Yesterday, 08:29 PM
As many of you will know, I have recently had a rebore with new pistons and a general top end overhaul, mainly to cure what I thought was a little end rattle. coming back from Wrexham last Tuesday, and after about 250 miles running, the noise started again.
To explain, although I recognise that it is notoriously difficult to diagnose a noise that one hasn't heard, it sounds exactly like a tin butterfly trapped inside the engine that is most apparent on light throttle and on the change from run to overrun - exactly where one would expect a big end noise to be but (at least to my ears) lighter than I would expect.
Accordingly, today I took the sump off and carefully checked that all was well inside. Removing the sump and gauze showed no sign of debris. Indeed they were as clean as when I assembled the engine. I checked that the gudgeon pin pinch bolts were tight and then dismantled the big ends in turn. The bearing surfaces were smooth with no signs of scoring or metal flaking. All the bearings seemed to be well covered in oil. I checked the crank pins. All were round to within 3 thou and, again showed no signs of scoring or discernible wear. I then checked the radial clearances and found all of them to be less than 2 thou. I reassembled the bottom end most carefully, making sure that the oil passages in the crank were clear and that both oil jets were unobstructed. (Not that I thought they were, as the oil pressure is about 3 lbs at 30 mph hot, but its best to check). I made sure that the bearing caps were the right way round (they are all marked anyway) and that the big end bolts were correctly seated and the nuts done up to 20 ft lbs torque. I checked everything several times, just in case I had missed anything, as all seemed to be as it should.
I have just got back from a 20 mile test run and my tin butterfly is still there when the engine is up to temperature. Oil pressure is just off the stop on tickover and about 3 lbs at 30 mph hot.
What have I missed?
To explain, although I recognise that it is notoriously difficult to diagnose a noise that one hasn't heard, it sounds exactly like a tin butterfly trapped inside the engine that is most apparent on light throttle and on the change from run to overrun - exactly where one would expect a big end noise to be but (at least to my ears) lighter than I would expect.
Accordingly, today I took the sump off and carefully checked that all was well inside. Removing the sump and gauze showed no sign of debris. Indeed they were as clean as when I assembled the engine. I checked that the gudgeon pin pinch bolts were tight and then dismantled the big ends in turn. The bearing surfaces were smooth with no signs of scoring or metal flaking. All the bearings seemed to be well covered in oil. I checked the crank pins. All were round to within 3 thou and, again showed no signs of scoring or discernible wear. I then checked the radial clearances and found all of them to be less than 2 thou. I reassembled the bottom end most carefully, making sure that the oil passages in the crank were clear and that both oil jets were unobstructed. (Not that I thought they were, as the oil pressure is about 3 lbs at 30 mph hot, but its best to check). I made sure that the bearing caps were the right way round (they are all marked anyway) and that the big end bolts were correctly seated and the nuts done up to 20 ft lbs torque. I checked everything several times, just in case I had missed anything, as all seemed to be as it should.
I have just got back from a 20 mile test run and my tin butterfly is still there when the engine is up to temperature. Oil pressure is just off the stop on tickover and about 3 lbs at 30 mph hot.
What have I missed?
