Compression test for 305 v6

[aharper33]

This past weekend I acquired my grandpas 1964 GMC pickup that has a 305 V6 in it. The last time the truck was licences was in 1979 so I would say it probably has not moved since then. I was kinda wanting to do a compression test on each cylinder just to see what they are reading. Does anyone know what the pressures should read? I have been looking around and have not found any exact values.

[AngryPirate]

Originally Posted by aharper33

This past weekend I acquired my grandpas 1964 GMC pickup that has a 305 V6 in it. The last time the truck was licences was in 1979 so I would say it probably has not moved since then. I was kinda wanting to do a compression test on each cylinder just to see what they are reading. Does anyone know what the pressures should read? I have been looking around and have not found any exact values.

If I recall, my 305A v6 (1960) was supposed to be around 120-125 psi .

http://6066gmcguy.com/EngineData.html

I guess the real key is that all of the cylinders should be within 10% of each other.

[truckster]

Before you do the compression test, I would make sure you don't have any stuck rings. Put a shot of penetrating oil in each cylinder and let it sit overnight. Then turn it over by hand. It should turn fairly easily with the plugs removed; if not, you might want to tear it down before you go any farther.

[aharper33]

Thanks for the reply's. Yeah that is the plan to make sure everything turns over first and see what we got. Just trying to get some info before hand so we can make some decisions.

[Alex V.]

Just me, but I'd see if it'll start and run before doing a compression test. If I pulled a compression test on a long-dormant engine and got anything, but not "good enough", I'd still want to start it and see if it's just sticky valves or gunk in the cylinders keeping it from pumping up. If it'll start and run halfway decent, you know it's blown at least most of the debris out of the cylinders. Barring obvious physical decay of the fuel or ignition system, it's not always really involved to just get the engine to fire and run for a few minutes.

[68gmsee]

Good replies above. Lubing the cylinders and letting it set overnight is a good idea.

After insuring it turns over freely maybe shoot some transmission fluid or light weight oil with oil squirt can and crank it without the plugs using a good, fresh battery. This will shoot out most of the excess oil out of the cylinders.

Then, also as mentioned above don't rely too heavily on the compression test until you are able to run the engine for a while. A lot of components may be sluggish like valves, lifters or rings gummed up that can give false readings.

[aharper33]

Definitely good ideas Thanks. This might be one of those projects that takes longer than some since we dont have a time limit but I want to do it right the first time. Still alot to learn.

[GRX]

120-125 psi sounds about right.

> IMPORTANT <

Be sure to blow the spark plug pockets out well with compressed air before removing them. As you will notice, the pockets are upward facing and often filled with debris. Saw a lot of these GMC v-6 for fleet service at the shop back in the day and scored cylinders from dirt was the demise of many.

[Dead Parrot]

Before any turning or cranking, I would change the oil and filter and check for water and/or coolant. And drain whatever used to be coolant and replace with water first. If it runs good, you can swap in the proper amount of antifreeze. Leave the cap off and check for oil in the water during your test run. You really don't want to put pressure on 40 year old hoses.

Take the distributor cap off and clean the cap contacts plus give the points a quick light sanding. A thin nail file or strip of fine sandpaper works great.

[TJ's Chevy]

Anything from 105 to 125 is what to look for(125 is the spec but due to wear 105 is about the lowest you'd want to be ok with.) I would also adjust the valves as it is common for them to be set to tight. .012 intake and .018 exhaust.

[68gmsee]

Originally Posted by GRX

.....

> IMPORTANT <

Be sure to blow the spark plug pockets out well with compressed air before removing them. ....

Spot on! The only thing I didn't like about these engines is the location of the spark plugs. Easy to access but debris collectors.

Back in late 60's while working in the oil fields, the boss had a GMC with one of these v6's in it. It developed a fuel leak and the fuel leaked on the spark plugs. Needless to say it was no fun but we did mange put the fire out before it did any real damage.

Keep a close eye out for fuel leaks!

[aharper33]

Originally Posted by GRX

120-125 psi sounds about right.

> IMPORTANT <

Be sure to blow the spark plug pockets out well with compressed air before removing them. As you will notice, the pockets are upward facing and often filled with debris. Saw a lot of these GMC v-6 for fleet service at the shop back in the day and scored cylinders from dirt was the demise of many.

This is the first thing the guy who had the truck told us. He has been a mechanic since the 60's and his dad actually sold my grandpa the truck originally. He is the only one who has ever worked on it.