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TR2/3/3A 87mm piston ring gaps

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jacksheehy

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I'm amazed at the body of knowledge you folks possess. Maybe someone here can answer my question. I'm putting a later TCF numbered engine into a TR3. The pistons are 87mm according to the number on the bottom. I honed the cylinders and bought new rings to freshen the motor up. I bought 86mm +040 Hastings rings from BPNorthwest because I was told they're equivalent to 87mm.
According to the TR4 service manual, the ring gap is supposed to be .010-.015 for the 86mm engine. I can't find a spec for the 87mm engine. I'm seeing about .035 gap for the oil rings and about .017 gap for the compression rings. I used a vernier to measure one of the cylinders and it measures 3.402" which equals 86.4108mm (less than 87mm). So, I have smaller than 87mm cylinders and yet my 87mm rings appear to be to small.
Am I going to have a poor running, oil burning pig when I'm done? The oil rings really worry me.
 
You say you honed all cyl did you check top to bottom of cyl for any taper plus or minus could eat up the .002 or .003. What did BPN say.
This is for a TR 6 but I think this may help . Top ring gape 0.012 to 0.017 2 nd ring 0.008 to 0.013 and your oil ring in question is much greater 0.55 to 0.38 Your crank case is for the most part neg. pressure so poor comp. rings that allow blow by would be a problem but when rings are fully seated it should have a good long life. Question steel or chrome rings. MF
 
We recently had exactly the same problem on a TR3A. Ended up using Jaguar rings which fitted well within spec.
 
I check with a club member who has built many motors. Due to heat in cyl the top ring needs more space that's the wider gap than second ring do not mix them up. the oil ring is just staggered and no big deal. MF
 
You can't measure cylinder diameter accurately with a vernier caliper, if that's what you mean by a "vernier." You really have to use a bore gauge. If you've reached the point where you need new rings, it would be a good idea to check the taper and ovality of the cylinders, make sure they are still within spec. You're also saying that the cylinder diameter is small, but the ring gap is too large, which is a bit inconsistent. I think you're confusing yourself with measurements that are not really valid.
 
I'm waiting for Hastings to reply to my email. It seems odd that the oil rings have a much larger gap than the compression rings. Since they came out of the same package, I'm assuming they're supposed to have a different gap. I can't find a gap setting for the 87mm rings. The 86mm are suppose to be .010-.015 so my .017 reading on the compression rings are close. The oil rings at .035 are way out of tolerance.
I know I can't measure bore accurately without a bore gauge but I expected the vernier to get me in the ball park. The vernier (calipers) measure about .020 less than I expected for the bores. I'm using a set of Starrett verniers so I expect they should get me in the ball park.
I'll let you guys know what Hastings says. Maybe their rings need more expansion room than the stock rings.
 
Update:
Hastings says my measurements are good to go. I still can't explain the .020. It must be some kind of measurement error. Thank you everyone for all your help.

Here's their response:
The End gaps per ISO spec. is
.010” – 020” for compression rings and
.010” - .055”for 3pc oil rings.
 
Dont over think this, the break-in is the critical part. Heartily recommend the use of break-in oil
designed for professional builders rich in ZDP. Amzoil is one. One trick to reduce oil consumption
is to knurl the valve guides(if worn) VS replace. The spiral groves hold the oil from sneaking by the
valve.
MD(mad dog)
 
Stock rings will always be on the small size. Most builders do not take the time to check the gaps, so the manufacturers want to ensure that a tight ring gap will not cause an issue. Thus the larger than spec end gaps. If you want to blue print your engine and set the gaps to the minimum for performance and/or long life, you must ask for the "file to fit" rings when ordering. Often they are marketed as slightly oversize. For example, if you have a standard bore, they will be sold as ".005 over".

86 and 87 mm are so close in size that the gap for 86 will be fine for the 87mm rings.
 
The next motor I rebuild, I'll order the +.005 set. I saw them in the catalog but didn't know what they were for. I'd rather have the rings gapped better but I'm putting this sucker together today so the bigger gaps will have to do. I doubt it will make a lot of difference in performance. It's not a racing engine so I think it'll be fine.
 
Just a side point with my TR6 motor I built in 1993. I checked compression yesterday it has been many years since. After all these years and steel rings one cyl 95 plus# all the others above 110# I think you will be fine. MF
 
Madflyer said:
Just a side point with my TR6 motor I built in 1993. I checked compression yesterday it has been many years since. After all these years and steel rings one cyl 95 plus# all the others above 110# I think you will be fine. MF
Click to expand...
I think so too. I'm setting up the cam now. I've seen so many Mickey Mouse things done to this motor, I don't trust the sloppy scratch mark on the cam gear and no scratch mark on the crank gear.
I wish Triumph would have made their timing gears/chain like most manufacturers with some factory markings. It sure would make things simpler.
 
jacksheehy said:
I think so too. I'm setting up the cam now. I've seen so many Mickey Mouse things done to this motor, I don't trust the sloppy scratch mark on the cam gear and no scratch mark on the crank gear.
I wish Triumph would have made their timing gears/chain like most manufacturers with some factory markings. It sure would make things simpler.
Click to expand...
It needed to be center punched at gears before removal and the new gears that are now adjustable for LEAD and LAG none when I built my motor. MF
Built it Drive it Love it
 
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