Diesel piston bore clearance

[hiho]

Hello all,

im rebuilding a 1.5 diesel and the bore piston clearance is 4 thou (on the worst cylinder). The workshop manual states that the wear limit is 2.5 thou, however it also states that the valves cant be faced, the seats cut or the head machined and we know that these things can be done. Other manufacturers have wear limits up to 4 thou, (for perkins diesels this is the standard clearance!!) other than perhaps a rattly motor, is there any reason i shouldnt reassemble the motor after honing and re-ringing? (The ring lands measure up ok and it used very little oil prior to disassembly)

Thanks for any advice

Cheers
Matt

[brackie]

Hello all,

im rebuilding a 1.5 diesel and the bore piston clearance is 4 thou (on the worst cylinder). The workshop manual states that the wear limit is 2.5 thou, however it also states that the valves cant be faced, the seats cut or the head machined and we know that these things can be done. Other manufacturers have wear limits up to 4 thou, (for perkins diesels this is the standard clearance!!) other than perhaps a rattly motor, is there any reason i shouldnt reassemble the motor after honing and re-ringing? (The ring lands measure up ok and it used very little oil prior to disassembly)

Thanks for any advice

Cheers
Matt

Did you mic the bores with an inside mic? Was the 4 thou in places or right down and around the bore? (If you used feeler gauges I doubt if you would get anything close to a true result.)

4 thou is a lot for a diesel. I cut my teeth on Perkins and they are a lot more tolerant than VW diesels in part because the bores are wider and so the wear allowance grows with the bore diameter. New rings will certainly help and if there's anybody local who can shot-blast pistons to expand their skirts then it may be an option, however if the bores are oval then forget it as the hard rings used in the diesel just will not bed in.

[hiho]

Thanks Brackie,

The bore measurements were made using an inside micrometer by the machinist (after honing) and the bores are within spec for ovality with a very sllight taper. Im very tempted to fit new rings and reassemble the motor, please stop me if this is a bad idea!!

Its No.1 and 4 cylinders that are the most worn, I assume that it is because 1 and 4 cylinders have only one adjacent cylinder to generate additional oil splash.

Cheers

Matt

[h100vw]

If you have spent money and are in a position to put it back together already, I would carry on.

Hunting down another motor, which will no doubt need the same treatment, will take time and more money

If you put this one together and get 50-100K out of it, I would call that a result.

In fact what was your motivation for pulling it down in the first place?

Gavin

[hiho]

Timing belt failure, my fault. I purchased another head which is also being rebuilt.

[hiho]

Ive decided to reassemble the motor with new rings. I have been advised that the first 30 minutes of running is key to the bedding of the new rings into the honed bores and that maximum combustion chamber pressure is necessary to achieve this. My plan is to attach a trailer to the car, start it, check for leaks and top up the radiator then take the car out for a drive. On the drive i plan to do repeated acceleration/deceleration runs using 3rd and 4th, first from 30 to 80 then 50 to 100 then 70 to 120 (if she goes that fast again, it bloody should!). The idea being that the combustion pressures work the rings into the honed bores, wearing (bedding) the rings in. This needs to be done before the honing pattern wears as it acts like a file on the new rings.

[Bazzamon]

PLease be aware that one must use the same measuring device for measuring the piston & the bore wear if you want to know the clearance. So one measures the piston just below the ring lands at the thrust or worn side with an outside mic. then write the size down. Then measure the bore approx 30mm down from the top with calipers or bore guage or internal mic. THEN transfer the the bore measurment to the outside mic.that was used to measure the piston. Thereby over coming the difference between the two separate measuring devices. Normal wear limit for bores max. is 8-10thou. You can also just measure the bore taper because very little wear takes place at the bottom of the bore. you can then relate that measurment to a rounded out bore size. Its the piston that has the clearence ground on it..not the bore. (20 years of engine reconditionong)... bazz