ovality in cylinder bore

[jocklow]

Chasing a poor starting problem, I checked compression. This gave a cr of 2-3 on the starter going up to 7 to 8 with some oil in the cylinder. I have removed the cylinder head and sump and taken out one piston. The piston is marked standard, and the bore is 86mm. i have now measured the cylinder bore with a tenths clock set with a tenths mike to 3.3858. The result is that there is little wear but there is ovality. Diameter varies from +0.002 to -0.002, measured at different angles at the same depth in the bore. Is this likely to be due to poor cleanliness when fitting liners or are they sufficiently rigid to maintain their shape. in which case it would imply a poor rebore.

If the rings are carefully ground circular as new, then presumably ovality will completely screw the seal and could explain my poor compression.

ANy observations/or experience welcomed.

[foster461]

How many miles are on these pistons/liners ?. They will wear more on one axis than the other, you will read stories on here of people rotating liners 90 degree to get a few more miles out of them. Not sure how much out of round they have to be before it starts to present problems though.

 

Stan

[Motorsport Mickey]

Don't frighten yourself. The bores move all over the place when the head is tightened, in the Technicalities book there is mention of what happens when the head is tightened down.

The measurement of the liners is what's important, unless there is evidence of excessive bore wear in one plane (look at the wear pattern on the pistons and liners) ie massive scoring it would be unlikely that the liners are significantly out of round when clamped. As a guide one of my reconditioning books talks of piston/bore clearances of up to .006 being acceptable, and as a norm I seem to remember .003 being quoted by AE somewhere

 

More likely that the bores are glazed causing poor piston ring sealing, have you been "easy footing" the car over the last couple of years ? . I should give it some severe running through the revs in top gear in the area of bmep. (Brake mean effective pressure). This is where the engine develops most pressure throughout it's compression stroke ie top to bottom and is normally found at maximum torque.

This causes the rings to expand and bed to the cylinder walls, giving the best piston ring bedding and cylinder sealing.

 

Hone the bores and refit the head and drive it hard - not maximum revs, but in top gear and with constant runs through the toque range, drop the speed and repeat. Give it at least 30 mins of this and see if any difference, I should repeat at least once weekly to keep the glazing off the bores.

 

Mick Richards

[Don Elliott]

I had glazing on the bores in the engine in my 1958 TR3A. I was getting 600 miles to a litre of Castrol 20W30. This had been going on for years till I decided to do something about it.

 

In 2007, I removed and tore down the engine and the bores were not worn - nor were they oval. So I had an engine rebuilt shop hone the bores (tightly secured into the block). He said I had the SUs set too rich and this was washing down the oil and caused the glazing. He could see just by looking. Then I reassembled it all using new Hastings piston rings which were available from near Detroit and they arrived by mail two days later.

 

Since this last rebuild, I have driven 18,700 miles using Castrol 10W30 and never had to add oil between 3,000-mile oil changes. I still have the original connecting rods but I installed a set of Hepolite pistons, rings and liners in a 1988 rebuild of this engine. These pistons and liners are still in the engine today with 112,000 miles and no issues.