compression check procedure

J

joe williams

New member
Oct 31, 2016
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Hello again,
I built a set of the 2 x 4 jack stands ( thanks Steve )and they work great, got the cv boot changed , front end greased, oil changed , and now going
to change plugs and thought I'd do a compression check to get a baseline for the motor. I've never done one before so what's the best procedure, hot,
warm, cold,manual says to disconnect the HEI,do I just remove wire clips on side of dist? I don't want to do something stupid and screw up the HEI
what should I expect for results on a 403 with 69,000 mi
thanks again for all your help
--
Joe Williams
1978 Royale rear twin
 
First have the engine warmed up. Remove all spark plugs. Unplug the primary
connector on the hei distributor. Remove air cleaner. Open the throttles
wide open. Crank the engine, starting with # 1, through 5 compression
strokes. Record reading. Proceed through the remaining 7 cylinders.
On a thoroughly warmed up engine, you should see at least 140 psi or higher
on your gage. If adjacent cylinders read low (under 100 psi) you more than
likely have a head gasket issue.
After you complete the test, then repeat except squirt a teaspoon of
30 wt. oil into the plug hole before cranking the engine. If the
compression jumps 20 % or more, you might have poor ring sealing. No change
or slight change is o.k.
Jim Hupy
Salem, Or
78 GMC ROYALE 403

Hello again,
I built a set of the 2 x 4 jack stands ( thanks Steve )and they work great,
got the cv boot changed , front end greased, oil changed , and now going
to change plugs and thought I'd do a compression check to get a baseline
for the motor. I've never done one before so what's the best procedure, hot,
warm, cold,manual says to disconnect the HEI,do I just remove wire clips on
side of dist? I don't want to do something stupid and screw up the HEI
what should I expect for results on a 403 with 69,000 mi
thanks again for all your help
--
Joe Williams
1978 Royale rear twin

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Warm the engine to normal temperature. Shut it down and remove the spark plugs. remove the power to the HEI (unplug the connector and leave it
loose). Block the throttle wide open. Test each cylinder in turn by spinning the engine with the starter and recording the gauge reading. You can
write the reading on the valve cover with a magic marker. All should be within ten percent.

Quick and dirty test. Remove the HEI power connector. Spin the engine on the starter with the spark plugs installed. If the engine cranks ast an
even speed, all cylindwers are close to the same. If there's a repetitive speed up every two revs, there's something causing low compression on a
cylinder.

--johnny

--
76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
"I forgive them all, save those who hurt the dogs. They must answer to me in hell" - ol Andy, paraphrased
 
Joe,

I reckon it would be interesting to see what compression pressures were with the engine cold and hot.

Regards,
Rob M.
Sydney, Australia
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426
USA '77 Kingsley - TZE 267V100808


-----Original Message-----
From: Gmclist [mailto:gmclist-bounces] On Behalf Of Joe williams
Sent: Wednesday, July 19, 2017 1:26 AM
To: gmclist
Subject: [GMCnet] compression check procedure

Hello again,
I built a set of the 2 x 4 jack stands ( thanks Steve )and they work great, got the cv boot changed , front end greased, oil changed
, and now going to change plugs and thought I'd do a compression check to get a baseline for the motor. I've never done one before
so what's the best procedure, hot, warm, cold, manual says to disconnect the HEI,do I just remove wire clips on side of dist? I
don't want to do something stupid and screw up the HEI what should I expect for results on a 403 with 69,000 mi.
thanks again for all your help
--
Joe Williams
 
Rob, On airplanes we usually run differential compression tests cold. If we do not like what we see we run them again hot. If it is still weak, we
tell the owner to go run the engine for 10 flying hours and we will check it again. If after 10 hours, it is still bad / weak, we try to determine
if it is valves or rings. This is usually done with air pressure and listening for leaks at the carb (intake valve), exhaust pipe (exhaust valve) or
oil breather (rings). We try to do the air thing with the piston at TDC.

Occasionally we also add oil to the cylinder when checking to see if the rings seal better.

If all of the above fail, we remove the cylinder (with piston) for further diagnosis / rebuild. Luckily for us we have a cylinder shop in the next
county. It is one of 3 in the country.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Jim H covered it well. Just got to the Twin Cities from Northern Minnesota. There's electricity, phone service all the time, and the internet HERE. So call tomorrow if you want. I've been having Chinese junk parts failing on a daily basis. At this point it takes Dave Roberts and myself to just start my GMC to get home tomorrow. REALLY. TWO of US. One more start in the morning. Dave, and Karen's GMC is still in Duluth with a broken FD, and trans. So their riding with us to go home to Cedar Rapids. Then their going back North. I even had a Chinese muffler hanger fail. And put my two new mufflers on the ground. This is a HOBBY. RIGHT? Bob Dunahugh
 
Ken,

The reason I brought up the cold compression test is that Dick Paterson noted there was nothing wrong with doing it that way; it
would be a bit low but as long as there wasn't a big variance from cylinder to cylinder (he defined big in percent but I don't
remember what it was) it would be OK.

Regards,
Rob M.
Sydney, Australia
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426
USA '77 Kingsley - TZE 267V100808


-----Original Message-----
From: Gmclist [mailto:gmclist-bounces] On Behalf Of Ken Burton
Sent: Wednesday, July 19, 2017 1:32 PM
To: gmclist
Subject: Re: [GMCnet] compression check procedure

Rob, On airplanes we usually run differential compression tests cold. If we do not like what we see we run them again hot. If it
is still weak, we tell the owner to go run the engine for 10 flying hours and we will check it again. If after 10 hours, it is
still bad / weak, we try to determine if it is valves or rings. This is usually done with air pressure and listening for leaks at
the carb (intake valve), exhaust pipe (exhaust valve) or oil breather (rings). We try to do the air thing with the piston at TDC.

Occasionally we also add oil to the cylinder when checking to see if the rings seal better.

If all of the above fail, we remove the cylinder (with piston) for further diagnosis / rebuild. Luckily for us we have a cylinder
shop in the next county. It is one of 3 in the country.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana

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Ken - we did tests on flat motors with one of us holding the prop at TDC and rocking it slightly. If we found a low cylinder, we'd 'stake' the
valves - whack the rocker with a soft face hammer pretty good. Still low, listen for leaks. We had the means of cylinder reqorking in-house (Mac's
Engine Service, now long gone).

--johnny
--
76 26' Eleganza(?) with beaucoup mods and add - ons.
Braselton, Ga.
"I forgive them all, save those who hurt the dogs. They must answer to me in hell" - ol Andy, paraphrased
 
Rob, there are a few good reasons for performing the compression tests on a
thoroughly warmed up engine.
1. Oil is thin and all parts are recently lubricated.
2. Engine turns over more readily, a bit faster perhaps, yielding higher
readings.
3. Valves are more likely to close fully and with more vigor, resulting in
higher readings.
There are perhaps more than this, but you get the point.
Jim Hupy

On Jul 19, 2017 6:13 AM, "Johnny Bridges via Gmclist" <

> Ken - we did tests on flat motors with one of us holding the prop at TDC
> and rocking it slightly. If we found a low cylinder, we'd 'stake' the
> valves - whack the rocker with a soft face hammer pretty good. Still low,
> listen for leaks. We had the means of cylinder reqorking in-house (Mac's
> Engine Service, now long gone).
>
> --johnny
> --
> 76 26' Eleganza(?) with beaucoup mods and add - ons.
> Braselton, Ga.
> "I forgive them all, save those who hurt the dogs. They must answer to me
> in hell" - ol Andy, paraphrased
>
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
>
 
Jim,

I agree with you.

The reason I asked for a cold and hot test is I was curious to see what the difference was for comparison and reference.

Regards,
Rob M.
Sydney, Australia
AUS '75 Avion - The Blue Streak TZE365V100428
USA '75 Avion - Double Trouble TZE365V100426
USA '77 Kingsley - TZE 267V100808


-----Original Message-----
From: Gmclist [mailto:gmclist-bounces] On Behalf Of James Hupy
Sent: Thursday, July 20, 2017 12:28 AM
To: gmclist
Subject: Re: [GMCnet] compression check procedure

Rob, there are a few good reasons for performing the compression tests on a
thoroughly warmed up engine.
1. Oil is thin and all parts are recently lubricated.
2. Engine turns over more readily, a bit faster perhaps, yielding higher readings.
3. Valves are more likely to close fully and with more vigor, resulting in higher readings.
There are perhaps more than this, but you get the point.
Jim Hupy
 
Hot should give a more accurate reading. We do it cold because it is easier. If it is good cold, it should also be good hot while the reverse is not
true.

Also since what we are doing effectively a controlled leak test (differential compression) we are not concerned with the condition of the battery, the
weight of the oil, or the rate that the engine is turning. We apply 80 psi of air and read the amount of leakage on a second gauge without the engine
turning the engine. In the end it is more accurate since all of the variables are gone. We also do this every year or every 100 hours and log it so
we have a history on every cylinder.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Joe. My 403 with 152,000 miles had 130 to135 PSI. The 1880 mile 403 was at 140 to 145 PSI. Both done cold with closed throttle. Just an FYI

78 Royale

Bob Dunahugh
 
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