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Been wanting to check the compression on the m38. So I took all the plugs out and such as the tm said. Installed the compression gauge and only got like sixty pounds. Took the gauge out to see if it could have been plugged since it hasn’t been used in thirty years. It was ok. Put it back in and got zero compression. So here’s where I figured out what an idiot I am. The compression gauge has a couple of adapters on it. The one I used was longer than the short one that should have been used. So now I have no compression on number one and 110 on the other three. The piston doesn’t look like it has any damage so I’m pretty sick about being so stupid. What happened to number one? Did it break a valve etc. oh if it does start, it’s pop crackle stuff. I will check the manual when I’m done pouting. Stupid John jr, arrrgh

Just about all the compression gauges I have ever seen (55 years worth) have only 5 to 10 threads. None of these would be long enough to strike the piston so what exactly did you stick in the plug hole? Post us a photo of this weird gauge.

Wes the gauge is just a regular gauge. The longer adapter was definitely hitting the top of the piston as there is a mark not a dent from it. Also the bottom of the adapter shows that it was hitting. Can’t post pics so let me know if there’s some other way to get them to you. Thanks John jr

Here's John's photos edited by me:

take off the tappet cover and see if the valves are still moving correctly.

If you done see a hole in the piston, there likely isn't one.


one step at a time

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Pretty sure I’m going to have to end up doing that but there’s a lot of stuff to take off to get to it first!

Pretty sure you bent the valve.

First off refer to the manual TM-9 1804A, graciously provided in the downloads above.

First off a long compression gauge adaptor cannot strike the pistons. Period.

Referring to the manual and looking at the cross section views (both). You can see that the spark plugs are located over the valves, not the pistons. You have bent a valve in cylinder number one. They actually bend fairly easily and since yo left a mark and you have zero compression that is what happened.

The good news is that most likely you will not need a full tear down to rectify. Follow the same manual and do the tear down as far as getting all the stuff off of the head and the valve side of the engine. The head will have to come off as will the tappet adjustment cover. But from there on you can replace the bent valve quite easily.

Hum, I cannot reach the pistons from the spark plug holes on any of my engines. Sounds like number 1 exhaust vale is not closing to get the compression. I would loosen that tappet and readjust the tappets

John,

This will illustrate the geometry of the spark plug hole/valves/piston and you should be able to see now that you hit the valve not the piston.

Thanks for all the info guys. John jr

And if you bent the valve the next step is removing the head. Then removing the side valve cover to expedite the valve change.