Next step?....
 

Okay Model A'ers, I have to pick your brains about what can be done about the following.

In my other topic I asked about the odd sized rod nuts which morphed into discussing valves and babbitt. Now my topic is pistons/rings/cylinder sizes. In short, this engine ran fine except for a nagging knock. I went ahead and removed the pistons and valves. I'm all set regarding doing the valves, but today I measured the pistons and cylinder bores. All pistons measure 3.85 and all cylinders measure 3.89. Measuring end gaps of each ring in the cylinders results in excess of .025~.030. This engine had good compression and didn't burn oil. According to Les Andrew's 'Red' book, the pistons should measure 3.8745 and the bores 3.875.

So...what to do. Is the .040 difference I have (actually .020 because of circumference) small enough where I can get away with putting oversize rings in and using my existing pistons? If so, would .010 oversize rings work, or will the end gaps still be too wide? I want to be sure I don't go too big or those gaps will be too small and bind the engine up!

Am I being too worried , or should I just replace the standard rings and put it back together...there are no markings anywhere to indicate the engine has been bored. Each piston is also faintly marked with a Ford script and arrow pointing to the front position orientation when assembled. Those are the only markings.

Experience? Suggestions? Comments? Thanks everyone.

Craig

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Are you confident in your measurements? The pistons would be very loose in the cylinders with these measurements.

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Where did you take the measurements? Top/Bottom or just top of bore?

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Before installing rings in an engine, always, always, measure and adjust (if needed) the end gap on ALL rings before installing them.
When measuring ring end gap, if it's too small one just files one end of the ring to increase the gap, simple.
Also, on used cylinders be sure to measure the ring end gap near the bottom of the cylinder, where the diameter is smaller.

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I measured each ring down in the cylinder the length of the piston from the top of the deck....so approx. halfway. I measured the pistons and bore with digital calipers since I don't have micrometers greater than 3 in., nor any inside micrometers. Measurements were taken after honing the cylinders.

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Craig, Modern pistons are not round. The expansion of the aluminum has to have some place to go. So the pistons are ground so that the fore and aft diameter is less than the side to side diameter (referring to the orientation in the engine). The side to side diameter should be fairly tight so that the pistons do not chatter and work correctly. The side to side parts of the pistons is where all the pressure is that resists the force of the rods when they are at an angle.

As others have asked, measure the pistons in several places. Fore and aft and side to side and top and bottom. Are the rings tight in the grooves in the pistons?

If your engine had good compression and was not burning oil, then you can just put the pistons and rings back in. The ring gaps seem a little large, but if they were working OK then they can go back in. Make sure you put the top of the ring towards the top of the piston as some rings have a slight cone shape.

If you put new rings in, make sure you adjust the gap according to the manufacturer's recommendation and do a slight hone on the cylinders.

As far as your knocking is concerned, make sure that you have the rods measured for twist and straightened if necessary.

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Thank you nkaminar, I did not realize that pistons weren't actually round! I will say that I believe these pistons to be original era production as they each have the Ford script on the top. My uncle built this engine in 1989-90 and was prone to use original parts when he could. He was a machinist's mate 2nd class during WWII on an LST during the N. Africa and Anzio landings. He also rebuilt two wrecked small airplanes..1947 Aeronca Chief and a later Cesna 150...that I've flown in with him. I mention this as I knew his mechanical prowess and that he wouldn't cut corners just to 'get it done'. He also guided me when I was a 15 yo kid in building my first car, a 1947 Willys CJ2A jeep.

I will remeasure everything and order new sets of rings, (few oil rings broke) among other things needed. Should I get a .010 oversize ring set to close up those gaps a bit?

Again, much appreciated,
Craig

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I think you could use the .010 rings and gap as needed. As for the knocking, check the play in your timing gears. If the mesh is not to spec they will hammer and sound like loose rods. I know this for a fact. :)

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Thank you flathead. I've already put in a new timing gear as the old one worked, but was 'partially' shredded. Thought that would solve the knock...no change. What town are you in down here? If you don't want to disclose that, a neighboring town will suffice.

Thanks, Craig

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Modern pistons are not round - now that is interesting.....

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Ford initially had round Model A pistons but slots were cut in the skirts so that the expansion could take place without the pistons binding up in the bores and scuffing. If I remember correctly, the tops of the pistons, were the rings go, were a smaller diameter.

The rings have different gaps depending on where they are on the pistons. The ones on top get hotter so they have larger gaps. Follow the ring manufacturer's recommendations.

The cast iron pistons used in the Model T's were round because the cast iron has a much lower coefficient of thermal expansion.

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I'm right next door, I was in Limerick today. Do you go to the Parsem car show?

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I've been going since it began back in the early 90's but either take my '41 Willys Navy jeep or my '48 Plymouth P15 Special Deluxe...unless I can get this Model A back together by then!

Let's see, right 'next door' would mean either Cornish, Parsonsfield, Limington, Newfield, or Waterboro...lol.

Craig

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#1 :)