Race Engine Forum UK

An acquaintance of mine experienced a loss of oil pressure on his Fiat 124 BC Coupe (1608 engine) after 100.000 km's plus. Warning light was activated and the gauge went to zero. Root cause was traced back to the drive gear slipping over the drive shaft. Splines look ok at first sight but as said the drive gear slips.
I have not seen or heard of this failure mechanism before but it seems appropriate to share his experience here as he's not active on any internet network. When overhauling an engine I would suggest to renew the drive gear and always check for proper seating on the shaft with a minimum on clearance.

Charles

Can happen, I always hold the driven gear up to the light to check. In prolonged use they lean over, bit like a tree in the wind..

Never seen a spline set completely worn out though...

You do not say if the photo is your friend's oil pump - if not are you sure the oil pump shaft has not sheared? If it is then the cause is simply prolonged use..

You could not tell without taking the pump off.

For readers: There is a nasty stress-raiser at the junction of the splined region with the main shaft, I always radius that region on overhaul. A concentration of stress will be present if there is a pronounced (sharp) change of section (shape) in a component subjected to repeated cyclical stress and if a fracture is going to occur - it will be there. A carefully shaped radius can prevent failure. There are engineering methods of analysis for this condition, nearly all rotating and reciprocating parts are subject to stress concentrations. They are generally referred to in stress analysis as 'k' factor, where two or three times the normal working stress is used in computation of the safe working environment for the part.

Fiat changed the form of the shafts on the Fiat TC units at least twice to improve strength. They never did it on the Lancia Beta though..!

Failure to check free-turning of the pump (via the aux driveshaft) during bolting-down of the pump can cause premature fracture...

GC

It's confirmed that, though splines look ok on shaft and inside gear, the combination of both is slipping. Photo is from the pump and gear with the problem but its does not clarify the problem itself.

Charles

check inside the pump.

The gear on the master-shaft is a shrink-fit. Maybe it is slipping..

GC

Will do that Guy.

Charles

REPORT, MARINE!!

GC