Piston sizes

[Tony Hillyard]

Good Afternoon all,

I have noticed that the spark plugs in my 1928 Model A Roadster seem to be always covered in oil, even when she has been running. I had a feeling I might have a worn motor so I popped the cylinder head off.

The bores seem OK, the pistons look quite newish. I will take the pistons out to check the rings and if necessary replace them. A question please....

Normally the piston sizes is stamped in the top of the piston.... something like +0.10 or perhaps just 0.10. On these pistons they are stamped 002 1/2!!

Any idea what that is all about? Is it 0025 for example?

[Tom Wesenberg]

I think you are right that the pistons are only .0025" over standard, though I've never seen this size before.

Does you engine misfire? Does it use oil?
If neither, then I wouldn't be taking it apart.

When you said the plugs were covered in oil, it sounded like you meant on the outside, and I couldn't imagine that happening.

[George Miller]

.0025 would be 2 and 1/2 thousands

[Tony Hillyard]

Thank you for your replies folks.

No Tom, the engine does not misfire, when I turn her off she usually backfires through the carb. At first I thought ignition timing. I re-checked that several times. Then I took out the plugs and they were very oily. So I popped the head off, found the inside of the head very very oily. So hence my thoughts of taking the pistons out to check that the rings were not broken. If they look a bit iffy I thought I would hone the bores and fit some new rings.

[Tom Wesenberg]

A common misconception is spark timing when you have a backfire or run on when the ignition is turned off.
Since the ignition is OFF, the spark timing should be out of the picture. The only connection might be if the timing is off and caused the cylinder to run so hot that some of the carbon is glowing red and makes the fuel ignite, but this is also very unlikely.
Run on is caused by too high an idle speed on most engine, but with the low compression of the Model A, it's doubtful you'd ever have run on from too high an idle speed.

How did the tops of the pistons look?
Oil getting past the rings should leave the outside diameter of the piston tops pretty clean.

[George Miller]

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Originally Posted by Tom Wesenberg

A common misconception is spark timing when you have a backfire or run on when the ignition is turned off.
Since the ignition is OFF, the spark timing should be out of the picture. The only connection might be if the timing is off and caused the cylinder to run so hot that some of the carbon is glowing red and makes the fuel ignite, but this is also very unlikely.
Run on is caused by too high an idle speed on most engine, but with the low compression of the Model A, it's doubtful you'd ever have run on from too high an idle speed.

How did the tops of the pistons look?
Oil getting past the rings should leave the outside diameter of the piston tops pretty clean.

Plus 1 to what Tom said

[George Miller]

Originally Posted by Tony Hillyard

Good Afternoon all,

I have noticed that the spark plugs in my 1928 Model A Roadster seem to be always covered in oil, even when she has been running. I had a feeling I might have a worn motor so I popped the cylinder head off.

The bores seem OK, the pistons look quite newish. I will take the pistons out to check the rings and if necessary replace them. A question please....

Normally the piston sizes is stamped in the top of the piston.... something like +0.10 or perhaps just 0.10. On these pistons they are stamped 002 1/2!!

Any idea what that is all about? Is it 0025 for example?

Is you engine burning lots of oil? Are you sure it is oil or just to rich. I would not take it all part unless I was sure. Did you check it after a long ride, it should be run at speed for awhile before being checked.

[Tony Hillyard]

The outside of the pistons are very clean. That's what made me think the engine had been rebuilt not long ago. It was all so clean. No apparent wear in the bores.

The exhaust does not indicate she is "burning" oil. Just that there is a lot of oil in the inside of the head. She does not smoke from the exhaust when driven.

Here in France we need to obtain a government certificate to allow us to drive on the road. As I have just imported her from the UK (before that from the US) I needed to get this certificate. Therefore I have not had the chance to drive her on a long run.

It is just that she always backfired when I turned her off. She scared the hell out of our dogs every time this happened.

A strange one this. What appears to be a rebuilt engine. Ultra clean inside the cylinder head, the valves seem to be shutting OK, the timing seems OK, but she still back fires when shutting down!

Odd!

[George Miller]

Originally Posted by Tony Hillyard

The outside of the pistons are very clean. That's what made me think the engine had been rebuilt not long ago. It was all so clean. No apparent wear in the bores.

The exhaust does not indicate she is "burning" oil. Just that there is a lot of oil in the inside of the head. She does not smoke from the exhaust when driven.

Here in France we need to obtain a government certificate to allow us to drive on the road. As I have just imported her from the UK (before that from the US) I needed to get this certificate. Therefore I have not had the chance to drive her on a long run.

It is just that she always backfired when I turned her off. She scared the hell out of our dogs every time this happened.

A strange one this. What appears to be a rebuilt engine. Ultra clean inside the cylinder head, the valves seem to be shutting OK, the timing seems OK, but she still back fires when shutting down!

Odd!

That would not because of the pistons. Some other cause. Late timing along with a exhaust leak at the manifold. Or some other thing.

[Benson]

Besides a leak at manifold ... a hole in muffler will suck air inside and backfire when mixture gets correct if there are any chunks of carbon inside the muffler that are glowing.

I would change the oil if not already done ... maybe the oil is thinner than it should be because it is diluted with gasoline from short trips and lots of starts with short trips where engine never gets hot.

[Tom Wesenberg]

My Cub Cadet used to let out a loud bang about 10 seconds after the engine stopped. Did this for a few years, then one days it just quit doing it.

When oxygen can travel up the exhaust and mix with unburned fuel, it can explode in the muffler. Check you manifold for any leaks.

[Tony Hillyard]

When I first bought her she backfired after she was turned off. So I changed all the oils, fitted a new exhaust manifold and gasket. Plus I fitted a new muffler and clamp.

I replaced the original style points with modern ones, reset the timing several times - always the same backfire when shut down.

It was after that I notice the oily plugs - and you know the rest of the story. So I'm stumped.

I will be away for a couple of days so I have time to think it through again. I will keep you all up to date on what I come up with.

Thanks again for all you help and advice.

[George Miller]

They use to have semi pistons, they would turn them to the size you wanted. In your case some one honed the cylinders enough to clean up. Then they had pistons turned to fit. They would do the same for bearings back in my early days.

[Tony Hillyard]

That's an interesting idea George. I wonder how you would order replacement piston rings for something like that?

[George Miller]

Originally Posted by Tony Hillyard

That's an interesting idea George. I wonder how you would order replacement piston rings for something like that?

For .0025 you would use standard. It would not be a problem. That is how thick one of your hairs are. Not enough to cause any trouble.

[George Miller]

There was a time when you would find a engine that came from the factory with under size bearings. Like .002 in some of the mains, or rods. They did that so they would not have to scrap the crank.
It was not a problem because you could buy .002 under bearings. That is one reason you should check every thing for size when you are working on a engine.

[Tony Hillyard]

Thank you George. You learn something new everyday. I think that is what makes our hobby so interesting.

[Tom Wesenberg]

Even in the 60's I was able to buy .001, .002, .003 bearings from Wards and Sears for my 55 Chevy. JC Whitney used to have a large selection of engine parts, but these days you don't see near as many engines getting work done to them.

[qstott]

In the 60's and 70's the Ford parts book used to refer to the .001 and .002 as "red" and "blue".
Can you imagine a customers facial expression when the parts guy asked if his rod bearings were red or blue?