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MG MGB Technical - Do 3-bearing crankshafts break?

Just curious - and a little anxiuos! I keep reading about MGA people breaking and changing their crankshafts, and someone has even made new ones, so there must be a market!

I have never heard about a broken 3-bearing MGB crankshaft, though it has no doubt happened. Are MGB cranks stronger than MGA cranks, or am I just not well informed?

Tore

Tore

Crankshafts fail fromm fatigue, so the older they are, the more likely to be broken. They are always in the process of breaking. MGA were geared lower, so they have more total turns on them than a B; they also have been running at higher RPM so may have been running in critical vibration harmonic ranges where the lifetime may be in minutes rather than years. Very heavy flywheels cause the critical periods to be lower down, and lighter flywheels move it higher in the range. A flywheels are more popnderous than B ones generally. I forget when it started, but there are also harmonic dampers fitted to Bs, rather than plain pulleys. The first 3 main engines didn't have them, later ones may have, Most 5 mains do. This is a good mod for any car with enough room, and improves crank life greatly.
FRM
http://www.usachoice.net/gofanu
FR Millmore

Bill Blydenstein used to race a 3 bearing crank MGB because the lower internal friction is worth about 5 HP . He always said they were quite tough enough. be interested to see responses to this thread .
S Best

I had one break while travelling on the motorway many years ago. Made a hell of a noise...
Investigation showed the breakage caused by a previous regrind had not radiused the machined surface as it meets the web....
Had a replacement crank fitted, suitably balanced, was one of the sweetest B's Ive driven....revved better than any 5 main, made good power with very few mods and standard SU's....Car is still in regular use with its new owner.....

Chris Cooper

"If you push something hard enough, it WILL fall over".

Sure, anything can break. With my three main, plenty of oil changes and take my sweet time while driving.

A little TLC goes a long way with things mechanical.
glg

Ive got a 3 main lower end in my shed, email if anyone is interested, I am interested in it leaving, and I don't have a boat that needs an anchor.
Jake

I always understood that the reason they went to a five main is that the 3 mains had splitting issues. Right or not, I grabbed a spare some time ago figuring when I needed it, it would be twice as expensive if even available. Like glg, I tend to baby it a little--never over 4500 rpm and usually shift at 3500 to 3800 rpm.
Paul Hanley

I have driven my 3 main bearing crank MGB for 33 years and covered 140,000 miles with out any crankshaft problems. I also entered the car in many car club competition events with some success.

I have always limited my RPM to 5 and the occasional 5.5, because I was told when I bought the car in 1968 that there was a possibility of crank problems if the engine was over revved.

Of course I aquired a spare, that way Murphy's Law doesn't apply.

cheers
Ian Buckley

Hmmm. I usually limit my RPM to 4.000, but who knows what the PO did? Guess I will have to start looking for a spare...

How do I see if I have a harmonic damper on my engine? Is it very obvious?

Tore
Tore

Gentelmen - Having driven a 3 main bearing TD for years, High RPM (within limits) is not going to hurt the crankshaft. What will do them in is lugging them and causing them to flex under load. I never let the TD drop below 2000 RPM and shift at 4000 - 4500 RPM. Since the TD has a 5.125:1 rear axle ratio, when I drive on the freeway (often), I consistantly run at 4200 RPM and on long trips, do so for hours on end (harder on the nerves than the crankshaft. In the past 30 years of driving the car, I have never had a cracked crankshaft.

Now the "within limits" on the RPMs qualification from above, the primary reason to not exceede 5000 - 5500 RPM on the 3 main bearing engines is the split little end on the con rods. This was always the Archillie's heal of the early engines. The reversing of direction at the end of each stroke of the piston puts a great deal of stress on the gudgeon pin pinch bolt used on this split little end arrangement. Even worse is down shifting to slow the car, which developes a great deal of vacuum in each cylinder and again subjects the gudgeon pin pinch bolt to termendous amounts of stress (A local retired race car mechanic told me of a driver with a small block Chevy engine dropping all the push rods out of the tappets by doing just that and developing sufficient vacuum to suck the valves open, dropping the push rods).

On any engine, wear is considered to get into the excessive range when the linier piston speed goes above 2500 ft/min. This figure is reached in the B series engines at around 4300 RPM (I can't remember the exact figure and am too lazy at the moment to run the figures). This being the case, one can run the MGB engines, be they 3 main bearing or 5 main bearing engines at 4300 RPM or under and not have to worry about hurting the engine.

I believe that like the engine Chris Cooper talks about, most crank problems in the 3 amin bearing engines was due more to sloopy machine work than to excessive RPMs (at least in the hands of a driver that knew what he was doing). I am sure that the cranks will eventually fatigue if run long enough, but I am still waiting for that to happen on our TD with over 200,000 miles on it.

FRM - none of the 3 main bearing engines had a harmonic blaancer on it unless someone retrofitted their own, so none were protected by that.
Cheers - Dave
David DuBois

Thats a good point about down shifting , I was taught to drive by someone who had driven drum braked lorries , brakes were a finite resource , you had to use the gears . I have consciusly changed my driving style now and use the brakes to slow down in any of my cars ( all have disc fronts. )although I do like to keep in an appropritate gear rather than selecting first when I have stopped .
BTW,
With a 3.5 inch stroke the 4300 rpm limit is close enough .
S Best

Tore-
The thre-main-bearing cranksgaft of the 1800cc engines were redesigned with larger bearings in order to enable them to absorb greater stress, but the crankshaft still had a tendency to "whip". Warranty claims for broken crankshafts forced the redesign to the five-main-bearing version. The five-main-bearing crankshaft is heavier than the earlier three-main-bearing version (32 lbs vs 25 lbs), so the three-main-bearing crankshaft revs quicker. David is right about the obliquely-split connecting rods being sensitive to high engine speeds.
Steve S.

While most of the preceding comments are mostly true, the real problem is specific harmonic vibration periods. I don't know what they are for the various B series engines, but for example, David Vizard states that for A-series tranverse engines with "typical"flywheerl clutch they are at 5750 and 6100. Lightening the flywheel moves them up range. The B series engines could be expected to have these at lower speeds. If you can get through these ranges, and don't get to higher harmonics, you can run forever, at least from the crank's viewpoint. Other stuff may still self-destruct.
Dave, I indicated that at least most 3 mains didn't have dampers, but fitting a 5 main one if you have room, should favorably affect crank life. The damper screws up the harmonic vibrations, preventing the catastrophic resonance.
FRM
http://www.usachoice.net/gofanu
FR Millmore

I have an MGA 1500 3 main. You guys are scaring me :,)

Ralph
Ralph

Just for the record, both of my 18G engines have factory harmonic balancers.
Paul Hanley

Had my TD for 30 years and while I have had the crank inspected during an engine rebuild, it has not had a problem. I also do a good amount of freeway driving and did change to a 4.3 MGA rear end to lower the rpm's.

The comment on downshifting reminded me of what my uncle once said. My uncle build sprint car and indy engines and when I asked him one day to teach me about downshifting and shifting without using the clutch like the race drivers do he stated.

This practice depends on the depth of your pocket book, brakes are a lot less expensive than engines or transmissions.
BEC Cunha

Adding to the record Phil refers to, my 18GAUH1411 engine had a harmonic balancer from new.

Interesting thread, especially about the small end weakness and changing down at high RPM.

I believe the real way to go, is a set of twin cam conrods which fit the 3 bearing and have a floating gudgeon.
Ian Buckley

Ahhh!

glg is familiar with Fudd's First Law of Opposition.

But can you tell us how we can best illustrate the stubborn consistancy of this eternal principle?
MMD

Understanding why a harmonic balancer is a good idea requires an explanation. Each time a cylinder fires, enough force is applied through the connecting rod to the crankpin to cause the crankshaft to not only turn, but to also twist the crankpin out of alignment with the others. Because steel is elastic in nature, this torsional twisting causes a subsequent rebound of the crankpin back into the opposite direction and into its original alignment with the other throws of the crankshaft. This creates what is known as torsional vibration. When the torsional vibrations of the multiple throws of the crankshaft combine, the result is referred to as harmonic vibration. At certain frequencies, harmonic vibration is damaging to the crankshaft, inducing metal fatigue and subsequent fracturing. Thus the actual purpose of a harmonic balancer is to dampen harmonic vibration, not to dynamically balance an engine. Replacement or refurbishment of your tired old harmonic balancer is highly advisable as it reduces torsional stress on both the crankshaft and the camshaft, as well as reducing wear of the camshaft drive chain, coolant pump, and alternator due to reduced oscillating stress loadings. Your Original Equipment harmonic balancer can be economically rebuilt to as-new specifications by a specialist (Damper Dudes, 6180 Parallel Drive, Anderson, CA 96007 (800) 413-2673). However, Advanced Performance Technologies' stainless steel version (APT Part # 18CSP-2) is even better as it has the additional advantage of having provision for easy removal.
Steve S.

Excellent Steve. You will note that I referred to this part as "harmonic Damper". Some cars, not MG, actually have balance weights built into the damper.
To answer Torr's question, if you have a plain pulley at the front you have no damper. The damper has a heavy ring as part of the pulley; this is attached to the hub by a bonded rubber ring. It is the oscillation of the ring on the rubber that kills the vibrations, because they get out of phase with the resonance.
The dampers are pretty bulletproof, but the ultimate evil is to put them in solvent for cleaning and leave them, it eats the rubber. A separated damper is a major disaster.
Ralph, don't worry; the cranks are pretty good as long as you don't run in a critical range a lot. Race cars may do so, and that is why it's a problem, but fitting the damper can never hurt.
Steve, thanks for the rebuild info, been looking for that for a while.
FRM
http://www.usachoice.net/gofanu
FR Millmore

Steve - Thanks for the new address for the Damper Dudes. I knew that they had moved, but didn't have their new address. Cheers - Dave
David DuBois

FR & David-
Glad to help.
Steve S.

Did 3 main problems force the introduction of the 5 main? I don't think so.

The 3 main had only been going in the MGB for a 3 years when the 5 main was introduced, with development time surely this was not long enough for crank problems to show up in customer cars.

After the MGB was launched the 1800 engine went into new saloons in the BMC range and it was strengthening the engine for towing and general misue in these cars that brought about the introduction of the 5 main. It was nothing to do with low volume MGB production.

Twin Cam conrods can fit later pistons in a 3 main engine, but these are short skirts pistons and can wear the block quicker than the original long skirt pistons. Also the rings are closer together on later pistons and piston slap is more likely when the engine bore wears.

Every factory racing 1800 MGB had a 3 main bearing engine, even after the 5 main was introduced, the Abingdon Special Tuning Department preferred the 3 main for racing over the 5 main!

Ralph, Essex, Ontario, Canada, don't be frightened, there is a good case for a 3 main, it drives better!
John Prewer

The factory liked Twin-cam conrods for their race and rally cars with the three-main, but engine bore wear wasn't an issue for them.
Wade Keene

3 main bearing engines were not favoured in the early to mid sixties. Vauxhall and Ford in particular had serious problems with bottom ends due to crankshaft whip which caused rapid wear of the main bearings. Ford therefore introduced the 5 bearing Kent engine which was virtually then bullet proof. Vauxhall introduced what they called a "flexible flywheel" wich was an inbuilt harmonic damper which was at least a stopgap until a new five bearing OHC engine appeared a couple of years later.

Five bearing technology had also become a major selling point and other manufacturers were soon forced down that line.
Iain MacKintosh

This thread was discussed between 03/06/2004 and 07/06/2004

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