Car passed its MOT yesterday, despite someone hitting me from behind on my way there (no damage luckily). It's been off the road for 18 months while I replaced big end bearings (for the second time), turbos and various other bits. Engine's now been out three times.

 

So happily drove the car to and from work yesterday. All was well. Took it out for an hour today and I'm starting to hear the tell tail knocking of the bloody big end bearing again!

 

There's obviously something else wrong that I'm missing. I've had the crank polished and replaced the bearings with same size once. I've had it ground and replaced with oversize once. And it's always the same bearing that goes.

 

What am I missing? The bearings hadn't turned in the rod and the rods appeared ok.

 

The only things I can think of is there is a blockage inside the crank carrying the oil from the main bearing to the big end bearing in question. Or that the rod is somehow damaged.

 

When the first bearing went, it didn't go big style. It gradually got worse over about 10,000 miles. I eventually decided to replace it. The new bearings lasted about 5,000 miles before one let go big time. The crank was reground and oversize bearings fitted. These have lasted a few hundred miles before failing again. I've not pulled the engine yet but I'm guessing it will be the same bearing again.

 

All comments and suggestions appreciated.

had this on my 309gti 5 years ago.if the cranks definitly fine then it will probaly be the con rod/cap has gone oval.on my 309 the engine shop repaired the rod by line boring it and fitting an oversize bearing.check the rod in question using a mercer gauge.

had this on my 309gti 5 years ago.if the cranks definitly fine then it will probaly be the con rod/cap has gone oval.on my 309 the engine shop repaired the rod by line boring it and fitting an oversize bearing.check the rod in question using a mercer gauge.

Thanks for the input mate. The rod and cap still have all the markings in them from the original machining. The machine shop said this indicated the bearings hadn't turned and everything should be ok. Does this sound right?

This sounds like the rod is at fault, I would have checked bearing clearance on the big ends before building the engine.

 

Also if that jeurnal (bad spelling, sorry) was well and truely done for then the grinder may have gone just that little bit too far to take out any damage and this will cause low pressure on that jaurnal, (I still can't spell that word)

 

I have also known too much oil pressure to knock out bearings, but that is very unlikely.

the bearings didnt spin on my 309.but the bearing with the most wear had an oval rod.the tolerances on a z32 engine are tight to say the least.just done one and if i recall the difference between a grade 0 and a 3 bearing is about 6 microns!!!!!!!!.another cause of premature failure is not blowing all the oil ways out fully after machining.but im pretty sure if the same bearing keeps going then that is probaly the fault.

Con rod I reckon,lack of oil tends to spin the bearing! :wack:

Did you flush out the engine oil cooler after the previous failure?

 

Daveman

good point dave.thats a common cause of auto boxes going quickly again as well.

Did you flush out the engine oil cooler after the previous failure?

 

Daveman

I can honestly say I didn't. Why? I presume you're suggesting some shit gets left in there?

Also, car was running quite hot for ages. I replaced everything (water pump, thermostat, etc) and eventually cured it with a bigger radiator. However, after putting it all back together this time, I needed the heater on hot all the time to keep the temparature down. Could this be related? Is there anything that would cause the bearing to fail that would also generate all this extra heat?

Bumped back to the top in the hope that more people have input.