Technical Chevy 235 leakdown test

I picked up a '58 235 a while back that I plan on using in my '51. It's still on the stand it came on. Anyway, I figured I'd do a leakdown test on it just for the heck of it. There wasn't any air coming from any valves or the oil fill hole in the valve cover. There was air coming out of the road draft tube however. It wasn't enough to feel, but you could hear it. Is this due to the engine being cold, which of course means the rings/pistons are contracted or, are all the rings shot and in need of replacement? I was going to start my swap yesterday but figured I'd better hold off till I figured this out

 

If there was a noise or air coming out the road draft tube, then is it safe to assume that air you are pumping into the cylinders is getting past the rings/pistons into the crankcase. How long has the engine been sitting ? Before I would run a leakdown test, when length of storage is unknown, I would like to put some light oil into the spark plug holes, let it sit for an hour, crank or at least turn engine over several revolutions. If rings/cylinder walls/pistons are totally dry, I don't think you can get a good leakdown reading.
You may get a very slight air sound out the draft tube depending how much air pressure you are using for the leakdown test. Been a really long time since I have done one.
Good news ........the 235 six cylinder is one of the easiest, most simple and forgiving engines in the world to work on........have fun.

 

It's been sitting for who knows how long. It was a Matco gauge but the air pressure gauge only went up to 35. I had the compressor on 100 psi. I've turned the motor several times but I'll try the oil thing.

 

Rings leak air, it's normal. Did you use a leakdown tester, which gives you an excessive percentage leak reading?

If not, don't worry about it...

 

Hey man, how ya been. That's the thing...I used a brand new Matco leak down tester. I followed the instructions to a T and did exactly what they said to do, but when I turned the regulator knob to where it was supposed to zero out the leak down gauge itself, it wouldn't get out of the 80s. This is before I hooked the gauge to the engine so I never got a leakdown reading. At that point, I just used it to force air into the motor and started listening. It wasn't enough air to feel it, I could just hear it. If you think it's ok, I'll go ahead and use it and if it craps out on me, I'll throw my original motor back in.

 

I would not expect to get valid readings until I'd run the engine for a little while. Have you run a compression test? If it gets more than 100 psi in most of the cylinders, it should start, at least

 

The guy I got it from said he did right before I got it and all the cylinders were between 90 and 95

 

give it a try, see how it works. Usually when a few are way down, it will have problems.

 

I suppose I'll give it a shot.

 

I would check compression in all cylinders. I bought a chevy 235 on eBay once. The guy said it ran great. I checked the compression in the first 3 cylinders and called it good. I got 120 in each of the three cylinders. Time went on and eventually I put the engine in the car. It cranked right up and sounded good but it was blowing smoke like crazy. Turns out it was only running on 4 cylinders because 2 pistons had a hole in each one. And I believe the head had some small cracks. The time to return it had run out and I was stuck with it. Now days you can't trust anyone.

Next time I bought an engine that I could listen to it run before it was pulled and it only had 2000 miles on a total rebuild.

On a side not at least I did get several parts I needed from the junk engine and several spare parts.


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Yeah, this guy sent me a couple videos of it idling on the stand but they were super fuzzy. They could have been made years beforehand for all I know, looking back. I guess I could pull the head and look things over, I just don't want to have to machine it to get it back together.

 

If the hissing sound was the same on all 6 cylinders, it's probably ok. If there was a hole in a piston, you'd have heard it

 

I think what I'll do is pull the old motor, put all my stuff on the new motor, install it without the trans and see how she goes. What would it sound like with a hole on a piston...whistling I guess?

 

I don't know but mine was smoking really bad from the oil coming up through the holes in the piston and smoking badly. As said above you probably don't have a hole in a piston. If you did it wouldn't have held any pressure in that one cylinder.


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Leak down your old engine in the car first so you can see it it's working on the first one.

 

Not a bad idea at all. The problem with the gauge is that it itself isn't working properly. The gauge for leakdown goes from 100 down to around 80 but no lower

 

So I went and bought a harbor freight leakdown tester today. I put 90 psi through it into the motor and it had 10 to 18% leakage for all 6 cylinders. I can live with that.