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TR2/3/3A pistons have 4 rings

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Well...I couldn't bring myself to install the 4 rings in a sports car. No matter how much I tried to convince myself, it just didn't "feel" right to me. I am swapping the 4 ring 83 mm's for 86mm's with 3 rings. The 3 ringers are even cheaper, so it pays for the return postage to Moss. I'll post some pics of each style, just so others see the difference in future projects.
 
Even though I'm working on (another) thread where I find myself interacting with myself (and a hi-jacked thread at that!?!), I have some follow up info for future gearheads interested in the 4 ring deal.

I broke down the old original TR2 pistons and compared to the new, 4 ring pistons. First, here is the design of the original piston from 1955. You will note there is an expansion crack along the non-thrust side of the piston. The oil ring is fed through small drillings through to the inside of the dome:





The weight of the original, including pin, rings and clips is 606.8 grams



I just had to include a pic of a brand new sleeve. Something about newly machined parts!



Now, for the pics of the 4 ring piston. At first glance, it is identical, with the exception of the fourth ring. With a closer look, however, there are 2 cutouts below the upper oil rings. These are really pretty large, and cover more than 50% of the circumference of the piston. This should eliminate any issues with piston oiling between the oil rings, as there is a large amount of the piston "opened up" to allow oil between them.










And, the final word on weight is that the new 4 ring weighs 622.8grams vs the original weight of 606.8 grams. So, only 16 grams, or 1/2 ounce. Of interest, that entire half ounce is in the weight of the 4th cast iron oil ring itself. I know from my days balancing pistons that 1/2 ounce can easily be removed from a piston, if it is a concern.

So what is my conclusion? Well, weight is not really a factor between the 3 and 4 ring designs. I also don't think lubrication is an issue any longer with the 4 ring pistons. Although I don't particularly like the large cutout with the 4 ring...I also do not think it is an issue on a relatively low RPM motor, like the stock TR's. If I planned to wind the motor to 5k regularly, or above 5k for any length of time...then I would not want the 4 ring, opting for the more modern 3 ring design.

I still have a set of 86mm on the way. I'll update this thread when they arrive...and I still have to choose which!!
 
I was reading the Haynes manual on installing pistons and noticed the engine they are working on has the 4 rings. One thing that struck me was that they said to have the pistons facing front. I know how the rods should go, but aren’t the pistons the same either way? I plan on putting the writing on top of the pistons all facing the same direction, but was confused about there being a front to the piston.
 
That's a good question. I know some pistons have the pin slightly off center. This allows the piston to "rock" over to the thrust wall prior to top dead center to minimize piston slap. I don't know if the TR pistons have off center pins or not. With symmetrical pins it does not matter which direction the piston goes. With off center pins it does, and the piston is marked to ensure the proper orientation. The Moss pistons have no markings on the crown, but the originals do.

I got the 86mm, 3-ring pistons today. I weighed them, and it turns out they are heavier than the 4-ring pistons...665.9 grams! So...stock displacement, lighter pistons with 4 rings, or 2.1 liters with heavy pistons and 3 rings. I think I'll use the 3 rings, but still a tough decision. I am sure either way will be fine.
 
Here is a photo of one of the Hepalite pistons with 4 rings that I bought from Peter Hepworth in Yorkshire in 1988. It was a full set of pistons, rings and liners for 1991 cc displacement. The piston shown is from 2007 after driving 94,000 miles. It was taken during my engine rebuild when I had the bores honed and put in new rings from Hastings near Detroit.
 
Don, that piston with almost 100k miles looks better than the brand new ones I have now with 0 miles! Good to see the design is identical. I think yours are made from a much brighter aluminum, so must be a different alloy.
 
I forgot to mention that since January 2007 when I rebuilt the engine, I have been running Castrol 10W30. This was with the honed bores and new rings at 174191 miles. It now has 194,154 miles - so in 20,000 miles during this time, I have had the oil level drop between 3000 mile oil changes - but never had to top up the oil level. I add a 4-oz. bottle of ZDDP Plus each time I change the oil (every 3000 miles) and I also add 4-oz of Lucas Gas Saver Upper Cylinder lubricant every time I fill the gas tank. I used to add Marvel Mystery Oil as the upper cylinder lubricant but I can't buy this in Canada.

If an engine is "old", many TR owners think that they have to use 20W50 grade oil. But if you have just finished re-building an engine, it is no longer an "old" engine - it's a new one.

I have a theory that if you use 20W50 oil, it is too thich (sticky) and stays up on the cylinder walls and then it gets burned off as smoke on the next revolution of the engine.
 
No doubt, Don. I always looked at the heavier oils as an extender to the next rebuild, not as a service oil.

Here are the 86mm pistons:







As you can see, Don, they come with more of a powdered looking hue instead of the bright aluminum I'm used to, and like yours have. Tonight I have to make my choice of the set I plan to use and send the other back. I'm hoping to make the return before the credit card cycle! And, every time I think I have made up my mind, you guys find a way to make me vascelate...again!

86mm...more power, 3 rings, heavier...

83mm...stock displacement, more original, 4 rings, lighter...


Hmmmm...
 
Gut wrenching decision...but you sold me, Don. I sent the 86mm pistons back, so will go with the 83mm 4 ringers. Here's my thought process:

From other posts, while adequate, it sounds like the TR2/3 crank is not overly strong. The 86mm pistons were a full 10% heavier than stock, so that is 10% more stress on the crank, which is .030" smaller on the journals to begin with. Plus, after studying the pistons for several hours (I know, I gotta get a life), I like the design of the 4 ringers better than the 3 ringers with the molded-in steel expansion plates. Plus, I'll gain experience with something new, which is the fun of this hobby.

Well, that decision is over and I'll never give it another thought.
 
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