Markings on Pistons

[aball36]

On the 49 cub, after cleaning off the top of the pistons, the markings became visible. There is a ".040-S" marking which I am assuming means one of the previous owners put in pistons that are 0.040 oversized. There is also another stamp that appears to say "97B", what might that mean?

Thanks,

Adam

[inairam]

The engine looks good. If it has oversized pistons it was rebuilt. Did you do a compression test before you pulled the head? That just may need a new gasket and the replacement head and you are in business.

[aball36]

Thanks for the reply. I did run a compression test first. It ranged from. 50psi to 67psi on the 4 cylinders. Based on some posts I read on this forum, it sounded like compression should have been up around 100psi. I was thinking of pulling the valves out just to look at them as long as I have gone this far.

[inairam]

Did it run? At those numbers I do not think it would start. The block does not look that bad.

Did you adjust the valves before the compression test?
Did you remove all of the spark plugs and open or remove the carb?

[tst]

engine is .040 oversize pistons, suspect 97B is may be a production date ? make sure your compression gauge is good, that engine does not look that bad for those low a readings

[ricky racer]

It doesn't matter what the engine looks like. With the low compression numbers, either the valves weren't seating or the rings or cylinders are worn. I'd measure the bores or drop a piston out, pull a ring off and put it in the bore to measure the end gap if the valves look like they've been seating.
Check the valve clearance and adjust as needed. If the valves were out of adjustment enough that they weren't fully closing, you can rest assured that's where the low numbers are coming from.

[aball36]

Thanks for the messages and confirmation on the oversize pistons.

For background, the tractor was gradually starting to run worse and worse over the last year and I thought I would check compression as long as I was starting to get deeper under the hood. When I gave up on it about 6 months ago, I could not get it started and got sick of charging the battery attempting to start it.

Unfortunately I did not read up on the compression test topic very much before doing the test and based on some of the responses, I am starting realize the test values may not have told the whole story without doing it the proper way. Live and learn the hard way. I went to Harbor Freight, bought one of their tester gizmos and starting popping one plug at a time testing each cylinder dry. I did not do any wet testing and didn't take the carb off as the one inariam had asked. Once I finally managed to get the head off, the block was rough looking, charred black, etc. before I wire wheeled it. I am surprised at how nice it did clean up however and was also surprised that one of the previous owners had put in oversize pistons.

Adam

[Glen]

Hi,
You don't have to remove the carburetor to do a compression test, just open the throttle fully, so the engine can get air.
Have all 4 spark plugs out of the engine at the same time.
Charge the battery fully for the test, and use the starter.

The Cub service manual says a Cub engine has 120 lbs of compression.
That is probably for a new engine. 90 lbs or more is ok, they have said on here.

I'm not sure if the engine ran while you have owned the Cub, or if it smoked when running.
Blue smoke is usually from burning oil.

Since the head is off, you could try moving the pistons side to side in the cylinders and see if they move, or are loose fitting in the cylinders.

You could also measure the clearance of the pistons in the cylinders, that will give us some idea of the wear it has.
Use a flat feeler gauge set, and push them gently down between each piston and cylinder wall.
Use as thick a feeler gauge as will go in the gap.

Do that with the pistons slightly down from the top of the cylinders, then turn the engine so they are down farther, and measure again.
Write down the measurements, so there is a record.

If it did run, and the results of the clearance measuring are good, and the engine has been rebored in the past, you could try putting on another head, and run the engine, and put in an oil additive, and see if the compression improves over time.
It is easier than rebuilding the engine.

They say on here to put Seafoam oil additive in the engine oil.
They sell it at auto parts stores.
Several people on here that used it said that it helped a lot.
They say on here to make the engine work while using the additive.
If the piston rings are stuck, it can help free them.

One of the experts on here said to use the additive for 2 oil change periods.
The 1955 Cub operator's manual says to change the oil every 150 hours of use.
If the rings are wornout, the seafoam won't help much.

[Clemsonfor]

As good as that engine looks at least I would make sure the valves are free. Put a new head on it and run it if it will start and put a load on it for a few hours to see if it comes around. As bad as you said it was I wouldn't be surprised but it has stuck rings.

[Glen]

Hi,
Below is a page from the Cub and LoBoy service manual showing the piston clearance in the cylinders.
When new the clearances are less than .003", so you would be using feeler gauges that can bend to fit in the curved cylinder.
It says skirt clearance bottom, so there's no way to measure the lower part of the pistons from the top, with the rings in place.
The top of the pistons that you can get to might be a different size than down farther, so maybe the only check you can do is moving the pistons side to side in the cylinders.


http://www.farmallcub.info/manuals/gss- ... 001-05.jpg