Bore and piston size questions

1crosscut

1crosscut

Well-known member
Let me get this out there straight away in that I am none too good at measuring things with the tools of the trade but have doing a lot of learning as of late so please bear with me if I have my numbers all wrong.

I am in the process of going through an engine that I am interested in purchasing. The person that has it has allowed me to take it home and check things out on it.
The engine has not been run.
It has new single lock adjustable valves and new valve seats. It's been
re-babbited, crank shaft turned down to -.010, new timing gear, re-built oil pump and a new one piece pulley. Clearances on the the three mains are good with plenty of shims. I did a pressure check and that showed no issues.
The engine has new .040 pistons and rings and that is where things are getting wonky.

While the babbit in the rods was "new" it was rather grey and just looked sort of off. Right away I noticed that there was barely any shim material in the rods. After checking them with plastigauge I came to the conclusion that they were too large and perhaps were for a crank shaft turned down to -.020 or such.
I ordered a set of -.010 rods from Burt's and was able to bolt one onto the crank shaft tonight without the piston just to check the clearance and it was spot on at .015
Earlier I had pulled one of the rings off a piston and placed it in one of the cylinders to check the end gap. It measured .070. From what I understand that is way too much of a gap. So along with my order of new rods I ordered a set of .040 rings too.
I put one of the new rings in a cylinder tonight and low and behold it still measured out at .070
Hmmm...
I measured the cylinders and am coming up with a measurement of 3.935
This has got me thinking that the correct pistons for this engine should be .060 not .040
One more bit of information is a feeler gauge of .032 fits between the piston and cylinder wall.
The guy that has it got the engine from a guy who got it from another guy so finding out the particulars on the engines history is probably out the window.
Will be cutting trees all day tomorrow so I probably will not be able to check back into the Barn until late in the day.
 
You need larger pistons and rings. Be sure you measure the correct side of the piston, which is 90* from the wrist pin ends, and be sure you measure the skirt and not just the top.

I hope you mean the rods have .0015", rather than .015".
 
I have an engine just like yours. The seller said that it was rebuilt. So I put it into my model A, and the piston slap when it was cold, was deafening. When warmed up, it sounded OK. I pulled the head and found excessive piston clearance. In the ring wear area there was way too much clearance. Something like yours, about 0.032". I would assume that my cylinder taper is excessive. I am having H & H engine rebuilders of Cresenton, Ca building me up a new Model A touring short block and I will exchange my old engine with them while I am attending the Bakersfield Swap Meet in April. Your engine probably will need a cylinder bore job to make it reliable.
 
Standard bore is 3.875", so if the cylinders are good, you need .060" over pistons. If the cylinders have taper, then you will need even larger pistons and a rebore. I'd be checking the diameter of the tappets to make sure they aren't too small and allow the cam lobe to strike the edge of the tappet. Check the timing marks and anything else you can think of while you have it apart.
 
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Tom Wesenberg said:
Standard bore is 3.785", so if the cylinders are good, you need .060" over pistons. If the cylinders have taper, then you will need even larger pistons and a rebore. I'd be checking the diameter of the tappets to make sure they aren't too small and allow the cam lobe to strike the edge of the tappet. Check the timing marks and anything else you can think of while you have it apart.
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Hm, I thot standard bore was/is 3.875 ...no ?
 
Hey Dave,
If your clearance is .015...you have problems with rod clearance. Maybe you are trying to say .0015 which would be great...with no shims needed ! But, hey I'm probably as bad with this engine 'math' stuff, as you say you are :)
 
Crosscut

Buy or borrow a bore gauge and measure every cylinder top, middle, and bottom. That will give you a clear picture. I just went through this. Turns out, the pistons were too small, bores had taper etc.
 
I think you should run from that one. To many things wrong. Who ever did this engine did not know what they were doing. Also the valve seats would worry me. They have to be done right or they will be coming out and breaking your pistons, plus they might put some new marks in your cylinder walls.
Also I think you mean .0015 not .015
 
Yes I did mean .0015. Thanks for catching that.
I'm not finding any taper to the bores they seem equal all the way through.
It's looking like the machining on the block was done by someone that knew what they were doing but the pistons and rods by someone else that didn't.
Appreciate all the help. Off to cut trees.......
 
I really admire you for getting out of your comfort zone! Precision measurements can be daunting to some one not familiar with working with them. As you said, you meant .0015 which is 1 1/2 thousandths, .015 is fifteen thousandths, or ten times larger. Watch the zeros!
With any mod.A engine, LOOK FOR CRACKS! Favorite places are the upper LH side of the block, The valve chamber. The upper rear of the block and the web between the cylinder and the valve seat. It can crack in other places but these are the most common. I've been told by rebuilders that to find one new block, they must look through four. If you have a good one, consider yourself lucky.
Terry
 
Everything else on the engine looks really good. I'll check to make certain that the bores are uniform and if they are I will probably pull the trigger and purchase the engine. The asking price is quite fair and it comes with quite a few other parts and pieces too so if for some strange reason the engine turns out to be a wash I should still be okay. Heck its better than going to the Casino and gambling.
Anyone interested in a new set of .040 pistons and rings?
 
I think you may be better off stripping it and have a engine shop measure it all for round and taper and square with really good measuring equipment
 
redmodelt said:
Nickle babbitt should be shinny. Gray babbitt would be lead based. Gray no good!
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This would be the deciding factor for me. Might want to run it by a scrape metal yard, where they have an expensive tool to analyze the babbit content.

As George pointed out watch the valve seats. I installed a lot of them in the 80's and never had a problem, but I would never install one unless there was a good reason to do it, such as a badly recessed seat. It's too bad so many were needlessly installed, and several have fallen out because they weren't installed correctly.
 
1crosscut said:
Everything else on the engine looks really good. I'll check to make certain that the bores are uniform and if they are I will probably pull the trigger and purchase the engine. The asking price is quite fair and it comes with quite a few other parts and pieces too so if for some strange reason the engine turns out to be a wash I should still be okay. Heck its better than going to the Casino and gambling.
Anyone interested in a new set of .040 pistons and rings?
Click to expand...

Hey Dave,
Well, if the price is right, I'd probably go for it also. No cracks in block would make it good block. That alone is worth some good $ ! And, I agree with your position that it keep a guy out of places where you can lose / lose, eh :)

Do you know what brand of pistons you have ? PM me if you wish.

BTW...I agree with Colin, in his statement regarding taking it (block) to a engine building shop for evaluation.
 
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