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mic Newbie
Joined: 11 Nov 2012 Posts: 3
245.08 points
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Posted: Sun Nov 11, 2012 1:15 pm Post subject: Leak Down Results on MHG before start up |
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Still learning here. Should my leak down results be erratic before start up with new MHG? Are heat cycles necessary for proper gasket seal? I'm trying not to complete assembly before deciding whether to deck the block again or do composite gasket. Head was tested, machined .008, new stem seals. Compression was good before dis-assembly. Thanks for sharing your experience. |
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10sec.et Member

Joined: 29 Mar 2006 Posts: 3483 Location: Houston,Texas 347040.52 points
1969 Oldsmobile Cutlass
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Posted: Sun Nov 11, 2012 6:51 pm Post subject: |
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is this a completely new build or did you just change head gaskets ? are you doing leak-down or compression test ? you mention both. perhaps a bit more detailed info on exactly what you are doing would yield a better response . _________________
af2 wrote: | It seems we can look at our magical Balls and come up with a fix?
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af2 Member

Joined: 01 Sep 2003 Posts: 5568 Location: grassvalley, ca 71528.86 points
1933 Willys Coupe
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Posted: Sun Nov 11, 2012 10:42 pm Post subject: |
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10sec.et wrote: | is this a completely new build or did you just change head gaskets ? are you doing leak-down or compression test ? you mention both. perhaps a bit more detailed info on exactly what you are doing would yield a better response . |
Not only what was said but????? What is erratic and what is MHG??
See 10sec's sig first though.  |
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Big Dave Moderator

Joined: 04 Dec 2005 Posts: 2660 Location: Tampa Florida 120444.52 points
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Posted: Mon Nov 12, 2012 12:46 pm Post subject: |
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The Composite Metal Head Gasket should seal completely once torqued . It doesn't require a heat cycle to seal.
If you are getting erratic readings (not sure what you are saying but I assume variation outside of 5% of the average pressure after throwing out the highest and the lowest reading) then your cylinders are not sealing. It could be a warped head or block out of square, or it could be the valves, or a cracked head (or block). If everything has been checked for cracks first you can eliminate that. If you have a new valve job that was done correctly then that can be eliminated leaving heat warped castings.
Big Dave |
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Paul P Member

Joined: 15 Aug 2002 Posts: 2436 Location: Townsend, Mass. 82671.86 points
1971 Chevrolet Chevelle
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Posted: Mon Nov 12, 2012 1:53 pm Post subject: |
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Maybe there is shmeg in the gauge or air line causing fluctuation? _________________ 2001 Focus 2.0 Zetec
stock cams, bolt-ons and tune
15.63@87 MPH 1/4mi
1971 - Chevelle 408 SBC N/A
6.86@102.5 MPH 1/8mi
10.78@122 MPH 1/4mi
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SMOKEmUP PostMaster

Joined: 30 May 2002 Posts: 3171
65149.84 points
1979 Chevrolet Camaro
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Posted: Mon Nov 12, 2012 2:14 pm Post subject: |
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New technical term? _________________ Stop running from your pain and embrace your pain. Your pain is going to be a part of your prize.
I challenge you to push yourself. |
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Paul P Member

Joined: 15 Aug 2002 Posts: 2436 Location: Townsend, Mass. 82671.86 points
1971 Chevrolet Chevelle
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Posted: Mon Nov 12, 2012 3:20 pm Post subject: |
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SMOKEmUP wrote: |
New technical term? |
I guess I should have said gurgling schmeg for technical clarity.  _________________ 2001 Focus 2.0 Zetec
stock cams, bolt-ons and tune
15.63@87 MPH 1/4mi
1971 - Chevelle 408 SBC N/A
6.86@102.5 MPH 1/8mi
10.78@122 MPH 1/4mi
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Big Dave Moderator

Joined: 04 Dec 2005 Posts: 2660 Location: Tampa Florida 120444.52 points
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Posted: Mon Nov 12, 2012 8:36 pm Post subject: |
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I think his finger slipped off the "T" key and hit the "G" key just under it. My hands are old and arthritic and I use that excuse all the time when typing on my manual typewriter. May have to go back to smoke signals if I get any older.
Big Dave |
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squeeezer Member

Joined: 02 Mar 2007 Posts: 2427 Location: new richmond WI 191524.76 points
1991 Chevrolet Camaro
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Posted: Tue Nov 13, 2012 2:00 am Post subject: |
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First q id have is why do you feel you need a leakdown or compression test???
What are you trying to diagnose??? _________________
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mic Newbie
Joined: 11 Nov 2012 Posts: 3
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Posted: Tue Nov 13, 2012 10:02 am Post subject: |
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Yes, I changed head gasket only and was wondering what other peoples leak down test results were before start up on their MLS or composite gaskets.
In my very limited experience it appears a system leak down test, before start up, with a new head gasket and less than perfect block deck may not be a good indicator of gasket seal. I assume that heat activates the cold sealers and some movement help the gasket to seal. Does anyone check a new gaskets seal before proceeding with assembly to avoid tearing it down again?
PS I try to ensure the rings have oil and rock cylinder away from and then back to TDC so compression ring will have some load when I LD test. Thanks |
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Paul P Member

Joined: 15 Aug 2002 Posts: 2436 Location: Townsend, Mass. 82671.86 points
1971 Chevrolet Chevelle
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Posted: Tue Nov 13, 2012 11:55 am Post subject: |
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Leakdown tests should be done when the engine is hot. You should expect less than perfect results when cold but they should be consistent cylinder to cylinder. If the rings are gurgling oil because of the large end gap when cold that could also give you strange results. _________________ 2001 Focus 2.0 Zetec
stock cams, bolt-ons and tune
15.63@87 MPH 1/4mi
1971 - Chevelle 408 SBC N/A
6.86@102.5 MPH 1/8mi
10.78@122 MPH 1/4mi
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mic Newbie
Joined: 11 Nov 2012 Posts: 3
245.08 points
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Posted: Tue Nov 13, 2012 2:53 pm Post subject: |
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Thanks to everyone for the pointers.
I've now tried 3 different HG's (1 used, 2 new) and the pressure leakage as Dave described is well beyond the 5% on all of them. Maybe the deck is warped more than my carpenters square or piece of flat bar steel is showing me. (Anybody know of a cheap decent machinist level to buy or have a better method?) Yet each cylinder showed different results on each gasket. Anyway, I'm going with the composite gasket I'll let you know how it turns out. I'll be trying to stay out my own way so I don't shmeg things up!
I've seen people whose new weeping HG's made them wanna cry. Trying to avoid all that drama of having to dismantle everything again. Hopefully, more torque and less boost, for a while. |
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af2 Member

Joined: 01 Sep 2003 Posts: 5568 Location: grassvalley, ca 71528.86 points
1933 Willys Coupe
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Posted: Tue Nov 13, 2012 7:07 pm Post subject: |
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mic wrote: | Thanks to everyone for the pointers.
I've now tried 3 different HG's (1 used, 2 new) and the pressure leakage as Dave described is well beyond the 5% on all of them. Maybe the deck is warped more than my carpenters square or piece of flat bar steel is showing me. (Anybody know of a cheap decent machinist level to buy or have a better method?) Yet each cylinder showed different results on each gasket. Anyway, I'm going with the composite gasket I'll let you know how it turns out. I'll be trying to stay out my own way so I don't shmeg things up!
I've seen people whose new weeping HG's made them wanna cry. Trying to avoid all that drama of having to dismantle everything again. Hopefully, more torque and less boost, for a while. |
I have never leaked an engine cold.
I have never leaked an engine with new head gaskets cold.
I will never leaked an engine with 100,000 miles cold on it because the effort will always be the same.
So the question was weeeeping head gaskets? I don't think so.
I run MLS gaskets and have had them off 4 times to date to clearance the dome from hitting and closing the plug gap.. Same gaskets by the way. |
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10sec.et Member

Joined: 29 Mar 2006 Posts: 3483 Location: Houston,Texas 347040.52 points
1969 Oldsmobile Cutlass
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Posted: Tue Nov 13, 2012 11:44 pm Post subject: |
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what he said ^^^^^^
spray on a bit of copper coat and torque em down. if you have leak down/compression issues, it wont be the head gaskets. _________________
af2 wrote: | It seems we can look at our magical Balls and come up with a fix?
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Paul P Member

Joined: 15 Aug 2002 Posts: 2436 Location: Townsend, Mass. 82671.86 points
1971 Chevrolet Chevelle
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Posted: Wed Nov 14, 2012 10:30 am Post subject: |
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Big Dave wrote: | I think his finger slipped off the "T" key and hit the "G" key just under it. My hands are old and arthritic and I use that excuse all the time when typing on my manual typewriter. May have to go back to smoke signals if I get any older.
Big Dave |
Schmeg is short for Smegma. Spell check just would not get it right.  _________________ 2001 Focus 2.0 Zetec
stock cams, bolt-ons and tune
15.63@87 MPH 1/4mi
1971 - Chevelle 408 SBC N/A
6.86@102.5 MPH 1/8mi
10.78@122 MPH 1/4mi
 |
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