I have what appears to be a low mileage 1915 T . I have measured the cylinder bore and I have .004 maximum run out with the cylinder mic . The bores are all still standard size . I'm wondering if they really need to be bored ? I was thinking of honing and then fitting aluminium pistons and new rings
If it has .004" runout I would have it bored .020" over. It would cost maybe $100 to have that done - then the rings will seal properly when you install new pistons.
What about the babbitt?
I personally would not bore it. Although I admit its a close call.
I spoke with Milt Webb at the Auburn Swap Meet some time ago and he recommended 7 thou clearance with alum pistons if memory serves. Its seems too much for me but he claimed no problems.
I don't know what you mean by "run out" but if you are talking about out of round then you are already at twice the limit.
(Message edited by ccwken on November 07, 2015)
I just put new pistons in a previously untouched 1914. It had .007 taper. I knurled the pistons to tighten them up. It runs great and is quiet. At .004 I would definitely not bore it out, and might not even knurl the pistons. I wouldn't worry about the rings - keep in mind that at least on the power stroke most of their work is done in the top of the bore where it is less worn.
Chris Brainard, can you discuss the knurling method you used---what equipment you used and where on the piston and to what degree of dimensional change was accomplished. I ask this as I may want to try it too. Thanks, Joe
Knurling was a common practice through the sixties and was done on a specialized piston knurling machine. I decided to try the common lathe set-up and it worked fine. The only problem I had was that I actually expanded the first piston too much.
Remember that the only engines without taper are those that haven't been run.
Because the earlier engines had "higher" compression pistons with less bevel (at least in 1914), I used the commonly available high compression pistons. As I did not want to actual increase my compression, I then cut them down as much as possible until the tops were about .220 thick. I am not sure if this applies to you, but I suspect that it is part of the origin of the folklore that you lose power if you switch to aluminum pistons.
All those confusing statements aside, Out-of-Round and Taper are two different maladies and neither is corrected by knurling a piston to a different size.
Out of round occurs at 90 degrees to the piston pin and causes the rings not to seal. The new rings can't follow the cylinder wall because the rings are round and the cylinder isn't. If the cylinder is out of spec, replace the piston and bore the cylinder.
Taper occurs when the top of the cylinder is larger than the bottom of the cylinder. This is caused by wear. Excessive taper will cause the new rings to move in and out and wear the piston ring grooves. Check the old piston with a new ring for piston groove clearance. If it's out of spec, replace the piston and bore the cylinder.
Changing the diameter of the piston does nothing to correct the problem.
I understand your points,and in agree with your science. For example, on critical motors I use torque plates and preheat the block to 200 degrees with circulating water to try to assure that the cylinders are round at operating temperature with the head torqued on. Do you recommend that for a Model T? I was only trying to add a practical perspective for this situation. If I were building an engine to drive 20,000 miles a year I would bore it, or perhaps hone it back into shape and size the pistons accordingly.
If my pistons wear out, which they won't since I drive it less than 500 miles a year, I will replace them.
You have to take into perspective what you are working on, a race motor, a car that you will drive every day, or something that you drive only occasionally.
Anyone can make an engine new again - decisions aren't required. It is easy to say "bore it out",it is much harder to understand what is really required in a particular situation and make appropriate and educated judgements.
If you're going to knurl pistons, you really need to have equipment to back up the skirt from the inside. Such was supplied by Perfect Circle(my favorite) and several others. DS in Bellingham WA
If the problem is taper, you will find that the rings will move in and out of the grooves as the engine runs, and you will have rapid wear on both rings and pistons. You will also notice some strange noises especially when pulling a hill. I would recommend that you either use the original pistons and put in new rings, or rebore to a larger size and replace the pistons. But it would be a waste of new pistons to use it as is without reboring.
Also, it will save you a lot of work to do it once than doing work in stages. Some work will need to be done twice if you try to save money and only doing some now.
Like some of the others, I'm not certain about what you're defining as "run out". But regardless, whether it's taper or out-of-round, I would use it as is. I would just give the bores a light honing, install new rings and go for a ride.
I"m with Norman & Jerry, if you are noting .004 taper, that's only .002 on each side of the piston. On a model T driven ~500 miles a year, I don't think you have a problem. Cast Iron pistons will last a long time, granted aluminum ones weigh less (and require more clearance!).
Do check the valves, two-piece valve should be replaced!!
Of course this is just IMHO, and your mileage may vary!