Oversize piston availability for 50 year old car


#1

I read a magazine article about a guy rebuilding a 1971 Triumph TR6. To get rid of some cylinder wall imperfections the machine shop overbored by 20 mils. Then he said he had to buy some new pistons, oversized to fit the new cylinder bores. I got to thinking: I wonder where he found oversize pistons for a 1971 TR6? It seems like it would be hard enough to find standard size pistons for a car of that vintage. Why are oversized pistons so easy to find for a 50 year old car?


#2

Go here, and choose your size.

https://www.rockauto.com/en/catalog/triumph,1971,tr6,2.5l+l6,1292251,engine,piston+&+piston+ring+kit,14008

Tester


#3

Wow! I see there’s +20, +30, and +40 oversize piston options. Out of stock presently at RA, but presumably still available. And for a 1971 TR6? Goes to show, almost any problem has a solution in the pre-electronic-module car repair world.

Sidenote: If anybody here watches the British tv series “The Detectorists”, the Lance character has a 1975 TR6 as I recall. His pride and joy. Sounds like he will have no problem finding pistons.


#4

Most any part for popular British cars are available in Great Britain, if not here.


#5

The Roadster Factory has been selling them for stock, 020, .030, and 040. for a few decades now. Parts for these cars, while not easy to get as say for a 1995 F-150, aren’t horribly difficult to come by. When it comes time for a rebuild (my 1974 is nearing that point), it’s common to bore the cylinders out, there’s plenty of material there.The 2.5L I6 has a long stroke but relatively small bore It’s also common to have the head decked to up the compression a bit. Stock was something like 7.5:1 in for the early-mid 70’s models. Bringing that up to around 9:1 helps both power and efficiency, and can still run on pump gas. When I have mine rebuilt there are few upgrades (that don’t “show”) that I’d like to look into, getting the head cleaned up, roller roller rocker arms, and a slightly hotter cam are all things that are on the docket. I was thinking about triple webers, but the more research I’ve done, and most of people I’ve talked to who have them say that for a street car, there’s practically no benefit to them other than looks/bragging rights when you pop the hood.


#6

I’m presuming whoever manufactures these, they just build a batch of them at 040, then turn a few to 30, a few more to 020, and the rest to standard +0?


#7

I can’t say for sure, but that sounds plausible/likely.


#8

I don’t know. Wouldn’t that result in different ring grove geometry for each? Are the rings wider for the 040?


#9

I’ll have to re-read the article, but I think for the 1971 TR6 they said they had to use oversize rings to match the oversize pistons.


#10

The diameter of the rings is larger, for sure. I’m talking about the cross section through the ring. If they turn pistons to fit smaller bores, all that has to come out of the ring cross section.


#11

I was thinking when they turned a piston to reduce its diameter, they’d also turn the groove the same amount, where the rings fit. So the cross section of a ring would be the same, or nearly the same, independent of it being oversized.


#12

So if the groove is shallower, then the ring sticks out more?


#13

When in need of British parts I’ve always used Victoria British in Lenexa, KS. They have just about everything one could need for MGs, Triumphs, Healeys, and Sunbeams.

I think they also offer new cylinder liners for these cars but I’m sure they’re pricy. And then some.


#14

If they turned the groove the same amount as the piston, then the groove wouldn’t be shallower, right?


#15

That would work.


#16

Been there, done that with a 6 banger, bored 30 over, 460 lift 280 duration cam, head milled .010 It wasn’t worth the expense. You need to go to a strictly mechanical advance distributor, need to get it dyno tuned to get the right jets in the carbs, and the advance curve set in the distributor. When all done I gained a whopping 38hp.


#17

Custom pistons can be ordered for virtually anything that was ever on the road. It takes time, and they aren’t exactly cheap but available. Most of the better piston suppliers can get you what you need in cast pistons and even easier in forged pistons.

If you start with a forging, they require a lot of machining anyway so its really not a huge deal to CNC whatever you want. More compression? shallower dish or change the piston-pin-to-crown height. Since the pistons aren’t perfect cylinders, they are “barrel” cut, oversizes or thinner ring packs are easy to program.

The museum I volunteer for recently rebuilt a 1950 Cadillac V8. The engine had never been over-bored and only needed a 0.010 cut. They custom ordered pistons as no one carried a 10-over piston. Took 10 weeks to get them but the engine is all back together and running now.


#18

I rebuild old motor scooters and motorcycles, and occasionally need something like an oversize piston for a 30 year old machine. The internet makes it possible. Sure, the part may come from Indonesia or Zimbabwe, but there it is when you need it. Many of us with a hobby of doing mechanical work end up with a supply of spare parts that we can’t throw into the recycle bin “because I might need it someday”. Now I can sell it on Ebay. I sell old, worn license plates that come with derelict scooters for $15 to $25 on Ebay, and every time I do it I’m surprised that someone actually wants it.


#19

Moss Motors was my go to for my '71 TR-6 and is currently my go to for my 70 year old MG.
Just about every part for my MG and more important, the expertise to give you the correct part for your specific car.


#20

I think that’s a good idea. I wonder if a person could make some spending $$ for their hobbies by grabbing appliances people put out on the curb to throw away, old washing machines, dryers, dishwashers, even stuff like old bicycles, taking them apart, and selling the parts piece by piece that still work. It seems like a used washing machine transmission might bring $100 from the right buyer.