Welcome to our resource for MG Car Information.



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 MGA - Twin cam con rods

Can anyone tell me what the diameter of the Twin Cam gudgeon (wrist) pin is please?

And might anyone have a set of twin cam rods they dont need.

Many thanks

Colin Parkinson


Someone might have a set of rods but they won't be selling them
William Revit

One of the guys in Melb restoring a twin cam used after market H pattern rods (and new pistons). I would imagine a lot better than original and undetectable!
Mike Ellsmore

Colin , I thought you were running a 5 bearing engine--?
William Revit

Willie, different engine! 1622 MGA rally engine, bored to 1800 using a Sherpa block.

Colin Parkinson

All good- just had a little thought that you might have been thinking they would fit a 5 bearing crank
Cheers Willy
William Revit

Get a set of these -----or maybe someone in the UK makes something but Pauter is a well respected name amongst the racers in the US

Be aware though that this type of rod have floating gudgeon pins so will need pistons with circlip grooves or gudgeon buttons
Same with T/C rods

By the way, just for your info T/C rods are quite heavy, a std one is around 890 grams and you can't get a real lot off them either, The set I have here are highly modified, lightened as much as possible polished and shot peened and still weigh 826grms

There's always the dreaded Maxpeeding rods on ebay--A few of the MGB racers here use them without an issue
they're resonably priced copy of a Carillo with ARP hardware, there's various stories about regarding the quality of the steel but anything new would be better than a std MGA rod---?

Both these brands and Carillo weigh in at around 600
William Revit

Hi Willie, yes I know they are floating pins. I have a new old set of flat top Powermax pistons I was trying to use up! That is why I am looking for a rod that isnt pinch bolt.
Maxspedding rods? I have a set for 5 bearing. They look really good, and value for momey, but I havnt had the balls to fit them into an engine yet!!

Colin Parkinson

Colin, you might just be the man to answer a question
The maxpeeding rods are advertised as having ARP 2000 bolts, which would make up a fair chunk of the asking price of the rods
my question is, do they look like genuine ARP bolts
They should have ARP stamped into the head and have the dimple in the middle of the head for the ARP tool ---- Any chance you could have a look please-
I expect they are , they'd be game advertising them as ARP if they aren't
William Revit

Willie, I cannot find these rods at the moment, so I assume I must have put them in an engine after all!!

They were a very early set, and I am sure they were marked ARP, but cannot prove that now.

Take care

Colin Parkinson

Yes bolts are ARP see image


M Dollimore

This is a pic of a Maxpeeding rod purchased last week in Melb. Are the ARP markings correct?

Mike Ellsmore

Mark , Thanks for that , yep they're ARP

Mike,Fairly sure your bolts would have to be ARP
These are the bolts that sparked my question
They must be a run of bolts from ARP specially done for maxpeeding-They would be in court by now otherwise I'd imagine
They've got the dimple for the ARP tool and stamped clearly as ARP, you'd have to be a brave man to do that and sell them as ARP if they weren't
I just pulled one of my young fellas Carillos out for a look but he's gone for Carillo bolts, which are etched instead of stamped and the centre dibot is smaller so I'll just put that away again
Sorry if I put any doubt in your mind Mike, I'm sure it has to all be legit.
William Revit

As someone who used to carry out stress and fatigue analysis on con-rods, I have never understood the logic of the H section. Theres a rule of thumb based on Euler strut theory that says that the second moment of area of the shank section in the trust axis should be 4 times that in the crank axis. To achieve this you need a I section.
I really dont like the stress raiser (or lack of radius) in the picture in the machining under the head of the screw)
Paul Hollingworth

It's a really interesting subject H beam versus I beam
Companies like Manley produce H beams for most uses and an I beam as their turbo rods
and then Carillo have I beams as their sports rods and all their good rods are H beams
Then there's Pawter with their X beams that seem unbreakable
I know for sure std I beams don't like a lot of boost whereas the Carillo H's will hack whatever you throw at them and they're quite lightly built up around the top half of the beam
Sometimes i wonder if I beams are too stiff up the top and can't soak up the shock of high boosts and a possible bit of random detonation
MX5 rods can't that's for sure whereas kitted out with Carillo H beams they're bullet proof
I'm a big fan of Bill Miller rods, have used them for years but there's not enough room in a B series block for them

Here's a sample of MX5 rod failure--I've got more

William Revit

Willie, thanks for the tip on Pauter.

I think their MGA rod will do the job. Just trying to get a sensible price from them!

Oh, I can beat your MX5 rod. I'll put a pic on tomorrow. It is an MGA rod though!

Colin Parkinson

Yep, just check up on the gudgeon pin size though
i notice in his list that he's got MGA pins listed larger than 3brg MGB---Might be a misprint but would pay to double check- and the floating gudgeon situation
They quite often have specials, but mostly chev
If you wanted a custom gudgeon pin size, you can order one-offs, it just takes a little longer
Brian is the boss-

Brian Pauter
Mario Tavares
Don Pauter
William Revit

As someone who has carried out computer optimisation of con-rods with those engines are still being in production, I have to say the Mazda design looks poor - very heavy big end and too abrupt transition at the base of the shank to the big end eye. That doest explain why its failed at the top of the shank though. In simple terms that's a load/cross sectional area calculation. It looks more like a overload failure rather than fatigue. Was the failure initiated by a surface blemish or an internal defect? had the gudgeon pin seized ?
The weakness of most mass production rods is the 'as forged' surface finish. For high speed use I would always recommend shot peening. Is the big end of the Mazda rod fracture split ? The failure gives the impression that the material isn't very ductile.
The rule of Ixx being 4 times Iyy (i.e. I section) comes from the end conditions in the strut calculation. The ends can be regarded as pinned in the thrust axis but are (nearly) fixed in the crank axis.
There is another consideration in high speed engines which is called inertia bending. This is the force generated from its own mass when the rod swings from one side to another. A second reason for the I section.
Paul Hollingworth

I'd say the Mazda rods are quite ductile, they usually get quite a curl in them before they actually break, you can pick it when they're idling a bit rougher/uneven than normal
No fracture splitting-machined
The bigend appears large because the rod is quite small and short but has a decent sized crank journal to match to
There are definately not any lube or seizure problems, it's just a load thing and possibly a bit of unavoidable detonation shock at times
The naturally aspirated MX5 don't have an issue but even a std turbo car which runs the same rods and only 5psi will have rod trouble eventually in it's life and on top of that the more boost you screw in the life is proportionally shorter
It's a disaster when one lets go and it's always up near the pin, so it wrecks the block ,cuts the windage tray and sump in half and basically makes an only too common expensive mess
We've tried everything with them ,polished beams ,shot peening --basically they're just not up to it and miles too short anyway at 133mm, we're on the max available to fit in now at 140mm H beams and haven't had an issue for 2 years with monster boost

William Revit

133mm rod x 85mm stroke =str/rod ratio = 1.565
way to short--?

Now a more acceptable ratio-would be nice to go longer but it's impossible to get any more into the piston
140mm rod x 85mm stroke =str/rod ratio = 1.647
William Revit

Willy- we usually talk about L/R ratio. Your Mazda figures give a L/R of 3.13. If we compare that with 1.8 litre K series we see it has a L/R of 2.98 because the block height was designed for a 1.4 litre engine originally. With a blank sheet of paper you would ideally go for > 3.3 to keep the piston side forces down.
Paul Hollingworth

Wasn't sure which ratio you would use in the UK
I spend most of my time talking to my US mates and they use S/R ratio as a std.---but you know what I mean--- We're stuck with what we've got now ,it's the longest possible rod that can be fitted, the stack height of the rings is at a minimum and the gudgeon pin is up to the top of the oil ring, nowhere else to go---------but it's working fine like this with monster boost and-----H beams
William Revit

The K 1.8 had a state of the art piston designed and supplied by AE in Bradford. They had made some advances in cast aluminium materials. The K piston although still 3 ring had a very low crown height and weight. We needed that to get a sensible L/R and to keep the 2nd order shaking force down as the engine had a reitively long stroke at 89 mm and 80 mm bore (spookily close to B series).The piston also proved to be very durable being able to survive a 800 hr durability test even in VVC guise. I wasnt there when the turbo was introduced but I think it was much the same but may have had a floating gudgeon pin rather that fixed (pressed)
Paul Hollingworth

This is getting off subject a bit, but ,The 4032 silicone aluminium AE chose for some of their cast and forged pistons was to try and reduce the thermal expansion rate of the pistons so they could sell forged pistons that didn't rattle cold.It was also a good alloy for casting and makes a fairly strong cast piston possible- For most road/sports drivers this is fine but 2618 alloy will always be stronger and a better choice for competition engines
William Revit

Willie here is one I prepared earlier!

The cam was in 3 pieces, and on removing the sump, a piece of block about 8ins x 8ins fell off.

I think we can say that was terminal!

Little end was ok, rod failed underneath the little end.

So back on topic, that is why I am trying to find a stronger rod! I think the Pauter may do it.


Colin Parkinson

That's broken Colin------
You won't have an issue like that with Pauter rods
William Revit

Colin, are you going to be using new pistons? If so you can just have them made with a pin size suited to whatever rod you are going to use.

For Twin Cam block you need offset 3 main style rods and aside from the Twin Cam, none of them were full floating pins. I imagine that you could run a set of original 3 main MGB rods and get Twin Cam pistons with the slightly smaller pin diameter used for those.

The fact that they were clamped rather than full floating wouldn't likely be an issue, nor should the size - I run press fit MGB rods with the slightly smaller pin diameter in my 1950 cc race engine.

Something like the Pauter, made custom for the correct 3 main offset plus the right full floating pin size would be ideal of course.

Bill Spohn

Bill I have an unused set of flat top Powermax pistons, fully floating pins with circlips. Pin size is 0.825.
Pauter do a rod that will do the job.

Should make the bottom end pretty bomb proof even with a 3 bearing crank?

Colin Parkinson

Willie, have you actually had any dealings with Pauter.
I have eventually had drawings and decided that there is a rod that will do what I want.
We have had 3 attempts on getting them to issue a pro forma that is correct, and it is still not right!
They must be very busy, or do not want my business.

I am not having a warm feeling about this.

Colin Parkinson

Not myself, Colin,no but my son did and had a little bother getting his order sorted ,ended up ringing them up (middle of the night here) and got sorted straight away without having to fill out any paperwork, just explained what he wanted, the guy on the phone understood exactly what was wanted and they arrived 6 days later
If you're going to call them have your measurements spot on (gudgeon pin size) All you'dhave to say would be MGA rod with floating gudgeon pin, bush size to suit your pins

Your-.825" is a bit of a weird size, have you got 21mm pins------------

13/16" = .8125"
21mm = 0.826772"
William Revit

Willie, thanks for that. I am sure they will not be metric. The pistons are Powermax for an MGB with circlips. I had better check exactly what they are proposing to supply. Colin
Colin Parkinson

Well after lots of pratting about the Pauter rods have been delivered.

Look good! and certainly not cheap!!


Colin Parkinson


With those conrods not being angle cut at the big end will they fit down the cylinder bore?

M F Anderson

Mick they are made to fit down a 1600. But mine is bored to 1820 so absolutely no issues there!
Colin Parkinson

Colin--you've done it
Considering what's going on in the world, a month isn't too bad, maybe frustrating a bit, but just look at them rods--bulletproof. You'll be happy with them-
William Revit

Willy, thanks for the tip. Very pleased with them. Colin
Colin Parkinson

This thread was discussed between 26/05/2020 and 30/07/2020

MG MGA index

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