Home » Public Forums » GMCnet » compression check procedure ( update )
| compression check procedure ( update ) [message #320646] |
Tue, 18 July 2017 10:26  |
joewilly
Messages: 40
Registered: April 2016
Location: Victor,Ia
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results of my compression check (hot)
#1-154 #2-146 #3-154 #4-152 #5-154 #6-146 #7-160 #8-160
overall seems good to me.
any ideas on why #2 & 6 are a little low compared to the rest.
installed new plugs ( E 3.52 ) the old plugs-Bosch platinum looked good but were gaped .080
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
[Updated on: Fri, 21 July 2017 10:19] Report message to a moderator |
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| Re: compression check procedure [message #320650 is a reply to message #320646] |
Tue, 18 July 2017 11:09  |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
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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
Foolish Carriage, 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
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| Re: [GMCnet] compression check procedure [message #320659 is a reply to message #320646] |
Tue, 18 July 2017 19:23 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
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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@list.gmcnet.org] On Behalf Of Joe williams
Sent: Wednesday, July 19, 2017 1:26 AM
To: gmclist@list.gmcnet.org
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
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Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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| Re: [GMCnet] compression check procedure [message #320663 is a reply to message #320659] |
Tue, 18 July 2017 22:31 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
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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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| Re: [GMCnet] compression check procedure [message #320667 is a reply to message #320663] |
Wed, 19 July 2017 02:07 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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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@list.gmcnet.org] On Behalf Of Ken Burton
Sent: Wednesday, July 19, 2017 1:32 PM
To: gmclist@list.gmcnet.org
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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GMCnet mailing list
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Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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| Re: compression check procedure [message #320668 is a reply to message #320646] |
Wed, 19 July 2017 08:03 |
jhbridges
Messages: 8412
Registered: May 2011
Location: Braselton ga
Karma: -74
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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
Foolish Carriage, 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
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| Re: [GMCnet] compression check procedure [message #320670 is a reply to message #320668] |
Wed, 19 July 2017 09:27 |
James Hupy
Messages: 6806
Registered: May 2010
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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" <
gmclist@list.gmcnet.org> wrote:
> 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
>
>
> _______________________________________________
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| Re: [GMCnet] compression check procedure [message #320685 is a reply to message #320670] |
Wed, 19 July 2017 18:15 |
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USAussie
Messages: 15912
Registered: July 2007
Location: Sydney, Australia
Karma: 6
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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@list.gmcnet.org] On Behalf Of James Hupy
Sent: Thursday, July 20, 2017 12:28 AM
To: gmclist@list.gmcnet.org
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
_______________________________________________
GMCnet mailing list
Unsubscribe or Change List Options:
http://list.gmcnet.org/mailman/listinfo/gmclist_list.gmcnet.org
Regards,
Rob M. (USAussie)
The Pedantic Mechanic
Sydney, Australia
'75 Avion - AUS - The Blue Streak TZE365V100428
'75 Avion - USA - Double Trouble TZE365V100426
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| Re: [GMCnet] compression check procedure [message #320696 is a reply to message #320685] |
Wed, 19 July 2017 22:31 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Senior Member |
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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
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| Re: compression check procedure ( update ) [message #320766 is a reply to message #320646] |
Fri, 21 July 2017 10:21 |
joewilly
Messages: 40
Registered: April 2016
Location: Victor,Ia
Karma: -1
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Member |
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results of my compression check (hot)
#1-154 #2-146 #3-154 #4-152 #5-154 #6-146 #7-160 #8-160
overall seems good to me.
any ideas on why #2 & 6 are a little low compared to the rest.
installed new plugs ( E 3.52 ) the old plugs-Bosch platinum looked good but were gaped .080
Joe
Joe Williams
1978 Royale rear twin
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| Re: compression check procedure ( update ) [message #320780 is a reply to message #320646] |
Fri, 21 July 2017 11:50 |
JohnL455
Messages: 4447
Registered: October 2006
Location: Woodstock, IL
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Is see E3 plugs. Personally I would never use those as you cannot set the gap. There is no way of knowing the equvelent working gap and how much stress you are inflicting on the HEI. My two cents as I like to calibrate and recheck my work before moving ahead. As far as engine I would say if no ticks or knocks leave it alone. You can tell a lot about health by sound.
John Lebetski
Woodstock, IL
77 Eleganza II
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