Piston Clearance

[BrandyTudor]

Hello. My #2 piston skirt decided to come apart the other day. Pulled the head and piston and the cylinder looks ok. Plan to put new pistons in all four but none of the pistons are marked with a size. How do I determine this. Also what is the correct way to check piston to cylinder clearance. The pistons seem to move around a good bit in the cylinder. I know I should do a complete rebuild but i just purchased this car and the budget is a little tight. Thanks

[Synchro909]

There should be a number stamped into the top of the pistons. You may need to clean them some to see it. Try a wire brush to remove the carbon and crud. I'd be very surprised if there is no size there. If you find, say 0.040, that means they are 40 thou oversize. If you have the equipment, you could simply measure it.
As for how do I check clearance between piston and bore, it sounds like the piston you have is too small. You need about 0.004 - 6" clearance which you either measure with an internal and external micrometer or, if you don't have that equipment, a 0.004 or 0.006 feeler gauge will give a good indication.

[BrandyTudor]

I only cleaned up the one I've taken out and could not find a number anywhere. The only thing I found was the Ford script cast in the piston.

[Synchro909]

Maybe check the others. They should all be the same. I'd take them all out anyway and at the very least, put a new set of rings in it. A valve grind wouldn't do any harm either.
Do you know any other Model A owners who might have an old piston or three laying around? If you have a look at the number on them and try them in the bore, you'll work it out.

[BrandyTudor]

Thanks for the info. I was planning to get new pistons from Snyders. I'll measure everything out and see what i can come up with.

[tbirdtbird]

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let us know how you make out

[Tom Wesenberg]

If you have the FORD script but no size marked, then you might have original pistons in a block that still has the standard bore.
That's pretty rare these days, and I'm still kicking myself for not buying a standard bore block I saw at a swap meet 10 years ago for $100.

[BrandyTudor]

The piston I took out measure's about 3.8505 at the top of the piston head. The car is at my father-in-laws house so I cant check the cylinder. Is this the correct place to check the size? I'd need to pull another if I need to check the skirt due to this one being broken.

[Special Coupe Frank]

If that Script looks like the classic Ford "F" with an arrow through it ( facing the front of the engine) , chances are the word is actually "Front", and those would be OEM pistons.

If you clean the whole piston crown off with some carb cleaner and a scotchbrite pad, if you do not find any numbers stamped into the piston crown, it is most likely standard bore. If that engine has more than 40k miles on it, I wouldn't be surprised if that piston "slops around" in the bore at TDC.

Tom - I have a couple of blocks that are still standard bore... trouble is, they also have "standard deck cracks" in them...

[BrandyTudor]

Special Coupe Frank - slops around quite a bit I think. I don't know how much exactly.

[midgetracer]

4-6 thousandths seems a bit much for clearence on a piston. I usually set the clearance at about 2.5 thousandths. What do the engine builders say?

[Tom Wesenberg]

Here are the .015" pistons in my temporary engine. I can move them quite a bit side to side, so when I give it a light overhaul I'll have to go with .020" pistons most likely. I set my inside micrometer by the car, but then bolted on the head before I remember to take the cylinder bore measurement. The piston size is good to know, but it's the cylinder bore that's the one you NEED to know.

If you look at the center of the piston, then read towards the number one exhaust valve, you can barely make out the ".015". Expand the picture, then turn the computer screen and it will be easier to read.

[Tom Wesenberg]

Do you have any pictures of your pistons or piston tops. My pistons also have excess clearance in my temporary engine in my 28 Phaeton. Due to excess clearance and worn rings it uses a little oil. It's not too bad, but you can see by the cleaned edge of the pistons tops that I have oil getting past the rings. The oil cleans off the carbon near the edge.

I've already cleaned the center two piston tops, and you can see the engine doesn't have a matched set of pistons. Only the rear two have centering pockets in the middle of the piston tops.

[Special Coupe Frank]

I will add that although my running engine in my 87,000 mile Special Coupe had "a lot" of sidewise piston slop at top of the bore, the engine was still pulling 60-65 psi compression (dry), and still "ran okay".

I pulled another engine down, also with std bore and original pistons, mileage unknown, and "a lot of slop at the top", and with long "ribbon" feeler gauges (0.002" + 0.008" - only sizes I could get locally), I measured at least 0.010" clearance between piston and bore.

I believe Ford's spec is 0.002" .

Do you have a machinist / shop close-by that has a boring-bar, and can bore the cylinders in the car ? Even if you pull the block and take it to a shop... my local shops run around $10/bore to re-bore a block.


What kind of shape is your babbitt in ?

If your babbitt and crank journals look good, and you still have shims in the bearings, I think you would be best served to bore the block, have the valves ground, and install new pistons and rings.

If your babbit is shot or really worn/no shims left, then this engine is on borrowed time... if you really want to just throw a piston in it and keep it running, I would look for another stock Ford piston (used), match its weight to the others, hone all the cyls, and throw a set of rings in it.

[Patrick L.]

Pistons are measured at the skirt 90º to the wrist pin.

Cylinders are measured at several locations along the length and with and 90º across the crank center line to determine wear and out of round and taper.

Clearance is another ball of wax. That depends on the piston. Original pistons were set tight, in the .002-.003" range. Most replacements are set looser depending on what the manufacturer says. General rule of thumb is .001" per inch of bore. I usually add a bit more just for good measure, I'd rather have a piston a bit loose than too tight.

[George Miller]

Originally Posted by Patrick L.

Pistons are measured at the skirt 90º to the wrist pin.

Cylinders are measured at several locations along the length and with and 90º across the crank center line to determine wear and out of round and taper.

Clearance is another ball of wax. That depends on the piston. Original pistons were set tight, in the .002-.003" range. Most replacements are set looser depending on what the manufacturer says. General rule of thumb is .001" per inch of bore. I usually add a bit more just for good measure, I'd rather have a piston a bit loose than too tight.

I agree and especially 3,4 need to have more clearance. If not you have a good chance to score them.

[Chuck Kuntz]

I have a set of NOS Ford script .005 pistons that may solve your problem. You would have to check the bottom piston travel limit of the block bore.

[kenparker]

As Patrick wrote - pistons are measured at the skirt. Cylinders need to be measured at top and bottom. Due to wear they are tapered and if tapered they should be rebored. Also feel at he very top of cylinder - top 1/8" - if your fingernail can detect a ridge or very slight "lip" that ridge should at the least be honed out. Of course reboring will take care of the ridge.

Pictured below are two different engine blocks. Each one had the "ridge". The ridge was removed with a Ridge Reamer - tool made for that purpose.

The one on the left is .060 oversize as indicated by the 060 stamped into the top. It had no arrow pointed to the front, thus the magic marker arrow. Another way to tell front from rear is the position of the slit in the piston. Slit to driver side of block.

The second picture - one on right is a .040 oversize cylinder and piston. It only has a 04 stamped on top but one can see the word front and an arrow at the front edge of the top. In this block the four pistons were numbered 1, 2 , 3 ,4. Cylinderr measures 3.920 at top and 3.917 at bottom. This one will get rebored to .060 oversize.

[100IH]

In your 4th post you said you measured the piston at the head - NO,measure the piston at the bottom of the skirt, the largest measurement at the bottom of the piston across the bottom edge, side to side, 90 degrees to the wrist pin. The bore is measured inside at the bottom of the bore, below the ring travel and at the top at the top where the ring wear stops its upward travel, the subtracted difference is the wear/taper in the bore. Get a machinist to do the measurements and tell them to figure out the piston sizes needed then have him order the silvolite pistons for the machinework. For a first time hobbyist, this is time to pay for the experienced help. Watch and learn. It will be worth the money.

[Jim Brierley]

As 100IH says. Ford said a max clearance of .002" This is not enough with today's pistons and our driving habits. On a stock engine I bore to .003" and then hone another .0005". Tighter than that and the pistons will seize in the cylinder, not good!