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TR2/3/3A Hastings Piston Ring End Gap Issue

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Joel M

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I'm current working on rebuilding my 3A engine, which was originally rebuilt 15 years ago and sat after only about 5-hours of run-time. The previous rebuild used a new 87mm County piston/liner kit with Grant rings. For this rebuild, I decided to use the same liners/pistons/rings due to there being almost no wear; however, I measured the ring gap to be 0.020" at 3" deep. This is 5-mils over the 15-mil maximum of the 86mm TR4 engine. I believe the 87mm liner bore to be good, as I measured it out at 3.425". So I decided to order new Hastings 86mm 040 rings to see if that would solve the problem, but they only brought the gap in to 0.018". Does anyone know the proper end gap for 87mm pistons? I'm thinking it should be similar to the 86's. Not sure where to go from here if 18-mils is too high. Has anyone else has this issue? These are new parts with almost no wear.

Joel
 
I don't know the answer to your question but would 89mm rings work, after fitting with a ring grinder?

Also, I had a problem similar to this. Wide ring gaps and the oil ring that came with the pistons caused problems with 3 rebuilds. IE: This is the 3 piece oil ring. two scrapers with a spring in between. The spring had 90 degree bends on the end and in two of the cars, the ends broke off with the result of smoke from burning oil . We replaced the oil rings with one piece Hasting rings and the smoke went away. I have not done the 3rd engine yet.
 
I bought moss rings in 2010 for my 86mm+.030 pistons. I used the gap in the Haynes book, which I recall to have been .017. The rings came long and required end filing to get to the spec. I recall from earlier engines that rings are usually long, though my last prior engine rebuild was early 1970s, so memory is not the best. I wouldn't accept a gap not in spec.
Bob
 
I know the TR4 service manual says 0.010 to 0.015 for 86mm pistons. I don't have the Haynes manual - does it list a spec for 87mm overbore pistons? And have you had any issues with the engine at 0.017 ring gap? That's only 1-mil smaller than what I currently have.
 
If you order the rings as a kit with the pistons, they will come with a “safe” gap...meaning they supply them so that there is no chance of the ends touching in service under the tightest possible bore, and assuming the majority of builders do not take the time to check the gap. In short...the gaps will be way large, as you discovered.

If you order the rings from a ring manufacturer, specify the bore and that you want “gap to fit”. Then you will get rings tighter, and which will require gapping. Again, only ring manufacturers, and some good speed shops, will have any knowledge of gapping to fit. Most general suppliers (Moss being one of those) do not deal with custom rings.
 
Thanks John, that's good information. Do you know what I can expect if I run the current rings at 0.018? Would I even notice the difference if I spend the time and money to get more rings and gap them properly?
 
Sorry to jump in here but I've put a few engines together and ran into the exact situation you have . The concern first is that there is enough gap so the rings don't expand and butt together which can cause them to fracture. Of far less concern is that the gap is too big. If yours measure .018" of and inch and the high side of the spec is .017" I'd worry about something else, it will be fine ! KB
 
Not an expert, but have rebuilt a few motors including a 4a motor. As mentioned too little gap is the bigger issue. You can return and go for better, but I bet if you calculated the size of the very slightly increased opening caused by the ring being a thousandth of an inch out of spec vs. the bore of the cylinder you would find it to be inconsequential in theory and I don't think I am going out on a limb by saying also inconsequential in practice.
 
I found a table that shows SAE recommended gap limits for various cylinder diameters that ring manufacturers are supposed to adhere to. A 3.425" bore would have a limit of about 0.020". General rule of thumb seems to be a minimum of 0.0035" of gap per inch of cylinder diameter, and a maximum of 0.0055" of gap per inch of cylinder diameter. That would put the 86mm 040 Hastings rings (and the Grant rings) near the upper limit, which is spot on with John's previous comment about manufacturers ensuring the rings are supplied with no chance of them touching in service in the tightest bore. It is good to hear that a few others have built multiple engines with this type of gap and seen no negative effects, so I will proceed with the Hastings rings as-is.

I do have one more slight concern for which I would like some feedback. The Hastings ring box arrived with the perforated cardboard ring dividers torn, so the rings were mixed up (no damage). This is not a problem for the oil rings, but 1 & 2 appear to be identical. Upon inspection, I can see no discernible difference in dimension, shape, or color, but I'm not sure I would be able to tell if there was maybe some special process on one versus the other. The ring box is generic with a sticker applied to indicate what's inside, so I'm pretty sure it would have 1 & 2 slots even if they are the same. I also seem to remember the Grant 1 & 2 rings were identical (they look it). Can anyone confirm if 1 & 2 are the same? If there is good reason for me to be concerned, I will buy another set. I'm afraid if I send these back to Moss they will end up in another engine rebuilders shipment, so I'm not going to do that.
 
Joel, on the top ring, if not labeled, there could (not should) be a small step down on the outer edge. This was done for the ridge caused by wear of the piston at the top of the cylinder. Not everyone cuts these out. Might want to check for that.
 
Thanks for link - both 1 & 2 are type 102 cast iron rings. Since I didn't see any difference in the color or coating, I will assume they are interchangeable. I guess I am ready to proceed with the rebuild! Thanks for all the advice!

Hastings Rings2.jpg
 
Just wanted to close out this topic with information provided to me by Hastings on the 6735 040 (87mm) rings. Allowable gap is 0.012" to 0.022", and the 1 & 2 rings are identical, and therefore interchangeable.
 
Joel M said:
Just wanted to close out this topic with information provided to me by Hastings on the 6735 040 (87mm) rings. Allowable gap is 0.012" to 0.022", and the 1 & 2 rings are identical, and therefore interchangeable.
Click to expand...
I know this is an older thread, but may I say how nice it is is to see people returning to close the loop and report back on outcomes. Thank you for your consideration, Joel.
 
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