Simple cylindar / piston clearance test

[pflaming]

To check out the cylinders on my block I was asked to load it up and bring it in. So I asked a different machine shop if that was the only way. He said: "Take a spark plug or other type feeler gauge, place it in the cylinder about an inch and a half, place the piston in upside down and measure how much clearance there is on the sides opposite the wrist pin holes, as the piston goes into the cylinder. If there is 15 thousands or less clearance your chances of a good cylinder are good, if less the 10 thousands even better. If over 15 thousands, too much."

I tested all six pistons with a 10 thousands feeler gauge on all six cyliders and could not get one piston into any cylinder so my tolerances are less then 10 thousands. Nice to have a machinist who gives less expensive options.

Now I know that this does not tell all, but it worked for a lot of people in the 40's and 50's and that is meritorious.

What success have others of you had with such an approach?

[grey beard]

Paul,

You're wasting your time trying to find cylinder wall taper without the right tools. Cylinders wear most at the very top of their travel. This is so because it is that area that is hottest - where combustion occurs - and where lubrication is at its least. It is for this reason that when blocks wear, a "ring ridge" is left behind to show the wear. The ridge is simply the size the original cylinder was before it began to wear - the point at which the top piston ring stopped in its travel.

For the same reason, the top ring always wears first and most, and the top ring land or groove always wears most. In order to check cylinder wall taper, it is necessary to get a zero wear reading from the bottom of the cylinder where there is no wear, and then compare it with that one spot just 1/8 or 1/4-inch below the top ring ridge.

Having said all that, when an engine exhibits a ring ridge sufficiently predominant enough for you to catch a finger nail on it, you know before you take it apart that the cylinders must be bored - the only cure for cylinder taper and/or out-of-round.

Paul, e-mail direct off-forum. If your need justifies it in your mind, I have a cylinder bore gauge that I can send to you, so you'll know what you're dealing with. Your only cost would be priority mail both ways - likely ten bucks or so.

Your call.

Good Luck

[TodFitch]

The test for piston clearance that you describe sounds like that given in generic engine repair manuals from the 1920s and early 1930s.

Another one, for piston ring gap clearance is to push the ring into the cylinder using the piston then measure the gap.

And finally, the 1920s way to measure taper was to do the ring gap test at different heights in the cylinder to see if they were all the same.

But as Grey Beard notes, there are better tools for doing that nowadays.

[MBF]

Having just done the engine in my 78 Dodge half ton, I can personally attest to what GB said. I did the ring gap test at 3 predetermined depths in each cylinder. No where near as accurate as doing it with a dial bore indicator, but it (along with the top ridge) showed that it needed to be bored and oversized pistons installed. Mike

[Tony WestOZ]

Personally 10 thou is to much. When you compare that to the Dodge spec of 2 thou with 4 to 6 pound pull force on the feeler gauge.

I pulled a flaty down last year. Genuine 1oo,ooo miles on it. Had between 6 to 7 thou wear. It had quite a ridge on it. Had to de-ridge it to get the pistons out without breaking the rings and damaging the pistons.

As the bore was in surprisingly good condition if I had to I could fit new rings and go again, but only if I had too.

Bore gauge is the only way to go to get an accurate measurement.

[pflaming]

I just had the pistons and wrist pins balanced. This shop offered to measure the piston cylinders on their professional machine, so the block goes in tomorrow. The cylinders have no ridge so that is promising. As a novice, as you all know, I ask a lot of 'common' questions and I am learning. This has been a great experience and I am fully appreciative of the help and offers made. I want to end up with a solid truck, but not a perfect one.

Special thank you to greybeard.