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The 39 120 had been running nicely, but yesterday fired it up and it sounded like a motor boat again. I pulled the plugs and ran a compression test on them (10 mm plugs). The results were:
1.75 psi
2. 60
3. 70
4. 60
5. couldn't get a reading as the adapter wouldn't thread!
6. 60
7. 55
8. 53

First question - are these excessively low? I am going to rerun the test tomorrow and squirt some oil in each hole and see if the rings are worn.

Second - it there a 10 mm, US metric heli-coil available?

I've got a bad feeling I have a valve job in the future. thanks

As a guy who is in the middle of one, I will tell you the valve job is not as hard as it sounds. I'm in the middle of doing an in block overhaul of my engine. I'm learning a lot and asking a lot of stupid questions but it is fun and not as hard as I thought it would be. The real proof will be in a few weeks when I fire it up again.

Ken - is the engine still in the car? That's the part I hate to look forward to. I pulled the engine on my 51 a few winters ago since the engine was toast, but at least there it's easy to remove the hood and yank the engine. The 39 looks considerably more difficult to get to the point of removing the engine.

PS - Go Navy! 3 years with the boat people and 21 with the CEC/Seabees after I found out I was a lousy sailor!

Dave,

1. Yes, your compression values are extremely low.

2. 10mm spark plug thread repairs were bread and butter jobs for repair shops and there were special inserts though not of the typical "Helicoil" type in that they had an upper flange that seated on the top of the hole. I don't know where to get them but suspect they are still readily available.

Rethreading for 14mms plugs is really a last resort method that I'd try my best to avoid if possible.

The drop off of the last 2 cyls due to batt voltage and cranking speed drop off or did you maintain batt state with a charger?

The engine is still in the car. I pulled it out a few years ago when I was sandblasting the frame, but my current garage is small so I decided to do an in-frame overhaul. Plus the block is cracked so I didn't want to put a lot of money into machine work since I don't know how long the block will last once I patch the crack. If you are just doing a valve grind, I think you could pull the hood, right fender and inner fender, and the head off to do an in-place valve grind. I'm sure the experts with more experience that I have will be able to give you even more time saving tips.

First, try a 10mm tap or thread chaser into the hole. From there determine how well a 10 mm bolt will screw into the hole. The threads in the hole maybe just carboned up. Put some grease on the end of the tap to keep debris from falling into the cylinder.

Lots of part sources and information on spark plug repair:

google.com/?gws_rd=ssl#q=spark+plug+thread+repair+kit

Good news - only problem was carbon in the spark plug hole. Ran the 10mm tap thru it and it cured the issue. I also redid the adapter for the compression tester, using a a 1/2" x 1/4" FPT coupling, a 1/8" x 2" brass nipple, NF, and a pipe thread to compression fitting adapter. I put the nipple in the vise and tapped it down to 10 mm, 1.25. I then cleaned it up and soldered it into the FPT fitting and screwed that into the adapter so that I could hook up the compression tester. Found out that you get low readings if you don't put the rubber washer in the adapter before screwing in the tester. Once I did that I got readings of 70-75 on all eight cylinders. I tested them again, squirting oil into the cylinders to see if the pressure would go up. It didn't change, so the good news is that the rings are still ok. The pressure should be closer to 80 or 85 if the engine was overhauled. But the good news is that is now running smoothly again.

I'll attach a few photos of the adapter for the 10mm plugs this evening when the photos show up the computer.

Here are some close up photos of the adapter I made for the compression tester. The adapter is the brass and chrome thing.

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