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MG MGB Technical - 3 bearing crankshaft cracks

Hi all,
I know the 3 bearing MGB cranks are good, but if they do crack/break is there a particular spot where this happens?
I want to crack test, using dye penetrant, a couple of cranks and it would help to know if there are any weak points. If a T-Type crank fails it is nearly always at the front web and sometimes at the back web. I am guessing it is probably the same with MGB cranks.
Has anyone out there got any information about this?
Bob Schapel
Bob Schapel

Hi Bob-yep, that's the only place I've seen them go--Had one break in a 5brg. right through the oil hole on the centre main bearing journal.
Just out of interest we broke quite a few on the old Jag. racer and it was always just in front of No6 bigend and the break was real carroty, not a clean break at all--decided it was a resonance/balance issue and made sure everything was balanced up, even to the extent of balancing the clutch plate and front gearbox shaft-still broke, so decided the front balancer wasn't up to the job(as in the 6cyl. Holden problem)----Had an old Traco 454Chev balancer, so modified that to fit the Jag. tapered fitting and bolted it up with the longest bolt that'd fit the front of the crank for a good solid grip of the thing(nicknamed the front flywheel by those who've seen it) and never had an issue since.
cheers
willy
William Revit

Thanks Willy,
It is the 3 bearing MGB crank in particular, which I am about to crack-test. Have you seen/heard of many of those crack or break? Sorry, I was not sure if your comments related to the XPAG cranks or 3 bearing MGB. It know it is a common problem in XPAGs.
Bob
Bob Schapel

3 bearing B ,Bob actually one was MGA the other MGB, both broke in that same spot on the front web.
and the 5 bearing B one broke right in the middle through the centre main oil hole.
An old mate engine reco fella here magnafluxes every crank he does and if they've been reground previously he really concentrates on the radiused area at the ends of each journal--he reckons he's seen plenty of cranks (not necesarrily MG) break from having the journal ground without a decently blended in radius there, specially high output Ford/Chev V8 things. Might be worth checking that area maybe to make sure it hasn't been ground into a sharp corner
You're doing well to have multiple 3bearing MGB cranks on hand, they're getting a bit hard to find here.
Cheers
willy
William Revit

Didn't have a problem with a 3 bearing crank, but the conrod was a bit of an issue!!!


Colin Parkinson

Wow, that is amazing ..... three pieces! I guess the message is to crack-test con-rods and then polish them to remove stress raisers.
Bob Schapel

What caused that Colin -?
William Revit

It was a few years ago, my MGA race car. Probably over reving on the rolling road, but it failed at a sedate 3000rpm while shaking it down on track. Cut the camshaft into 3 pieces as well!!
Colin Parkinson

Colin-It's crazy where/when they break
Had an MX5 Mazda one break right up near the gudgeon pin--Usually a sign of detonation,but i doubt that in this case, and the rest of the rod cut a great slice through the block and cut the end off the sump--at 2000rpm





William Revit

Wiliam- You are a trip!
Stephen Strange

Cheers Stephen--Merry Christmas
William Revit

Please refrain from calling the Torsional Vibration Damper a harmonic balancer, its no such thing. In my experience most four cylinder engines have a torsional resonant frequency of around 300 Hz which means the 4th harmonic of firing excites that frequency at around 4500 rpm. There aren't many cranks that can live with this if its undamped. Generally torsional failures are through the crankpin either initiating in the fillet radius or at the oil hole. With steel cranks you get the classic 45 deg failure.
Bending failures tend to be through the web, fillet to fillet. Sometimes this is due to just lack of static strength but can be caused by bending resonances and flywheel whirling. This resonance generally occurs at the top end of the rev range. You can help yourself by lightening the flywheel and clutch. In a well formed fillet radius we would expect the stress concentration factor to be about 3, but as said above if the radius isn't smooth it can be much higher. There are various fillet treatments to increase strength - Nitriding or for a much larger effect rolling and induction hardening.
Paul Hollingworth

Not so sure Paul-plenty of harmonic balancers on the market---As i'm sure you are aware the problem area for 6cylinder engines is around the 6200-6300 rpm mark and that's where the Jaguar was having issues-
The magic balancer for 6cyl Holden racers here used to be a monster mercury filled self balancing unit but impossible to get now, possibly because of the mercury content.
William Revit

Willy, its not possible to 'balance' the excitation coming from the engine (gas and inertia torques) only modify the systems response to it. The term tuned absorber with damping better describes how the TV damper works. It splits the resonant peaks into two smaller peaks by dint of the damping from the hysteresis in the rubber element. The cheapest and most widely used form of damper are what are called rubber in compression dampers. Certainly that's what's on B series.
In a four cylinder engine the major harmonics are 2,4,6 but often the minor orders 3.5 and 4.5 show on the torsiogarph test (a device for measuring the amplitude of torsional vibration at the front of the crankshaft)
Its some time since I worked on a straight six engine for one of my employers, so my recollection is a little hazy. With the crankshaft being longer the torsional natural frequency will be lower say 200 Hz, so the major harmonic of firing the third order will pick this out at 4000 rpm, the sixth at 2000 rpm. Its also possible that the minor 1.5 order will cause problems at the top end of the rev range.
Industrial diesel engines use untuned torsional dampers which have a impellor floating in heavy oil, typically made by firms like Holset. They are generally too expensive for automotive use.

https://rimmerbros.com/Item--i-LHG000070L
Paul Hollingworth

The thing is though---we call them balancers here.
If i went in and asked for a torsional rotation damper no-one would know what i meant---It's just we've been a bit Americanised here
Several Ford/Chev engines, that I'm used to working on here use the flywheel and front pulley to help balance the crank, one of them being the Ford small block injected engine runs 50oz out of balance both ends with a bare crank and uses 50oz flywheel and front balancer/damper to get the balance correct---There are traps with this as well as the earlier Ford ran at 25oz and parts will swap over with disasterous results.
I agree with what you're saying , it's just that I've always called them balancers-

The Callies racing damper(just for you) has the balance weight built into the undamped section of the hub and then the rubber mounted ring around the outside--nice balancer as you'd expect
BUT there's some to stay away from like this summit one which has the balancer section as a part of the outer ring--I'd hate to be the rubber bit with a heap of revs on

Good one------

https://eclassics.com/eclassics-1969-ford-mustang-crankshaft-damper-3-bolt-hub-302/?gad_source=1&gclid=CjwKCAiA-bmsBhAGEiwAoaQNmuYPfLVfC2YweJNo6_wqO85CH8FseQ5R2bD4eZALQ33yyfAsDTqnQBoCdO4QAvD_BwE

Other one---

https://www.summitracing.com/parts/pbb-pb1084ss
William Revit

Willy, I'm sorry I got so uptight but I hate Americanisms especially when they are not accurate. I regret that Australasia is becoming so Americanised after all its a former colony and the King is head of State (for now). Certainly lots of Aussies and Kiwi's working over here. (often in bars)
Yes, it is common to balance the crank (half reciprocating and whole rotating mass) externally in the front pulley and flywheel. Often Used on inline 3 & 5 cylinders and V6 & V8. Resorted to when the capacity of the engine has been stretched and there isn't enough space in the crankcase to get all the weight on the crank. It does mean that the crank system assembly has to be balanced together. Generally this is not done on mass produced engines in Europe. The TV damper and the Flywheel/Clutch are balanced by the supplier before assembly. It only needs a small clearance on the spigots to generate a large out of balance as these parts are quite heavy. I recall that a typical tolerance for parts was 0.5 oz-in (cant be bothered to work out what that is in SI units)
Here is the B series part :-
https://rimmerbros.com/Item--i-12H963
Paul Hollingworth

Wow, Willy and Paul, that is all too complex for me to understand, but I do have a question to ask.

If an engine has a supercharger fitted and driven by V belts from the front of the crankshaft, does that have any of the same effect as a damper on the front of the crank? I know it would be unlikely to be "tuned" to the engine but should it help at all?

The plot might thicken with toothed belts running at engine speed which could be "timed" ... but ignore that scenario for now.

Happy New Year

Bob Schapel
Bob Schapel

Welcome back troublemaker--look what you've started-lol

Don't get me wrong, I enjoy chatting with Paul, he's full of good experienced knowledge

I have a correction to make as I mentioned the ford crank as being 50oz out--incorrect, As I was talking to Paul mostly I knew he would know what I meant, but, the correct value is 50"oz/in" as Paul has quoted so in fact the 50oz gets divided by the number of inches it is away from the centreline of the crank to roughly explain that---

On your supercharger question, If the drive pulley is mounted solid on the front of the crank then yes you'd think there'd be a constant load on the front of the crank which could only be a good thing to balance out the flywheel end that normally takes all the power being produced
In the case of v belts driving off the outside of a damper i'm not so sure, there would be a constant load still but damped out by the damper a bit but hopefully would also help with crankshaft harmonics-
Hope so anyway ,that's how i'm driving mine-

An interesting observation with our old chev v8 which broke the occasional crank or three was that if there was an issue with the front damper the crank would break at the rear but if it was flywheel or clutch related it'd break up the front AND if you crashed and came to a sudden stop you could just about guarantee the crank would break near the back during the next race meeting or two

The ford cranks with the 50 oz balance tend to break the crank just behind the number 1/5 bigend journal if they're constantly driven high in the revrange, and if you measure from the centreline of the balancer(sorry, damper) to the centre of the front main bearing the break in the crank will be the same distance back from there---Although the crank is balanced with it's counterweighted damper, you can't get away from the fact that if you're revving the nuts out of one of these that counterweight has to be causing some sort of weird strain on the crank compared to a normal zero balanced crank
Stupid thing is if you buy a good crank like a Callies it's internally balnaced with zero balanced flywheel etc----Why couldn't ford do that in the first place

Happy New Year guys--that's all from me this year.
I'm about to knock the top off one--CHEERS
William Revit

Bob and Willy,
I don't think you would want to drive a supercharger from the ring of the TV damper. The amplitude of torsional vibration there is typically 5 times that of the hub (inner bit). I would recommend driving from the hub. The small increase in moment of inertia of the hub pulley will lower the torsional natural frequency of the crank a little so may require the damper to be re-tuned.
I've argued many times with my employers over the years that we should adopt the inside out damper design like Honda where the seismic inertia is inside the poly-vee pulley, thus subjecting the Front End Ancillary Drives to lower torsionals. To get enough inertia at this smaller radius the mass has to be higher of course. I've designed several like this but never got any of them to take it up.
Failures due to torsional vibration are normally in the last crankpin. This is where the relative twist is the biggest. We call the torsional mode shape the elastic line and that end of the crank the gradient is steepest due to the very large inertia of the flywheel and clutch (order of magnitude bigger than any other inertia in the system)
Paul Hollingworth

Paul- At the risk of moving Bob's thread even further off track-
years ago in the very competitive speedway sedan racing here we were forever trying to go faster and faster to stay in front, we'd broken the odd crank or two and I was talking to a fella in the pits one day about it, he seemed interested and he turned out to be a top drag car racer from the US, and recomended alloy conrods, we were using Carillo forged steel rods at the time and he reckoned because of the comp.ratio and the ign timing/revs we were running the steel rods were too stiff for the combination and were putting too much shock through the crank, So a phone call to Bill Miller and he agreed and made a set of custom rods for us--and only half the price of the Carillos---I was a bit sceptical, but as it turned out, the engine did a whole season without an issue and it was fast--The only problem we had was that it lost a couple of important races because the fuel pump belt went for a walk
So with the last meeting of the year and being big prizemoney this had to be fixed-
Noticed that the sprintcar fellas were running their pump direct off the front of the camshaft-no belt-hmmmm--------let's do that
So new timing cover and mounted the pump to the camshaft and the supplied water pump then bolted up direct to the crank---very tidy setup BUT no front pulley at all
Was a bit concerned about that but the sprinters were the same--they had no flywheel though
Our flywheel,ringgear/clutch assembly weighed just on 8lb so because it was so light I thought it would probably be ok ---big mistake.
Off to liverpool with the new killer setup--went like a rocket ,qualified right up the pointy end and all set for the final----Off they went
Full hook down the front straight and it's the BIGGEST engine failure ever, broke the crank just in front of the rear bigend crankpin and took the two rear conrods and the piece of crank straight out the back of the block, broke the bellhousing to pieces and spat the gearbox out under the lh rear wheel--very impressive
Unbelievably the engine was still sitting there running on the remaining 6 cylinders
When we pulled it apart the crank, a forged steel one had just shattered, the end where it broke looked crystalized like sawdust
Lesson learnt--If you've got a flywheel you need a front damper
We ended up with the same setup but with a front damper between the engine and the waterpump
The modern sprintcar setup now is a damper and electric waterpump---wish electric pumps had been about back when---------

Useless rambling but there you go-
willy
William Revit

This thread was discussed between 17/11/2023 and 31/12/2023

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