VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1912-1928 Compression Tester

Hello All,

Does anyone know where to get a compression tester for the older 4 cyl. engines. I have been searching, but can not find one with an adapter large enough to fit the in the cavernous hole in the head for the huge diameter spark plugs. I have some run out plugs that I have been thinking about drilling out and threading. Any suggestions?

Go to a plumbing store and buy pipe reducers and stack them to get the size down to fit your compression tool.

After reading your question, I know that years ago they made adapters for fit 14MM spark plugs to fit the older 7/8 spark plug hole. I checked online and found this website https://www.sparkplugadapters.com/products-1

Excellent! Thank you so much for the info. I hope you have a very HAPPY NEW YEAR!

I took an old spark plug, broke out the ceramic centered and used the spark plug base into the head leaving a much smaller opening to press the compression gauge into

Qman, I was thinking about that as well. I purchased the plug adapters that max-parts recommended. I purchased 5, 1 for the compression tester, 4 more to play with a better plug setup than the champions I am running now.

maybe sacrilege posting this here but i got my adapter from the model a store since they use the same sized plugs

https://modelastore.com/index.php?route=product/product&product_id=12635

It is crazy how many parts were interchangeable back in the day!

So is the thread in the head an actual pipe thread? I am just trying to get educated on this!

In the link posted above by MarxParts there is a 1/2" NPT and a 3/4" NPT adapter listed so apparently there were vehicles that used plugs with those threads but I have no idea of the years or manufacturer. My research tells me that early on European Mfgs used straight threaded plugs with a gasket but American cars at first were made with an "iron-pipe" thread and the reason being because it was cheaper to manufacture. Source : Automobile Engineering by The American Technical Society-Chicago USA 1934 Vol. 5 pg 78-79

The Chevy 4 engine uses a 7/8 straight fine thread spark plug. The 1/2 inch pipe thread spark plugs were used in a lot of the small hit and miss engine. The 3/4 inch pipe plugs were used in very old large industrial engines. The most common engine to use 1/2 pipe thread spark plugs was the Ford Model T engine.

Just remember that the plugs on these old style cylinder heads do not go down into the combustion chamber nearly as far as newer engines. Any spacer/adapter only removes the spark out of the combustion chamber further. It will burn much slower, reducing power and fuel efficiency on an already low hp powered engine. So it will be a trade off, and may require some experimentation. You may be able to compensate some by advancing the spark timing a little further.

I believe the discussion about adapters is just to be able to connect a compression tester hose to the 7/8 straight fine thread spark plug hole.

Not to use other spark plugs in the engine.

Cheers, Dean

I made an adapter some years back. I used the bottom part of an old Champion C4 spark plug. The C4 is a three piece spark plug that was used back then in those old engines. Simply plug the inside with a piece of 3/4 rod. I just JB Welded it in. Then drill through and tap to fit the new spark plug. If I were doing it today, I would 3D print the insert and glue it in. Should be strong enough, I think. Another method would be to melt it full of lead or solder and drill and tap that. Even filling it with JB Weld should work.

Dean, I mentioned experimenting with hotter plugs. I have found the engine flattens out in 3rd. and any further acceleration is nil. Timing is very close to spot on, the flywheel was incorrectly installed when I purchased her. I can only time her by ear, until I pull the motor and trans. The pull should be coming soon, until then I want to see if a hotter spark would help.

I am planning on doing a compression test this week, any ideas on what the proper range should be?

The flywheel was installed incorrectly in my '28 Canopy Express too.

With the oil pan off, I hand cranked the engine until cylinder one was in the top dead center (TDC) position during the compression stroke. Then, I carefully painted a fluorescent pink line on the flywheel through the sighting hole so that the pointer was pointing to the new pink line.

I've been timing the engine for the last 10 years using that pink mark. ;-) I set my timing with the timing lever in the full retard position and turn the distributor so the points are on the brink of opening to send a spark to cylinder one.

This procedure results in an easy starting engine. Once started for a few moments, I then move the spark lever fully advanced and drive away. I've never worried about the exact timing in the full advance position. The engine seems to run fine.

I have to admit, though, that I don't push it to see how it performs at the top end.

BTW, I'm using the Champion W89D spark plugs gaped at .035".

Cheers, Dean