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MG Midget and Sprite Technical - Goes like stink. Cause an exhaust manifold leak?

In the last couple of weeks or so two things have increased.

My sickness and headaches. And the power of my Sprite; which I am NOT imagining. There's a very noticeable increase in acceleration. Really, it's going like stink.

The only other change is my manifold gasket leaking. Manifold gasket, see picture.

Could just a gasket leak, contribute *that* much to getting the exhaust out?

If so, what I though was already a pretty free flowing exhaust system, -- a single box almost straight through as I've punctured the baffles and it sounds well fruity --- , must in fact be pretty restrictive.

So can it? Can an exhaust leak boost power?

One thing I do know. Those fumes get right up my nose, sting my eyes, and make me feel bloody unwell. New gasket going on now.










anamnesis

Quite a blow.



anamnesis

A leak on the centre port, is in fact two leaks, kinda !
Malc Gilliver

Oh yeah. I hadn't thought about that.

Put a new gasket on this afternoon. So I'll soon know if my power increase has gone. If so, I need a better silencer.

Good thing is, my hearing is now crap, so extra noise doesn't matter so much. 🤣🤣🤣🤣.
anamnesis

I'm warming to the idea of two RC40's !!

One under the axle and one behind the petrol tank, chances are the biggest loss is the 90 degree at the front
Malc Gilliver

Go to the beginning of this thread.

I had a blown gasket. It wasn't long after I put on a new maniflow 3 branch. I assumed it was a crap gasket. So I bought another, supposedly better one.

The 'better' one began to leak and gas me too, until last week when I could stand it no more I bought yet ANOTHER gasket.

And behold, today, after fitting it yesterday, within 20.miles, I was being gassed again.

So off it comes again tomorrow. It can't just be gaskets. Something must be amiss with the maniflow manifold. My previous 3 branch never blew a gasket, and that was 30 or so years worth, before rust killed it.

I saw a post by Guy about welding his maniflow, and how he used his spare head.

Yep always worth using your head, even the spare.

I'll do the same tomorrow. I'm going to bolt up the inlet/exhaust and see what kind of fit it is.

anamnesis

I had a Maniflow LCB that had a leak on the centre port from new. It wasn’t quite flat.

I put some exhaust putty on the face, where it was leaking. It cured the leak and it also adhered to the manifold so well that I never had a problem with it again, despite removing the manifold dozens of times - it was my race car.
Dave O'Neill 2

The problem I’ve had especially the centre blowing has been down to the manifold flanges (inlet and Maniflow exhaust) not being the same thicknesses.

Alloy flanges often much thicker - I’ve brazed up the thick washers with a half washer packer attached to hopefully give equal clamping pressure.



richard b

As per Richard, both manifolds need to be the same thickness to get a decent clamping happening and yeah sounds like your silencer is a bit suspect.

On the exhaust fumes in the cab issue, I'd be having a good look around the firewall/bulkhead for holes, missing grommets, sealed up pedal box etc--if it's got a heater is the ducting in good nick--does your model/year have the rubber seal across the rear of the engine bay to seal the bonnet, I think only some do.
I hate exhaust fumes, once you get touchy to them it takes ages/years to get them out of your system. I got so bad at one stage that if I was out on the bike and a car went past I'd get real close to throwing up just on the passing smell of it. Had a mate that used to race a Dodge Viper and unknown to him with his helmet on couldn't hear the exhaust leaking and it nearly got him, Luckily he pitted and they pulled him out only semi conscious and off to hospital, hasn't raced since and can't bear the smell of exhausts even now, probably 7-8 years later. He was a really well known serious racer and did quite well and no softcock, it really stuffed him.
William Revit

Friend of mine didn't make it from exhaust gas. He fished a ten metre creel boat with his son.
One day he hurt himself. Not badly, but enough that he wasn't fit for deck work so he left Kenny (the son) to look after that and sat in the wheelhouse all day.
Sick as a dog after the first day and put it down to seasickness.
Passed out the second day. Kenny realised something was wrong, went in, stopped the boat and dragged his old man out on deck. Got on the VHF to the coastguard and started CPR.
Too late. The lifeboat got him ashore to the ambulance boys but he was DOA.
Leaky manifold gasket.
RIP Henry. Great bloke and not even 60 yet.
When I stripped the manifolds off Don's Triumph 6 there was clear evidence of exhaust leaks on the head. So I checked the manifold flanges with a straight edge. None were parallel to each other. The manifold is factory original so I tend to assume the casting has warped, maybe from overheating or maybe it wasn't correctly finished at the factory. Who knows? The warp is bad enough that it's scrap though, can't be corrected by machining - it would require too much metal removal.
Greybeard

Carbon monoxide poisoning kills off brain cells so even before you go unconscious, and recover, long term damage has already been done.

With the arrangement on an A series, being non cross flow, it is really difficult to check adequately around the exhaust manifold, especially underneath, behind the carbs. The first indication I got sounded like someone tapping with a spanner. Not at all what I might have put down to a chugging exhaust. A harsh metallic tap tap tap.
GuyW

Yep, irregular flange surface is just what I'm thinking too.

I already tried the weld on washer trick to even up thickness, to no avail unfortunately.

Tried a bit of holts exhaust paste, that didn't work either.

I'm going to try brass nuts, currently using steel. But used steel for nigh on 50 years and not blow gaskets.

Manifold not the silencer Willy. But yep backwash can be a problem. Going to look for how it gets into the cab too. But again, not been a problem for yonks before now, and nothing other than the manifold's been changed.

Interesting you say about sensitivity though. I was thinking exactly that. Have I become extra sensitive? But the stuff makes me feel unwell, not just preceived unwell. But sick, headache, as if 'drunk'. It's definitely leaking. Serious stuff as you all say.

And exactly Guy. I can't see the fit, esp' with the HIF inlet. The twins inlet manifold lets you see the inner flanges. I may have to resort to going back to my twins if I can't resolve this.

Have some lunch, then I'll strip it again and have a look.







anamnesis

This is a minefield! I bet many Spridget exhaust manifolds are slightly leaking. Maybe maniflow are worse? I have a standard manifold on a stock 1275 and check nuts fairly often. But this means I also have the exhaust joint just below and this is difficult (imv) to completely seal. I also have a gap or two into the footwells.

Got me worried now.....
Bill B

Bill, when I was using a standard exhaust manifold. To get the bell shaped joint to the pipe gas tight I would put a thin layer of sealer onto the cup shaped mating surface and use a jack under the bend of the pipe below, where it exits though the chassis opening. Then jack it up just slightly, to squeeze the joint together before fitting the clamp. This worked well for me. Just don't jack it up too much and strain the manifold !
GuyW

Guy
Thanks, I think (!) I may have done something like that so maybe it's OK. Maybe we should all carry CO detectors.
Bill B

RTV Copper based is good to 700c spikes. Good for below the cylinder head from what I read, but not at the cylinder head because the exhaust ports getter hotter.

As for mine, I stripped it down and bolted the manifolds up -- minus the gasket -- to my spare head. It doesn't look obvious where the problem is. I can't get a 2 thou feeler blade in.

But there's no doubting the smell, my stinging eyes, sickness, and headaches. Nor the noise for that matter. Each time I fit a new gasket, it's much quieter. This time it lasted about 20 miles on my way to Croydon. Then as it 'blew', I could hear the exhaust note change as well as start to feel unwell.

My best guess is the siamese port again.

I'll bolt it up with the gasket tomorrow, I'm wondering if the flange distorts when it's bolted up tight.

I examined the rest of the system. No signs of any leaks at all.









anamnesis

Centre branch of manifold not blocked ?
Tim C

Anamn, does your system have an exhaust pipe support bracket underneath at the back of the gearbox. Earlier cars had it. I think that the length of the vertical down pipe with effectively an integral manifold must put a lot of leverage on what is a narrow band where the bolts, all in a line, secure to the head. Add a bit of inertia reaction whenever you blip the throttle, plus perhaps soft rubber in the engine mounts and maybe it is opening up an exhaust leak pathway which you dont see when it is static, off the car.

My car has a head steady to stop the engine tilting. I wonder if that helps, though I hadn't considered this might help before.
GuyW

Yep, I do have one Guy. Although I didn't on my old 3 branch. But when I bought this new maniflow, I reasoned as you do and reinstated it.

More investigation yesterday revealed an intake leak as well.

My inlet manifold flanges are slightly concave. Both flanges I can get a 6thou feeler blade under the centres. The location rings help of course but clearly there's a leak. Maybe that's the source of my popping exhaust on the overrun. So I'm lapping them flat again with a file and some wet/dry paper.

And my exhaust flanges are the same. They ain't flat. Mostly the centre port. The gasket isn't being compressed enough in the centres. The gasses are being blown out through the gasket layers. That's why it's not obvious where the leak is. Split the gasket open where the layers had started to seperate. Plenty of signs of sooty exhaust gasses blowing right through the gasket layers and into the engine bay.






anamnesis

This is my last but one exhaust gasket. Supposedly heavy duty. It was mostly only being compressed properly at the stud locations, it was blowing internally through the layers.

Then I improved things by welding half washers to the flanges on the manifold. But the flange centres still weren't clamping the gasket fully enough, because the flanges are effectively concave. Hence another gasket has blown through.

So I need to make my flanges flat.

And, given I've had this problem since I bought it not that long ago, and having read through the archives and seeing I'm not the only one with this problem, I now conclude ----

------ MANIFLOW ARE EXPENSIVE ******CRAP******.

Have you seen the price of the f*cking things now?

AH Sprite & MG midget 1098-1275cc Exhaust manifold Maniflow part no LM031
£422.26 **PLUS** DELIVERY.

****£422.26.**** Jeez. Just for bent sodding tubes that aren't as good as they were 30/35 years ago.

https://www.ebay.co.uk/itm/204076289045

It's taking the piss, esp' since they can't even make the pissing flanges flat!!!!.

Almost everything these days is JUNK!!. And to think people used to complain about poor quality from our nationalised brit' car industry.

Well at least BLMC still made good spares, -- including decent exhaust gaskets.

Because even if it was clamped properly, the gaskets I've bought recently aren't a patch on the quality of what we used to buy.

Dave, anybody, do you have an old or new ORIGINAL bmc/unipart/bl exhaust-inlet manifold? I'd like to compare thickness and surface area dimensions.






anamnesis

This too.

The gasket is barely 8mm wide where it needs to be clamped. It's not even wide enough cover the area of flange that it could cover.





anamnesis

Add to that, some of the gasket overlaps the port on the manifold. The black sooty area is about 2mm at it's widest. It's sits uselessly inside the port doing bugger all. So all there is, is circa 6mm width of gasket being clamped, and even that isn't being clamped properly.

The gasket exhaust ports are bigger than the ports on the cylinder head (it's a 1300GT head). The exhaust ports on the manifold are even bigger.

The whole bloody lot is mismatched, and the manifold flanges aren't flat.

So now I think I know the problem(s). Here're my solutions.

1. With a bit of welding, I'm going to reduce the size of the manifold exhaust ports, to only just a bit larger than the head ports, to match the size of the gasket exhaust ports.

2. Grind/polish the flanges flat.

3. I might even try to make very slightly raised 'fire rings' (as per my red square) that will be pressed into the gasket when the manifold is clamped up. Easier if you could buy an exhaust gasket with a fire ring, like head gaskets are made.

4. Start praying. 🤣🤣🤣.






anamnesis

IIRC the manifold being larger than the exhaust port is intentional as the step effects how a pressure pulse coming back at the step behaves and how it effects what goes on in the combustion chamber. You'd have to check the bible AKA Vizard for more specifics.
David Billington

Thanks David, I'll have a read since I have the book.

If that's the case it makes sense, since the gasket is already slightly larger than the head exhaust port.

But I don't see why the manifold port should be even larger than that.

Just found this. Anybody used one?

https://www.minispares.com/gug704053mg-manifold-gasket-for-fuel-injected-cars


anamnesis

And this one too.

https://www.minispares.com/gug704063mg-turbo-manifold-gasket-with-exhaust-inserts


anamnesis

I’ve just found my old gaskets
They are called ‘big bore’ probably got from Moss years ago before they closed Bristol !

62mm wide. Top 14/15mm with a 9mm clamping witness marks for the manifold at the centre port.

Also a clearer pic of my new spare in my superheated roof spares dept !

Mine blew badly on the M5 on a long run to Devon - it was manifold thickness difference even though I thought I’d got it right. More work on the HIF manifold was required.

R.





richard b

Thanks Richhard.

That's just what I thought. They used to have a lot more meat on them.

How about thickness?

anamnesis

I’ll have to venture in the roof again !

MED list this one
https://www.med-engineering.co.uk/products/med-inlet-exhaust-gasket

Probably would not need the big ports version as it looks full race

Anybody used one ?
richard b

Ok thickness - this is from an old one but from a non compressed part (having removed the edge burrs from the punching process !)
1.2/1.3 mm

Port sizes :-
Inlet 39mm
Exhaust 33x 38mm
richard b

Hi Richard. Thanks for going back to the roof. 😉👍.

Thickness is about the same then.

That MED gasket looks like it may be ok. I'll ring them and ask about the width measurements.

I've found a few more on ebay offering 5 year guarantees. Either confidence in their offering, or stupidity.

https://www.ebay.co.uk/itm/155258490910



But I suppose to be fair, given the crap state of my exhaust manifold flanges, I doubt even a proper old stock gasket would last long.


anamnesis

Sorry exhaust should have read

33x28mm
richard b

Anam

I used to have some NOS Metro Turbo gaskets. I'll have a look, see if I've got any left. I may have ebayed them, though.

I think I also had some of those 'injection' gaskets.
Dave O'Neill 2

Thanks Dave.

I found the vizard stuff David. Yep you're right it is mentioned. A bit over my head but I get the idea of it.

Frankly though, I'm past worrying about a bit of hp or mpg. I'll settle for not being gassed as a decent swap.

What is interesting though reading that exhaust section, is the title I gave to this thread and my comment in the first post I made.

"There's a very noticeable increase in acceleration. Really, it's going like stink."

Each time my exhaust gasket blows on the centre port, I really do notice an increase in ooooomph. It picks up revs noticably more freely.

But as I say, I'd rather not be gassed than be able to accelerate a bit quicker. Lol.






anamnesis

Looking back at your earlier pic of the gasket the inlet manifold is well bedded into the gasket - the exhaust not.
That is similar to mine when it blew - exhaust not clamped as well with the manifold thicknesses being different.

Is your exhaust at all restricted ? That could slightly improve power to an extent if it leaks I guess.
richard b

Yep, that earlier picture is before I welded half washers to the exhaust flanges to improve exhaust clamping, not realising the flanges were concave.

I checked for any obstructions in the manifold, none there. Then just the straight tube to my single box, which I whacked the baffles throug. So I'd say not restrictive, but I wondered exactly that too Richard.

anamnesis

I suspect that my Manifold on the 1275 is leaking like that. It's pretty noisy. I was considering putting 2 gaskets in,but if the mating surfaces are concave I don't think that would improve things. The fire ring versions sound like the way to go.
GuyW

I can't find any Turbo gaskets, but do have the injection versions.


Dave O'Neill 2

I'm considering two gaskets too Guy. The problem though is as you say. Because the gaskets aren't thick enough to deform into the concave flange to seal it. There just isn't enough soft material in the gaskets to compress/deform.

Also, I remember in the past having to scrape the gaskets off the head and manifolds, such was the seal. These newer gaskets don't stick/seal at all.

Dave have you tried those injection versions yourself? Are they good?

And, supplementarily,

1, how many have you got?
2. Are they for sale?
3. How much?

Lol

I see the top and bottom port widths are pretty 'thin' though; compared to Richard's old stock.

But perhaps those fire rings obviate the need for full width coverage.






anamnesis

Just to point out I did say mine were classed as ‘big bore’ gaskets which are bigger than the original standard ones.

Seem to be available on e bay.
richard b

"Dave have you tried those injection versions yourself? Are they good?

And, supplementarily,

1, how many have you got?
2. Are they for sale?
3. How much?

Lol

I see the top and bottom port widths are pretty 'thin' though; compared to Richard's old stock.

But perhaps those fire rings obviate the need for full width coverage."



1. About a dozen, I think.
2. Everything's for sale ;o)
3. email me - daveo138 at yahoo co uk

They're original Rover parts from the '90s, when the Mini was still in production.

Rover obviously thought they were OK for the job.

I don't know if I ever fitted one myself. I think I probably ordered Metro Turbo gaskets, and these were supplied as an alternative.

If it was me, I think I would probably want to address the curvature of the flange rather than trying to find a workaround.
Dave O'Neill 2

<<Rover obviously thought they were OK for the job.>>

Except of course their job at the time wasn't to match up with a less than perfect version of a Maniflo !
GuyW

True Guy.

Thanks Dave.

Yep, I agree, I've got to sort out the flanges whatever gasket I use. Today I did a bit of welding and a bit of grinding. It's a whole lot flatter now, and now there's metal under all of the available gasket surface.

Flattened the flangrs on the inlet manifold too.

Tomorrow I'll bolt it up, again, and give it a try with yet another of the bog standard gaskets. Maybe with flat flanges it'll be ok.

If it blows again, I'll email you Dave.

I suppose I could wear a gas mask while driving. 😅.






anamnesis

Where's the surface plate and marking blue to check the flatness?
David Billington

Haven't got either. Have got a straight edge, light source, and feeler gauges. It was very obvious how concave the flanges were. Now they are definitely flat by comparison. The question is, will they stay flat, or will they warp with heat?



anamnesis

This thread was discussed between 29/04/2026 and 03/08/2026

MG Midget and Sprite Technical index

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