, just that the 3 bearing pin has a slot in it for the pinch bolt.

Can anyone offer me any advice? Which pistons and why?

By the way, I am fitting a Moss supercharger kit, bit these are designed for the 8.75:1 pistons, so I am happy to stay with them compression ratio. Peter Burgess is doing the head for me, I had told him I was sticking with the 8.75:1 pistons, so he is keeping the chambers appropriately large.

Many thanks,
David
David JM

David,

I do not know the answer but in such case why not fit total seal piston rings?

Cheers,

Jean
Jean Guy Catford

David

A while ago I was thinking of supercharging my 3 bearing engine using a Shorrock blower, this was before the Moss supercharger came along and not many people had supercharged an MGB so it all came to nothing.

I wanted to lower the compression and was told that the only pistons for this were 5 bearing engine (18V) ones, but that to fit these to a three bearing block I would need mga twin cam conrods.

These conrods are EXPENSIVE and RARE!

I have no problem with original pistons in my 18G engine, tuned up nicely she flies!

Do Moss now do a kit for an 18G/GA engine? When last I looked they did MGAs and 5 bearing MGBs but not three bearing. I would like to know more of how you are doing the conversion please!

John.
JW Prewer

David Look up the ACL sight they still show the 3 main pistons and there here in OZ (Tasmania) Denis
Denis4

Thank you Denis, however, at the moment I am living in the UK. At least this means that I will be able to go to the big MGB 50th event in a few weeks time at Blenheim Palace.

I can get the 3 bearing pistons here, however, the wisdom is that the 3 ring pistons used in the 18V engines are a much superior piston. It would be good to use better pistons without need to spend out the big money on forged pistons.

John, regarding the supercharger kits, I have gone for an early 5 bearing kit. I believe that the only difference is the water pump. They don't do a kit with a 3 bearing water pump. The 5 bearing water pump has a larger impeller and you will need to grind out the area behind the water pump to allow for this. The later water pump is a good move anyway as it increases water flow through the engine. A few guys in the USA have done the conversion already. It seems to be a lot more popular there than it is in the UK. Feel free to contact me if you want to discuss it further.

Regards, David
David JM

WSM say:
Clamped pins through 18GA are 3/4"
Floating or press fit pins GB-on are 13/16"

FRM
FR Millmore

Thank you Fletcher,

My engineer assured me that they were all the same! Presumably then I could possibly machine a groove in the bigger pins and open up the top of the conrod by 1/16". I can't believe that I am the first to be looking at this?

David
David JM

I've thought of it. not done it.
Don't like those clamped pins; though there were/are millions running, some disasters have happened. You need to consider how thick the pin walls are, which may vary by piston manufacturer as well as by engine type. And you would have to bore the eyes on a mill with single point tools. Seems better to just use the right pistons.

FRM
FR Millmore

David,

All that is being said agrees with the view I was told - MGA Twin Cam conrods are required.

The 3 bearing engine was unique to the MGB and an upgrade of the 1622 MGA engine (as fitted to other BMC cars). It makes sense that some MGA parts would fit.

The 5 bearing was not an MGB engine at all but was built for the Austin 1800 and built stronger to tow caravans. The block obviously is substantially differed to the 18G/GA but it used the same cylinder head, probably for cost reasons. I guess, but cannot prove, that MG were ordered to scrap the old 3 bearing and use the new engine.

Of the 50 FIA MGBs at Silverstone this year none used a 3 bearing. This is probably not because they need five bearings (the 1622 MGA racers put out considerable power and don't seem to have a reliability problem) but because the pistons are a lot smaller and lighter and far more options are available.

I would go to the experts. I had my MGB rebuilt at Beer of Houghton and Malcolm Beer will know the answer (he knows every MGB/midget/V8 answer!). Neil Cawthorn in March, Cambs has a motor repair business (try googling it) and a top class MGA racer. Neil also builds MGBs for people, has some early ones, and might be persuaded to give some info about pistons which could help.

John.
JW Prewer

Just use proper 3 main pistons. Drilling the rods won't do it - the clamping bolt won't fit properly if you do that and there are weakness issues. Using Twin Cam rods isn't a good solution - expensive and different wrist pin size again, means honing out your brand new pistons for oversize pins, assuming you can find the pins.

All a whole lot of work to accomplish a very notional improvement. I've raced many laps with 3 main pistons. If it really worries you, spend all that creativity and money on a set of forged 3 main pistons that will be even better than the 18V slugs.
Bill Spohn

Bill,

Please go ahead and post some pictures of the racing for us all to appreciate! This BBS needs to liven up, there is less going on here than a few years ago...

John.
JW Prewer

Or spring for some aftermarket con rods with the right size holes and a few pounds less recip weight!

FRM
FR Millmore

You can have pistons made up for the 3 main in any size you want - I ran special factory .080" Hepolite forged that used to be available from BMC (now probably long gone).

I'll attach a small image of a clamped writs pin still gamely held by the rod after it was bent in an 'S' shape after the piston departed for parts unknown (to the detriment of the crankcase). If I remember I'll post a larger version when I get to my home computer with the parent image. BTW, the valve beside the rod is from another engine (ahem) difficulty. It describes perfectly the angle of the valve cut out on a high compression MGA Twin Cam piston when the two unexpectedly met....


Bill Spohn

Quote "The 3 bearing engine was unique to the MGB and an upgrade of the 1622 MGA engine (as fitted to other BMC cars). It makes sense that some MGA parts would fit."

The 3 bearing engine was the standard B series, as used in many cars, f'r instance. Morris Major, Austin Lancer, Wolseley and all the others I don't know.

I agree that the 1800 engine was designed for the Austin 1800, and tuned for the B. Whether the Austin ever ran a 3 bearing I don't know, but I wouldn't be surprised if it did.

Herb
Herb Adler

Ok, so somebody prove me wrong, and I mean it, please - if I am wrong I'm happy to be proven wrong!

Was the MGB 1800 3 bearing was the first 1800 B series and was it only fitted to the MGB?

Did other B series engined car at the time (Magnette, Wolseley, Riley, Morris, Austin) staye at 1622cc until other cars started getting the 1800 B series when they got the five bearing version was produced?

I know Wikipedia isn't that accurate but it says the 1800 applications were:

1962–80 MGB
1964–75 BMC ADO17 ("Landcrab") Austin 1800
1967–72 BMC ADO17 ("Landcrab") Wolseley 18/85
1966–75 BMC ADO17 ("Landcrab") Morris 1800
1975–78 Princess 1.8
1971–78 Morris Marina 1.8
1964–67 TVR Grantura Mark III and IV
1964-65 Elva Courier Mark IV
1974–78 Leyland Sherpa van

So there we are - 5 bearing designed in '64 for the Austin 1800.

Wikipedia also lists an 1800 B series from 1954–80 in something called a Probe 16, but another page contradicts this by saying that Probe 16 production only began in 1969. It is perfectly possibly that 1800 3 bearing engines made their way into low volume cars made by manufacturers who bought engines from BMC (TVR Grantura?).

Another statement to be proven wrong is that every works MGB race and rally car used 1800 3 bearing engines even after it had stopped being used in production cars. The only five bearing 1800 works engine might have been the race engine put into the MGB GTS for its early outings before the 3000cc engine was available.

Incidentally another gem of wisdom to be proven wrong is that the MGB in 1977 only got a rear anti roll bar because one was made for the Sherpa van. So if true this is another example of the poor MGB only getting a worthwhile part because there one became available by chance - chronic lack of investment in MG again!

John.
John Prewer

The 3 main 1800 was, AFAIK, only used in the MGB. The other listed cars used a 1622 or a 5 main 1800.
Bill Spohn

Apropos nothing at all, my father in law told me that in the early 60's he built a tuned 1800 B series engine by overboring a 1622 engine, using the MGB pistons and putting the lump c/w HRG Derrington crossflow head in a tweaked Riley 1.5. He subsequently sold the car with a cooking engine and put the hot motor in an early TVR Grantura. This was sold a few years later with impending arrival of my wife - more space being needed. He then purchased a Gilbern Genie, which he's still got. I'm guessing that trying to get hold of a pukka MGB motor back then was difficult and/or expensive.

BTW now got his old crackle black alloy HRG rocker cover on my B roadster. Unfortunately, the head went way back when!

Peter
P A Allen

Peter,

The Gilbern must be very rare now - wikipedia says only 197 made.

Can you post a photo of the rocker cover? They have re-made the HRG head but I've never seen a rocker cover.

For my money rare after-market bits like these are just as relevant as original parts and certainly more interesting.

I have a Heron GTB Fastback hardtop which I am getting all fired up to restore and I am thinking an HRG head would be well in keeping with it - it's been done before of course, see http://www.ten-tenths.com/forum/showthread.php?t=124679#post2760430

John
John Prewer

John,

Tried to attach photos of rocker cover, but it says they're too large. I could send them by e-mail, if you were interested?

Love the hardtop! So much prettier than the Works one and so evocative of old Car & Car Conversions ads, maybe even '60s Motor Sports. I guess to fit you'd remove the bootlid and have some sort of internal cover.

Peter

P A Allen

Peter,

john dott prewer att hotmail dott com.

I've only ever seen two other Heron GTB hardtops, one on a black car in Essex in the mid 80's and another at MG Car Club Beaulieu for sale off a car shortly afterwards.

A mate of mine from Dereham came across it (he collects such things as well as an A30 or two, a Y type, a Midget and two MGB GTs!) and I got it from him.

You remove the boot lid and the hardtop has a shelf under the back window that cover the boot space. Being from the 60s the shelf needs a nodding dog and the rear window has to have some bullet holes!

John.
John Prewer

Couple of things:

Punching a 1622 out to 1798 is a full 4 mm punch, not something I'd recommend based on my experience with wall thickness, unless you sleeve the block. There is also little point as you are still stuck with the weaker 1622 crankshaft instead of the beefier 3 main MGB item.

The 5 main MGB blocks will take a .120" punch but it gets iffy going beyond that. One of my race engines is a 1950 cc.

Don't have a handy picture of the HRG valve cover - the spare pushrod race engine with HRG head is living under my work bench.

The Gilbern was rare, but also a bit slab sided, I thought and certainly not an improvement on the MG syling - an uglier MG that didn't really weigh as little as they claimed. There is a claim that a few GTs were built using a 3 main 1798 but quickly switched to 5 main. Haven't seen that substantiated. If anyone has the Motorsport issues form 1963-64 with the reviews of the cars in them, that might confirm either way.

In any case, that would be about the only non-MG use of the 1800 3 main I can think of. My 1963 TVR Grantura was built with a 3 main 1622 and my friends car, a dozen numbers later was supplied with a 5 main MGB (production was slow and the 5 main started coming out in 1964. They only made 90 cars, so it is a rare one too.
Bill Spohn

Chasing my ambitions of putting the original 18GA back into my MGB instead of the 18G, and making it ssomething special, I went to see Malcolm Beer who showed me conrods that will fit the pistons from a five bearing engine on to a 18G or 18GA three bearing crank & block.

David, I left you a voicemail about this.

For anyone trying to supercharge a three bearing with a Moss kit this just might mean that you can fit a five bearing water pump to a three bearing engine and use the Moss 5 bearing (18GB on) kit with all the internals of the engine uprated.

Perhaps there is your anwer David!

John.
John Prewer

This thread was discussed between 05/09/2012 and 11/10/2012

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.