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TR2/3/3A TR3 Piston Surprise

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doc50

doc50

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Have not seen this before. Got my TR3 motor apart today, ready to take to my machinist for 'spec-check'. #1 and 2 pistons, good shape. #3, 2 cracks emanating downward from the relief hole/slots in the sides of the piston. #4...whoa...badly scored and a HUGE crack starting from the same relief hole/slot (which is just below the oil ring). I've had a bunch of TR3s and have never heard of this problem. Should I get a certain brand of piston to keep this from happening in the future? Thanx, Thom, 1959 TR3 #34909L
 
Thom, We LOVE pics on this forum
No help with the pistons but I'm sure you'll get more help than you need
 
Almost sound like you were experiencing detonation on those two cylinders.
How were the sparkplugs? Color OK?

Merlin
 
I got this as a basket case, engine out. No idea about even how long ago the motor was rebuilt...but rod and main bearings look real good, as if it was not run too many miles...all else is conjecture at this late stage. I've decided to replace the 'G' pistons (86mm) with new 'H' liners/pistons (87mm). That will bring me up to 2138cc, I believe. Also, does anyone know a good source for a mild cam? I read in the posts about a .270 grind... Thom
 
My guess is you'll find the block full of crud around #4 liner. Lack of water circulation allows it to overheat (which could also lead to detonation).

BTW, the piston letters refer to the actual piston size relative to it's nominal size. In other words, a 'G' piston could be either 83mm or 86mm nominal, same goes for 'H'. In either size, the difference between G and H is only about .0004"
I've never heard of 87 (or 89) mm liners having grading letters, as they were strictly aftermarket parts; but anything is possible.

Ted Shumacher does indeed sell stuff for these old cars; but doesn't have a 270 grind listed.
https://www.tsimportedautomotive.com/camspecs.html
I think you might be referring to Larry Young's new street profile, which sounds promising.
https://www.tildentechnologies.com/Cams/TriumphCams.html
Unfortunately, last I heard, his cam grinder had quit and he hadn't found another one. You'll have to contact him for details.

Another source to try would be Ken at
https://www.britishframeandengine.com/parts1.html
 
Thanx, Don, Merlin and Randall! I'm taking the (apart) motor Wed to my guy who, of course won't tell me when he'll have it back...but he's real good, so that's ok. I'll look into those links to see what cam is out there. Interesting facts about the stampings and sizing, never knew that. I am now thinking that the small difference (86-87mm) should not affect performance very much. Any real world opinions out there? I appreciate the input.Thom
 
doc50 said:
I am now thinking that the small difference (86-87mm) should not affect performance very much. Any real world opinions out there?
Click to expand...
Not much difference between 86 and 87. Displacement only goes from 2138 to 2188, about a 2.5% change. So you'll get about 2.5% more torque at lower rpm. Might get a little more power at top end but probably not much as peak power tends to be limited more by breathing than displacement.

I certainly wouldn't bother changing 86mm to 87 if they are in good shape otherwise. But if you need to change them anyway, might as well go for the larger ones.

In all honesty, my "real world" experience seems to show that general engine condition makes a bigger difference than even 83mm vs 87mm. My TR3A had 87mm liners in it; but my current TR3 seems to make more torque down low with 83mm liners. The 3A did pull harder at top end, though, just a few mph more than the TR3 will do.

What I would really like to have back is the engine I had way back in the 70s, with 87mm liners & a "3/4 race" cam (plus a few other goodies). It didn't idle so good, but sure woke up and pulled like a train out beyond 3300 or so. Of course it also had a habit of breaking other things (gearbox, rear end) so there are always tradeoffs :smile:
 
Interesting point. I noticed (by memory so might be off) that the 86 kits are priced at approx $500 vs the 87 piston kits going for approx. $900. I recall this from the Moss Motors catalog pricing. If the pricing difference is true, then the 86 piston and cylinder kit might be an attractive option if someone's budget is also being considered in the purchase decision.


TR3driver said:
Not much difference between 86 and 87. Displacement only goes from 2138 to 2188, about a 2.5% change. So you'll get about 2.5% more torque at lower rpm. Might get a little more power at top end but probably not much as peak power tends to be limited more by breathing than displacement.

I certainly wouldn't bother changing 86mm to 87 if they are in good shape otherwise. But if you need to change them anyway, might as well go for the larger ones.

In all honesty, my "real world" experience seems to show that general engine condition makes a bigger difference than even 83mm vs 87mm. My TR3A had 87mm liners in it; but my current TR3 seems to make more torque down low with 83mm liners. The 3A did pull harder at top end, though, just a few mph more than the TR3 will do.

What I would really like to have back is the engine I had way back in the 70s, with 87mm liners & a "3/4 race" cam (plus a few other goodies). It didn't idle so good, but sure woke up and pulled like a train out beyond 3300 or so. Of course it also had a habit of breaking other things (gearbox, rear end) so there are always tradeoffs :smile:
Click to expand...
 
You said; "...the piston letters refer to the actual piston size relative to it's nominal size. In other words, a 'G' piston could be either 83mm or 86mm nominal, same goes for 'H'. In either size, the difference between G and H is only about .0004"..." Randall, I am not getting the gist here, I always thought that for instance "H" pistons were the big pots that gave total volume of 2138cc, "G" would have given a little less volume, etc. When you say actual piston size relative to it's nominal size...please elaborate. Also, I think I had a similar engine set up, mild Crane cam, "H" pistons, TR4 tranny. sweet combo, and I'll be trying to go to a similar setup on this one. Thom
 
The "grades" F, G & H were applied to both the pistons and the sleeves. The process is called select fit. The difference between the grades is, indeed, just .0004". There are slight differences in the manufacture of these parts -- tolerances. By measuring the pistons and liners and grading them, then matching the bigger liners with the bigger pistons -- smaller liners with smaller pistons -- proper fit is maintained without requiring the machinists to hold to a tighter tolerance. It's cheaper to select the parts for fit than to maintain the tighter tolerances in manufacture.
 
Thanx for the clarification, MOSESO. That clears up 40 some years of mis-understanding on my part Thom.
 
Hmm, sorry I missed the question. Anyway, Moseso's reply is right on target. Here's the chart for 86mm :


untitled.JPG
 
In addition to the above query -- I'll ask the question that is really on my mind:

I am replacing one piston in a stock TR3A engine that has four original Grade F pistons. Is it a given that the replacement I use must also be a Grade F?

.0004" isn't much and the engine is a bit worn -- but too tight is too tight.
 
My guess is that your bores will be worn enough that you can get by with any grade piston. But if not, there is nothing to say that you couldn't work the piston skirt down a bit to get the required clearance. Somewhere I've got the information on how to turn them to size in a lathe, but .0004" in aluminum alloy should be easy enough with crocus cloth.

FWIW, the best running TRactor motor I ever built had mismatched liners & pistons. (I got the set really cheap from Warshawski's, apparently someone had returned the set because the letters didn't match.)

The 86mm chart above is from the TR4 workshop manual; here's the chart from the TR2/3 workshop manual:

untitled.JPG
 
Just wanted to update the thread: Got the motor back in Dec....7 months later... and all my guy had to do was put in cam bearings!
Well, plus clean up the block and send the rotating parts to the balancer. Anyway, it has a .270 cam, and 86mm pots. It really runs strong and I will have a lot of fun soon. Still finishing the last things such as; OD/lights/horn/signals, etc etc.
I'm finding that the closer one gets to finishing a resto, the more one has to run through (rapidly cooling) molasses!

Thom
TS34909L (O)
 
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