I have a question for you guys who actually put miles on your cars. What piston clearance do you run? My '26 sees a lot of action and has been making odd sounds lately. I removed the head and found scored cylinder walls, so I dropped the pan and pulled the pistons. Three of the wrist pins were so tight that it takes effort to pivot the rods.
This engine was totally redone by a highly regarded specialist in New England three years ago and has about 8,000 miles on it now. My bore gauge shows a clearance of 2.5 thou. It's bored 60 over and has import Pistons from Langs. The wrist pins are a nice dark blue color from getting way too hot and the Pistons are beat. I got new ones from Egge and am going to take the block to a local shop to have it honed to remove the scoring. I really like that the Egge Pistons are relieved in the area of the ring lands by 30 thousandths, giving that area of the piston room to expand without causing scoring of the bore.
Should 5 thou be my new target clearance, or is that still too tight for a 60 over bore? That crap about 2.5 might be ok for a parade car, but not one that gets used. I'm kind of disgusted to be opening this engine up so soon after paying nearly 8 grand for a "top shelf" rebuild. But, roll with it....
I use the rule of .001 per diameter inch. So about .004 for a T block is my minimum clearance. Besides this you need to get rid of the scoring, so may end up with more!
The scoring is more than likely due to the fact that the wrist pins were to tight in the pistons. Have the rods checked if you are reusing them and for sure if you are replacing them. Bent rods can also cause piston skirt scuffing and more.
I haven't found a set of rods that were straight from a couple of different vendors.
On a good clean block (water jackets) you may get by with .0025” but I like more, I have my blocks baked and shot peen and use .0035” to .0004”.
There are pistons and then there are better pistons and that is a fact.
Yep I drive, we take a trip in the T model, upwards to a month and anywhere from 3,500 to 5,000 miles most years.
Needs to be four thousandths or as close to that as they can get.
If you use 5-30 or 10-30 motor oil you can get by with .0025" like the instructions say, if you never overheat the engine. Best to go for.004".
If you insist on roofing tar ( straight 40, 30 or 20-50) motor oil you had better have AT LEAST 4 thousandths of an inch clearance, or more.
Those are very large pistons, they expand a lot when the engine gets really hot.
You only need to overheat the engine once to score the cylinders and pistons if you have too little clearance.
I've had mine set up to 5 to 5 1/2 thousanths. Has worked fine for me on several motors that get used and occasionally abused. No wall scuffing issues. Pistons used are from Egge. My machinist told me I was setting it up half way worn out and I relayed problems friends had experienced with 3 and 4 thou clearances. He did what I asked and was happy with the results when he heard it run and rode in the speedster.
Oh, and the wrist pins should not have been tight. I go with push fairly easy with a thumb but won't slip out without help.
Half worn out engines really run great, as a general rule.
All the T Fords I have heard about that would go 55MPH with a stock engine and rear end were half worn out, or they had engines that had a lot of miles and a lot of years on them.
I know of two that still had original standard size cast iron pistons.
I would go to 5 1/2 on my own engine, even 6 if I were gonna run it hard all the time, but I would not on a customer's engine because it may burn some oil.
I am pretty sure my own model A engine has .005" clearance.
It is hard to get a machinest to bore and hone them that loose.
The instructions that come with the pistons for model T and model A usually call for .0025"
Mike Bender's point about piston pin fit is worth paying attention too. New Pistons usually come with .0005 clearance between the pin and the piston. In the splash lubricating system used in a Model T, this is not enough clearance to insure enough oil gets to the pin.
Some engine rebuilders will hone the piston pin bores to provide .001 clearance with the pin. This seems to greatly reduce the problem of the piston pin seizing, which in turn will cause the piston to scuff.
I have also found it necessary to at least check, and usually straighten rebabbited connecting rods. We shouldn't be too harsh on the people pouring new rods. It is a lot of work to pour and bore connecting rods, and straightening must be done quickly in order to deliver the rods at the price we can buy them for. If you don't believe this, just do your own rods some time.
Thanks guys. Hopefully, when the machinist hones the scoring away, and the bores end up at 5 to 5.5 thousandths over 3.807, all is good. I'll have him check the rods for straightness and hone the piston wrist pin holes on the Sunnen machine. I always ran Rotella 15w-40 and changed it about every 400 miles. How about that TW Timer!! Love it.
The man who taught me to bore cylinders and grind crank shafts trained me to install each piston upside down in the bore while placing a feeler gage between the piston skirt and cylinder wall. Newer engines such as 302 ford with cast pistons should be .002 inch. My 427 side oiler with forged pistons required .004 inch clearance. I fit my flat head model T's with aluminum pistons at min .003 inch. Sunnen type cylinder hone must be used to achieve clearance after boring.
And more importantly what do you end gap the rings at?
The rule of thumb is .004" per diameter of piston.
3.750 piston dia. X .004 = .015".
However most rings furnished by Grant or Hastings will have more gap, but should be checked.
Mike, I agree for the center and bottom ring, but the top ring needs .020" to be safe.
I have seen some that scuffed the cylinder and wore the top ring down fast when they were only given .015". Even seized the engine for a few minutes.
I also shoot for .017 on the second ring but .015" on the oil ring.
Never assemble the engine with ring gaps above the piston pin ends. They are best to be on the sides with the oil ring gap on the driver's side.
Here too it makes a difference what weight of roofing tar the owner will run in his engine.
All parts stay cooler with lighter oil and you can run smaller ring gaps.
I always figure that if you can't manage to measure/file, etc and establish the exact correct ring gap, it's certainly better to have a "thou" or two too wide a gap than too narrow. Remember, not enough ring gap (even one thousandth less than ideal) equals scored cylinder! Too much ring gap (even several thousandths) well,....at least, no cyl. damage!
And another way to think about it is, even if the ring gap is a few thousandths too wide, in the time it takes for a piston to travel through the upward compression stroke and the downward power stroke, especially at higher than idle rpm's, just how much do you suppose can leak through that little opening in just a literal fraction of a second, right?
I can testify if you do have a little too much clearance you can use the next larger OS rings to make sure you can get the proper end gap. That's what I had to do.
Gene is right, oversized rings that are .010 larger than the piston size should be used if the ring gap is too large.
Well as usual we have a difference of opinions, but all are very close as to a minimum ring gap.
So what is a maximum gap, I agree with what Harold is making reference to.