Piston broken in half(top-bottom) 5

[dirk489]

I reconditioned a friend's engine which broke later on during running in.The piston broke just on the pin line,conrod bolts snapped clean off, smashed the whole block.
My question is: is this always due to overreving or is there another possibility?
Dirk/Antwerp

 

[flavioct]

Apart from what is written above, we started from the assumption that the engine has been rebuilt with new pieces and according to the usual techniques for this engine that for certain verses had one serious problem.
Everyboby has a MGATWIN CAM must know MGAguru.com and what is written in his site:

Apparently the single most important lesson to learn about the MGA Twin Cam engine was not figured out by the factory until after this model had ceased production. The engine had been plagued by burned pistons, quite mysterious at the time. The factory first retarded ignition timing, then lowered the compression ratio (both reducing power output). They finally gave up on production when the engine could not be made to run reliably.
Sometime later avid driving enthusiasts were to discover the cause of the problem to be a resonant vibration of the SU carburetors at certain select engine speeds. Such vibration would cause foaming of the fuel in the float chamber, which in turn causes a lean running condition, which in turn leads to low power output and burned pistons. The problem may be overtly obvious if the car is run on a rolling road with power monitored as the engine speed increases, and there may be a sudden unexpected dip in the power output right in middle of the sweet spot on the power curve. Using some modern technology, an O2 sensor in the exhaust may also disclose the lean running condition.
The solution then is to install vibration isolating flex mounts between the intake manifold and the carburetors. Once this was resolved, Twin Cam enthusiasts have enjoyed many years (even decades) of high speed motoring enjoyment with a generally reliable engine. If this problem could have been known by the factory it might have been fixed during production, and the MGA Twin Cam model may have survived for a much longer production life. The Twin Cam engine might also have followed on to subsequent MG models. Can you say MGB Twin Cam without a flutter? There was at one time a Twin Cam engine MGB prototype. These pages then are dedicated to the trials, tribulations, and final solutions to the Twin Cam carburetor vibration problem.

 

[Tmoose]

"Has anybody here ever encountered an out-of-round crank pin?"

Yes, but generally from wear presumably from absolutely dismal maintenance. Many manufacturers permitted a wear limit of 0.001" out of round.

Crank grinding errors generally were not ovality, or oversized journal diameters, but taper, crappy fillets, heat "checks" (cracks !) from loaded up grinding wheels and bad technique, excessive journal width (rod side clearance or crank end play).

The bearing manufacturers spec the heck out of journal ovality and rod big end ovality, and warn specifically against combining components each with maximum ovality.
Probably for fear of creating some of reduced or overpowered hydrodynamic effect.

 

[tbuelna]

The big end bore of conrods are usually honed round/cylindrical. But the bearing shells often have a tapered wall thickness to improve the operating hydrodynamic oil film properties. The crank pins on high-performance engines are often ground with a slight barrel profile to reduce the effect of edge loading in the bearings due to pin bending.

 

[dirk489]

To :gruntguru: the rebuild with Arias pistons is what I did recently.
The failure history of the Original engines is interesting:
All the engines I took apart (about 10) with the Original H/C pistons showed seizure marks on the sides.
When it became apparent at the factory,after the American G.I.'s broke their engines on the autobahns in Germany, they simply gave more clearance in the bores,causing another problem;oil consumption.It was only when they used the later, L/C piston with GREATER ovality that the problem was solved. All this happened in the period that Healey took over the factory and I am certain that the Healey guys did their best to torpede the MGA twin cam.
I had several Original H/C pistons reground to the later spec ovality with perfect results.The set in the broken engine was one of them.
A second problem were the heavy conrods. In the Le Mans races several engines succombed to rod bolt breakages.
A third problem was and still is that the distributor drive gears are positioned on a half speed shaft replacing the original camshaft in the block.This half speed shaft needs to be shimmed perfectly to avoid axial movement of the shaft. I came to the conclusion that this movement is the cause of sometimes very erratic ignition,hence detonation problems.On my cars I shim to a practically nil clearance.The ignition point will move only a few degrees on the stroboscope.
The race engines I built with Arias lighter pistons and 7" rods will rev freely to over 8000 and are reliable.Only trouble is head gaskets when running with high compression,12:1(now using Cometic with copper rings inset)
Dirk