Mercury big end failure

[gareth9702]

Can anyone suggest why a big end would fail on a Mercury 90 two stroke? The big end on cylinder 3 failed abruptly with no prior warning signs. There was plenty of oil in the tank and no other apparent damage to the engine. Any suggestions will be useful. I don't want to rebuild if there is a reason why it could happen again.

[neil.mccrirrick]

Correct oil? Oil pump failure? Water pump failed and overheated? Something blocked the water intake and overheated. Blown head gasket

There will be a fault somewhere but if it’s old it could just be fatigue if run at high revs for a prolonged time

[jwalker]

Lack of oil in that cylinder/crankcase. Could be a lack of oil in all cylinders but that one may run hotter anyway so it's the first to show. Put your sleuth cap on and get investigating the lubricating system.

Good luck!

Edit: There should, of course, be a roller in each groove of the cage, I presume you know that. Just checking since you haven't shown any in your picture.

[gareth9702]

The rollers are basically flattened and welded into the cage. I have never seen a bearing fail that badly before.

[Davie]

Can’t be lack of oil in bottom cylinder as this engine has oil injected petrol which is common to all carburettors. Cant be head gasket as it’s a blind bore engine. Unlikely to be the water pump/ cooling as the cooling water can’t cool the top and middle pistons without cooling the bottom piston , I would take a guess that this engine at one time has been low mounted and water has gone up exhaust and into The Crankcase which has pitted the big end bearing, needle rollers and conrod or same problem if bottom crank seal has gone and is drawing water. The daft thing is the bottom main bearing might/ will appear to be perfect.

[Davie]

Originally Posted by gareth9702

The rollers are basically flattened and welded into the cage. I have never seen a bearing fail that badly before.

Bearing fail… I had one of these that punctured the hood when it let go. Your lucky if it’s not wrecked the crankcase, needle roller went up side of piston. I wish I had taken pictures of some big time failures over the years. Had one of these engines and the conrod snapped. Fisherman kept it going to get home. It actually cut the front crankcase on half, the conrod was working like a strimmer

[gareth9702]

Thanks, Davie. That is an interesting suggestion. It was one of a pair on a Revenger 23 so was sitting low in the water. I hadn't considered water penetrating the exhaust.

[gareth9702]

A follow-up question. I cannot find any dimensions for the crank or the con rod in the service manual. Anyone know where this can be found?