Allowable Piston Ring Gap?

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Model T Ford Forum: Forum 2013: Allowable Piston Ring Gap?
Top of pagePrevious messageNext messageBottom of page Link to this message  By Zachary Carrico on Sunday, September 29, 2013 - 12:02 am:

I have lurked the forum for a long time and would like to thank everyone for the indispensable information gleaned over the past two years or so, but I have reached the point where I think I need to ask a question.

My TT doodlebug would smoke pretty good if the oil was filled up all the way so I figured it was time for some new rings to help with oil control. The new rings I purchased when squared in the bore have a gap of .038 well above the .008-.012 specified in the Ford manual. My cylinder bore is slightly oversize owing to wear and the honing that I did but is still very close to factory specs at ~3.753" where I was checking the rings. my question is are the rings wrong, or is this considered acceptable? The old rings looked like a "Z" where the ends met so even a big gap still couldn't leak. Maybe I should just try to reuse them they seem fine just the cylinder walls were polished like mirrors which I assume accounted for the oil loss. (new valves wouldn't hurt either, but I am putting them off for the time being)
Anyone have any thoughts? TT_bug


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ken Kopsky, Lytle TX on Sunday, September 29, 2013 - 03:53 pm:

How did you measure the bore? Most bores will be worn to a barrel shape. Honing will not fix this. It's hard to believe with wear and honing that the bore is only .003" over sized. Keep in mind that for every .001" increase in bore, the ring gap (for the same ring) will increase over .003".


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ken Kopsky, Lytle TX on Sunday, September 29, 2013 - 03:58 pm:

Forgot to ask; Please define: "filed up all the way".

Oil should NOT run out of the top petcock. If it does, it's over filled.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Zachary Carrico on Sunday, September 29, 2013 - 09:41 pm:

Thanks for your thoughts,

I remeasured tonight just in case and came up with 3.758 and 3.759 at 90 degrees, should have thought about the 3 thou, last night and realized that was too small of a number but I was greasy, tired and apparently not so inclined to think!

-So figuring nearly 10 thousandths dia, that would equal 30 in circumference, which then completely makes sense. This still begs the question how much gap is allowable, do I need to ensure all the cylinders are 3.760 and put in .010 over rings (and pistons)? I would like to avoid this as money is always tight with three little ones running around.

I measured each cyl in several places in the bore for both out of round and taper, the worst cylinder was only 4-5 thousandths of wear before honing and generally less than two in taper(if I recall correctly,with three young girls in the house I only get an hour or so to work at a time and didn't write down the pre-hone #'s just measured to verify that things looked hone-able). I tried not to remove metal, just break the glazing with a med stone, then fine.


-To keep from excessively burning oil I had to keep the oil only as high as the lower petcock. On flat ground the oil level could be a bit higher than the low petcock, but on hills it smoked out my wife and kids in the trailer if it was run much above the lower petcock.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ted Dumas on Sunday, September 29, 2013 - 10:45 pm:

You might get 0.010" oversize rings and fit them. You need to check the gap at the bottom of the cylinder, there it should be the smallset in diameter.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Aaron Griffey, Hayward Ca. on Sunday, September 29, 2013 - 11:15 pm:

If you can not exchange the rings for a set that is .010" bigger just use the ones you have and don't worry about that large of a ring gap.
It may be of some advantage to put the pistons back in with the oil ring gap on the driver's side (left) and the second ring with the gap to the cam side and slightly to the rear and the top ring with the gaps above the oil ring and slightly to the front.
Ring gaps should never be directly above the piston pin.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ken Kopsky, Lytle TX on Monday, September 30, 2013 - 12:16 am:

To tell you the truth, I don't want to give a ring gap number. I don't see clear specs on the wear and taper. We already see that one set of numbers is .002". That's .006" in gap variation but it's unknown where it's occurring. That's a lot to ask for a ring. Especially when you factor in engine heat that will close the gap. The top compression ring should be run wider than the oil ring and second compression ring.

Throwing out a safe number may not cure your oil burning and your time will have been wasted. For accuracy, you need to know the temperature of the materials at the time you set the gap. Is it 30 degrees or 100 in your shop? It can vary the setting by as much as .005". The normal cylinder taper limit is .001" and out-of-round limit is .002". Over that and the cylinders should be bored or you won't "fix" anything.

Perhaps someone will come along shortly and throw a number out there.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Kerry van Ekeren (Australia) on Monday, September 30, 2013 - 02:08 am:

Actually, the old engines with cast pistons are a little more tolerant than that to be out of spec, ford gives a cyl taper and out of round is 'more' than .0025" and only slightly scored, to just hone the bore. 215 Ford service.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ken Kopsky, Lytle TX on Monday, September 30, 2013 - 12:28 pm:

Good point Kerry but how many T owners have machinist tools that will accurately measure to .00005". And no, I didn't add too many zeros. That's what's required to measure to .0025. I'd be surprised if most could accurately measure to .001". :-)

By the way, Zachary is waiting on your suggestion for ring gap on a barreled out cylinder. Also, it's the rings that are cast iron no matter what the piston material. It's just the opposite since cast iron pistons retain more heat for longer periods.

Bump for more help.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ted Dumas on Monday, September 30, 2013 - 02:18 pm:

Zack needs to set the ring gap at the bottom of the cylinder, scuff up the cylinder walls and install his new rings and put it back together.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Kerry van Ekeren (Australia) on Monday, September 30, 2013 - 04:53 pm:

Well Ken I do have all the right tooling, but it is not rocket science to check with no more than a set of feeler gauges, piston to cyl wall clearance, as for the large ring gap, yes it's far to much but in saying that it was only a few weeks ago, I pulled down a 15/16 for a fellow club member, clutch trouble and just check every thing out, that had large ring gap, it was running sweet and blew no smoke. But at our end of the world, we don't use 5w oil.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Zachary Carrico on Monday, September 30, 2013 - 11:38 pm:

Hmm..
. I actually expected to open up the head and find things worn such that any work would be an improvement, this was a 300$ T after all. Seeing that things werenít so bad came as a surprise.

Ken you are right, the engine should come out, if this was three years ago before my three girls were born, I would have took the engine out and brought it to the machine shop to get bored, fit new pistons, rings, ream valve guides, etc.. Unfortunately that kind of work just isn't going to happen, between kids and job there just isnít much left over, time or money. I donít want to be that guy who takes in good parts and turns them to scrap, buys more stuff just to do it again, but the bottom line is my girls love to ride around in the woods when I get home from work, thus makeshift repairs. Some day, with luck one of them will be interested enough to want to make a project out of it, I have a bank account set up that gets ten bucks detoured from my main account each month it is not much, but there will be money when the time comes.

The measurement point is well taken; getting a square measurement inside a cylinder is tricky especially with limited skill/practice. Fro reference was using a telescoping bore gauge but not one with a dial indicator so I only took measurements to get some idea of condition, not to get definitive numbers. I am sure that using calipers to measure the gauge didnít help with precision or accuracy either. (and it was 70 degrees when the measurements were taken, but I suppose that is moot:-)

Aaron, I can see how having that lowest ring turned such that oil is not so likely to be driven toward it would help, canít hurt to set the rings that way when it is assembled

Kerry, I think you have a point about these old engines, they do pretty well over a wide variety of lousy conditions that seem absurd by today's engine standards. This engine ran pretty good once I replaced the capacitors in the coils and adjusted them a bit. Using the crank the engine will start pretty well even in 15 degree F weather (actually I have never even used the starter, it was missing the bendix from day one). With some luck the level of smoke will be cut down if it is not quite gone.

If any one is curious I do have pictures of some of the work that I have done to keep this thing going here:
http://memebeam.org/zcarrico/offroad/t_doodlebug/t_doodlebug.htm


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ted Dumas on Tuesday, October 01, 2013 - 11:57 pm:

Lots of nifty projects and equipment. Looks like your doodlebug is a lot of fun.


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