Here are the numbers from my compression check.

1 = 380 - 2 = 340
3 = 380 - 4 = 320
5 = 400 - 6 = 320
7 = 280 - 8 = 360

I have read that all cylinders should be within 10% of the highest. Is this number a magical rule of thumb or an actual value from GM?

#7 is obviously a concern as it is only 70% of the best cylinder. # 4 & #6 are 80%. All others are within the 10% allowance.

What are the possible causes of the lower compression values?
I will be performing a head gasket/cracked head check on the weekend. Could a bad head or gasket influence these numbers to this extent?

As it shows in my signature – this is a rebuilt engine with just under 7,000 miles.

Thanks

NOTE: These were values were taken from the glowplug hole with all other glowplugs removed.

#7 is very low. could be a bad head gasket that blew out there causing low compression. other than that i don't know. just throwing in my $0.02

Here are the numbers from my compression check.

1 = 380 - 2 = 340
3 = 380 - 4 = 320
5 = 400 - 6 = 320
7 = 280 - 8 = 360

I have read that all cylinders should be within 10% of the highest. Is this number a magical rule of thumb or an actual value from GM?

#7 is obviously a concern as it is only 70% of the best cylinder. # 4 & #6 are 80%. All others are within the 10% allowance.

What are the possible causes of the lower compression values?
I will be performing a head gasket/cracked head check on the weekend. Could a bad head or gasket influence these numbers to this extent?

As it shows in my signature – this is a rebuilt engine with just under 7,000 miles.

Thanks

NOTE: These were values were taken from the glowplug hole with all other glowplugs removed.

I'm no expert, but it may be because your engine isn't fully broken in yet. How does it run?

Nick

#7 is low and may be a problem, but I had similar readings on my last 6.5, a GM long block with about 100K on it and never had any problems with it. If yours is noisy, smokes bad, burns lots of oil, has coolant in the oil or is low on power, then its time to worry. If it runs well, then I'd just drive it.

If were I & there was a valid warrantyi would be getting the truck to the rebuilder.IMO the compression is poor & not going to normalize. It's your money spent & you deserve value for money spent. Not a time to be kind & gentle,but direct with the provider about the problem..