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MG MGB Technical - Big Bore Headgasket

As I'm always looking for better options----
Has anyone got a supplier of a superior quality big bore 85mm headgasket that I should know about
Or what are you using with reliability-------
Thanks in advance
William Revit

I am using Payen. The bore is large enough for my 1950cc St 2 and strong enough for a supercharged engine I built for my mates 1840cc . Neither are used in competition But I have used these gaskets for years on all my customer cars without a single failure.
Preparation of head and block faces is important as is counter sinking the stud holes by 1/2 thread.
The 1950 is running 10:1 compression and 129BHP up to 6500rpm. Why pay more? Jim
jim soutar

Payen is 83mm, so is close to 1859 but not quite there (overlaps the bore by 10 thou). That said, a lot of people have used it successfully in that application.

We have a locally made one here, comfortable clearance on 83.5mm, many have used it successfully, but I have had a couple go at one of the the water holes at the siamesed junction.

I use for customers' cars and also my own car a big bore (85mm approx) copper/filler/steel gasket which I get from a UK wholesaler. Not sure who makes it. It' works well if coated with VHT copper gasket cement, and also modified by the addition of some additional water holes (being ones which are usually in B series gaskets but which have been deleted.) I get this doen by a local gasket maker. My car runs 1950cc with 11.8:1 compression and is used in all manner of competition events including the week-long Silver Fern historic rally here, which sports 700-1000 miles of special stages over closed roads so no speed limits apply :-).

There is also a cometic gasket available but I haven't had the need for it.

Paul Walbran

Paul The Payen available here comfortably fit 83.5 without overlap. I mush prefer them to copper sandwich types, the silicon rings and lacquer finish work extremely well on the Payen. jim
jim soutar

I'm on 85mm and been using the copper/steel mls gasket but occasionaly get a failure between 3'4 and suspect it is a coolant flow problem
With the added coolant holes in the copper gasket, are you blocking the two centre holes or leaving them open
I've been blocking them and opening the rear holes but might be on the wrong track doing that
If you are blocking them are you drilling steam holes in the plugs
William Revit

Willy, what torque are you using?

Peter Burgess Tuning

I've had a set of head studs made up using Caterpillar bolts, with the ends chopped and threaded, don't know what grade they are.But they are known to be top quality fasteners. The block's been drilled and threaded and the studs go deeper into the block. I've been doing them at 60ft/lb and they feel as solid as a rock doing them up- no give at all and keep their tension.
I've been thinking of giving stretch studs a try but feel it would be a backward step
A few years ago on the Chevs with steel shim headgaskets we used to soak a piece of cloth with locktite and wipe it all over the gasket ,then bolt it up while it was still wet. worked great but a bit of a prick to get apart--That was a single shim though, I don't think it would be as successful with a mls gasket
What tension are you guys using----more than 60 or less is better ????
William Revit

50 Lbs/ft on ARP Studs and nuts
jim soutar

The Payen gasket we stock is AK660 which as far as I've been able to find is the only 1800 B series one they do. I like them as a gasket, but they do overlap the 83.5 bore slightly once compressed.


I have opened out the holes on the back to 9/16" but have left the small hole (8mm IIRC)in the core support between 2 & 3 under the exhaust port. I've plugged the hole in the head and redrilled with just the small hole in the centre.
That made a big difference as before that the gasket had scorch marks in that area when removed, now it doesn't. It also had a tendency to slightly pressurise the cooling system at high revs, maybe half a cupful into the overflow bottle per special stage. After adding the hole it doesn't. I have had to have the holes put in with a proper ferrule, just punching them through and doing nothing else doesn't work as well.

The other holes I've added are on the spark plug side in the vee between 1&2 (5mm) and also 3&4 (8mm) that are in most standard gaskets as the late rubber bumper had holes there and reduced holes by the plugs. The first batch of local big-bore gaskets I had done locally 30-odd years ago sometimes blew between 3&4 initially, but that drilling solved that.

I've also plugged the bypass hole at the front to help re-direct more water to the other end of the engine.

I haven't had trouble with my car blowing between 3&4 on the current gasket. We have had a customer's in recently with that problem, I'm not sure which gasket he used as he'd had it set up elsewhere and stripped it before bringing it too us.We suspect it may have been set up with a bit too much advance, but it's still work in progress so can't confirm.

Torque: We I use ARP on mine following their recommended torque of 45 lb-ft with their own lube for the nuts.

Paul Walbran

Thanks for the info
I'll have a look at the new gasket when it comes
The 5mm and 8mm holes aren't in the gasket I've been using , but they will be this time round, and a steam hole in the centre
I suspect I'll be doing some drilling
Your excellent info sort of confirms what I'd been thinking was the cause, so all good news to have proof from someone (You) that has had success
Thanks again
William Revit

Cheers Willy. Can you get that VHT copper gasket cement product that I mentioned there? It works well and is easy to apply - it's all we use between head and compression lowering shim on the supercharged K midget engine.
Paul Walbran

Hi Willy

We encountered problems two seasons ago with heads lifting and blowing gaskets on endurance race engines after twenty plus minutes flat out. The head lifts and the gasket goes mainly adjacent to number 2 cyl and water loss ensues and damage to block and head if not pulled up quick! We feel this is caused by the make up of fuel with the ethanol addition. To overcome this we fit ARP head studs and Cometic gaskets covered with Wellseal. Torqued down to 70 lbs ft and have hit 74lbs ft on stubborn ones! The problem is exacerbated by the FIA regs stipulating radiator surrounds, ducting oil cooler mounting positions etc etc.
For the Cometic gaskets we need a finer finish on the faces so we have (with tears in eyes!) spent 1500 on a 14 insert cutter for the faithful old Milwaukee miller (800 for whole machine in 1988).

Evolve or die :)

Peter Burgess Tuning

Thanks again guys for your interest--
yes i use the VHT copper ,it's the pressure pack one- interesting I used a solid copper gasket for a try and used the blue Hylomar spray but decided it was acting like an insulator and preventing the heat from getting soaked out of the gasket-melted a couple then swapped to VHT and there was a marked improvement and I've stuck with it since
Don't know if I'm game enough for 70lb I'll give that a think. Haven't seen Wellseal here, I'll check that out
Is it a metal based sealer??
Nice expensive set of cutters, be brave,we don't want grown men weeping around the place--
The machine shop I use here has a multi cutter head on his machine but I must admit I wouldn't have a clue how many tips on it BUT I get on really well with him and if I ask real nice he swaps the head out for a diamond wheel that he keeps hidden away - It gives a fantastic finish on cast iron but not quite as special on alloy, it sort of fluffs the surface a bit like a freshly sharpened stone does--

Anyway the plan at this stage is some new holes in the gasket/head, a dose of VHT and maybee 70lb tension if I take some brave pills

I'll keep you posted
William Revit


Wellseal is a non setting gasket jointing compound which looks a little like golden hermatite.
label says it contains perchloretylene and pulverised vinsol resin.....I am non the wiser :)

The cutter inserts are for aluminium, apparently this allows us to take fine cuts off ally and cast iron whereas the coated inserts are only happy taking big chunks off! Mind you we can traverse at 1 3/4 and 40 thou cuts when we 'go for it' on such as TR4s where we are taking up to 180 thou off.

Peter Burgess Tuning

Sounds like a nice setup Peter
A few years back I had heaps of trouble with headgaskets and it turned out to be that the machine shop I was using at the time had his wheel set up with too much clearance at the back of the wheel and it was machining with a very slight concave finish
Getting a head back from being machined, the last thing you think of is that it's not flat, but after the gasket blows and the straight edge comes out--bingo
They would be straight length ways but hollow across --Having worked in a machine shop for a while I knew what was causing it but he wouldn't listen to the fact it was bad machining so we've moved on to my current guy, who certainly knows what he's doing AND his ears work fine--
William Revit

I'd throw a straight edge across the block to see if you have issues you weren't aware of.

I use the Payen on an 83.5 mm bore and have no issues (the Twin Cam engine has different cooling, though - the water pump isn't in the block and just circulates through the head; coolant from the block basically percolates up through holes in the head).

I run 12:1 with no issues.

I looked at bore sizing extensively when I did the big bore Twin Cam and after trashing a couple of blocks (intentionally) concluded that 83.5 mm was the largest I could trust without linering the block.

BTW, this thread reminds me of the good old days when BMC offered a proper competition composite copper head gasket for the MGB. I ordered one and someone back East (where the Canadian parts depot was located) kindly folded it in half so it would fit in the large envelope they sent it in.....
Bill Spohn

I've had a good run at 84mm without liners in the past, but very particular when offsetting the bores to get a good landing between 1-2 and 3-4 . After boring the block I don't cut a leed on the top of the bore, just lightly take the edge off with a hand scraper and this leaves a good 3mm wide surface for the gasket. It's a bit fiddly getting the rings started in the bore but I've made a 84mm funnel and that works good--
On this current problem engine it's got liners and 85mm bore, but still 3mm between bores----I'm convinced after what Paul/Peter have said that there's a coolant flow problem causing it but we'll see once the gasket turns up and we have a good look at it all
The thing I like most about posting on here is the fact that there are some truely knowledgable dudes on here that know there stuff and there's always a little reminder comes up that jerks your memory into gear,to think outside what's staring at you-
In this case it's reminded me the dizzy hasn't been out of this thing for yonks and could well be out of spec.
William Revit

William - I had one block bore nicely to 84 mm....and then when tested it broke through with pinholes into the water jacket due to core shift.

Unless you sonic test the bores all the way down in a block you want to take out that far, it is really Russian roulette.

While core shift tends to be more of a problem on later blocks, the one that 'holed' was an early 18GB!
Bill Spohn

OK Guys--update time

Pulled the dizzy and checked it-
8deg @ 1600rpm (16@3200) so all good there

Started cleaning the head up and found it straight as a die but it's getting a light swipe just because--

The interesting bit is that along the edge of the combustion chamber for about 30mm from the spark plug area along to about level with the centreline of the exhaust valve the edge has been hot, there is a blue line about the width of a match along the edge/face of the chamber and the same on the other side from the beak area to the exh.valve centre--same on all four
So I've been into it and increased the radius a bit on the exposed edges and shot the head off for a very light skim
So now I'm thinking that with the dizzy being ok, using BP7ES plugs, so they should be cool enough and nothing else has changed really apart from now going to increase water flow through the head. I'm thinking more along the lines of it has been caused by dodgey fuel, either stale or the servo had some ethanol crap in there instead of proper stuff

What Id like to know at this stage--is
If anyone out there has a Cometic gasket that they could measure across the bore for me
They are listed as 83mm but I've been told they are bigger than that
Also if they are 83ish , is the pressing for the fire ring right on the edge or is there enough flat surface there to open the gasket up to 85mm
William Revit

I'm not sure on those BP7ES plugs. What CR and cam timing are you running? Plugs are very CR sensitive. My experience is that BP7ES is OK for 10:1 on a fast road cam (just) but higher CRs call for BP8ES or cooler. In my case (11.8:1 on a fairly lumpy cam) I run Bosch plugs are a grade colder again (W2CC I thin but not sure - sorry, too far from home at present to have immediate access to plug type, did the Anzac centennial at Gallipoli in the weekend and still on the way home). Fouling on these is never a problem. If I go back to BP8ES they scorch a little on the ends.
Paul Walbran

Thanks for that
Might just go for BP8ES then - to be safe
It's running 10-1 comp and a crazy cam that a mate that used to work at a cam joint made up for me
Heaps of lift and cut off inlet timing to give the same result as a scatter pat. cam --- that bit works fine
I'm still thinking the fuel caused the heat
William Revit

Interesting sounding cam. Yes, at that combination of cam and CR BP7ES is unlikely to be the cause. The 8s could be worth a try though, the worst that can happen is they foul which would confirm the current 7s as good.
Paul Walbran

Stepping back and relooking at it
There are probably two seperate issues here

First the headgasket ,which is why it's apart
It's nearly four years since it had one so not really an issue. A bit of extra coolant flow will probably help and the extra holes are happening there so all happy with that.

The other issue of overheated combustion chambers is probably more of an issue ,but after reworking the edges of the chamber that were marked out in blue for me to make sure I got the right bits done ,and checking the dizzy etc I'm convinced it was dodgey fuel that caused this.

The car performed faultlessly at the National meeting in Victoria over Easter and didn't show any sign of a problem touring around the following week,But died after two runs at the historic sprints the following weekend. The sprint was just over 800metres long but there is a full throttle, full revs section uphill in 3rd that tests things a bit, but having said that, it's run this event every year for a while now so I'm thinking there had to be something out of the ordinary to cause a problem and fuel is all I can come up with so it's all going to go back together with these little mods and hope for the best

Thanks for your interest gents; this is the first job I've done for over a year now and I'm feeling a bit rusty so your input has been very much appreciated.
William Revit

Cheers Willy, let us know how you get on
Paul Walbran

All back together and happy

Really, A thrash around the countryside in a worked over B takes a bit to better, there is definately something about these cars that gets you going
Time will tell as far as results for work done but it feels good and that's the main thing for now

BIG trip to Perth for the nationals next year so there will be plenty of pre trip trips (sounds funny) to test reliability

Over and out

Just read back through all this--
Just to clarify--"This is not my car" it belongs to a good mate, I help him out where I can, Built his engine for him probably 6 years ago and the car lives a mostly competition life
Sealed road rallies
Speed events
MG natmeets
Historic club level events
William Revit

This thread was discussed between 19/04/2015 and 19/05/2015

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