The 1947 - Present Chevrolet & GMC Truck Message Board Network







Register or Log In To remove these advertisements.

Go Back   The 1947 - Present Chevrolet & GMC Truck Message Board Network > General Truck Forums > Engine & Drivetrain

Web 67-72chevytrucks.com


Reply
 
Thread Tools Display Modes
Old 10-31-2017, 02:27 PM   #1
aharper33
Registered User
 
Join Date: Jan 2016
Location: Laramie, WY
Posts: 41
Compression test for 305 v6

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.
aharper33 is offline   Reply With Quote
Old 10-31-2017, 02:48 PM   #2
AngryPirate
Senior Member
 
AngryPirate's Avatar
 
Join Date: Jan 2012
Location: Austin, TX
Posts: 428
Re: Compression test for 305 v6

Quote:
Originally Posted by aharper33 View Post
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.
AngryPirate is offline   Reply With Quote
Old 10-31-2017, 03:10 PM   #3
truckster
Senior Member
 
truckster's Avatar
 
Join Date: Jan 2001
Location: Orem, Utah
Posts: 3,757
Re: Compression test for 305 v6

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.
__________________
If I didn't build it or fix it, it's probably not mine.
truckster is offline   Reply With Quote
Old 10-31-2017, 05:47 PM   #4
aharper33
Registered User
 
Join Date: Jan 2016
Location: Laramie, WY
Posts: 41
Re: Compression test for 305 v6

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.
aharper33 is offline   Reply With Quote
Old 11-01-2017, 10:01 AM   #5
Alex V.
Registered User
 
Alex V.'s Avatar
 
Join Date: Mar 2013
Location: Campbellsville, KY
Posts: 765
Re: Compression test for 305 v6

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.
__________________
Alex V.
------
1967 C10 Suburban, 350/NP435, Green/Green, PS, PB, HD cooling, charging, shocks, and springs.

1985 GMC C3500 SRW, Sierra Classic, 454/TH400, white/blue.
Alex V. is offline   Reply With Quote
Old 11-01-2017, 11:15 AM   #6
68gmsee
Active Member
 
68gmsee's Avatar
 
Join Date: Jul 2007
Location: Centrally located between Houston, Austin and Waco. BCS area.
Posts: 7,739
Re: Compression test for 305 v6

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.
__________________
68 GMC 250/3 speed Saginaw p/b p/s
69 Chevy 350/350 currently in pieces still lookin for a cab
06 Trailblazer
I just want a vehicle that I can work on, that won't talk to me, leave error msgs or keep track of how I drive...
68gmsee is offline   Reply With Quote
Old 11-01-2017, 11:25 AM   #7
aharper33
Registered User
 
Join Date: Jan 2016
Location: Laramie, WY
Posts: 41
Re: Compression test for 305 v6

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.
aharper33 is offline   Reply With Quote
Old 11-01-2017, 07:31 PM   #8
GRX
Registered User
 
GRX's Avatar
 
Join Date: Jul 2010
Location: MD
Posts: 1,835
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.
__________________
1969 c-10 Step Side Long Bed. I-6 250cid = = 1969 Pontiac GTO hard top. 400, 4-speed.
GRX is offline   Reply With Quote
Old 11-01-2017, 08:25 PM   #9
Dead Parrot
Registered User
 
Join Date: Aug 2012
Location: Oklahoma City, OK
Posts: 754
Re: Compression test for 305 v6

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.
Dead Parrot is offline   Reply With Quote
Old 11-02-2017, 12:21 AM   #10
TJ's Chevy
Chevy I6 and GMC V6 Nuts
 
TJ's Chevy's Avatar
 
Join Date: Mar 2013
Location: Next to my bench at the shop, Oregon.
Posts: 10,013
Re: Compression test for 305 v6

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.
__________________
"6 in a row makes it go!"
"I prefer a buzzin half-dozen"
1966 Chevy C10 "Project Two Tone" http://67-72chevytrucks.com/vboard/s...d.php?t=596643
1964 GMC "Crustine" semi-build:http://67-72chevytrucks.com/vboard/s...d.php?t=665056
My youtube channel. Username "Military Chevy": https://www.youtube.com/channel/UC_h...fzpcUXyK_5-uiw
TJ's Chevy is offline   Reply With Quote
Old 11-02-2017, 09:02 AM   #11
68gmsee
Active Member
 
68gmsee's Avatar
 
Join Date: Jul 2007
Location: Centrally located between Houston, Austin and Waco. BCS area.
Posts: 7,739
Re: Compression test for 305 v6

Quote:
Originally Posted by GRX View Post
.....

> 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!
__________________
68 GMC 250/3 speed Saginaw p/b p/s
69 Chevy 350/350 currently in pieces still lookin for a cab
06 Trailblazer
I just want a vehicle that I can work on, that won't talk to me, leave error msgs or keep track of how I drive...
68gmsee is offline   Reply With Quote
Old 11-02-2017, 09:19 AM   #12
aharper33
Registered User
 
Join Date: Jan 2016
Location: Laramie, WY
Posts: 41
Re: Compression test for 305 v6

Quote:
Originally Posted by GRX View Post
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.
aharper33 is offline   Reply With Quote
Reply

Bookmarks

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -4. The time now is 07:17 PM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Copyright 1997-2013 67-72chevytrucks.com