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Junkyard engine- lower compression on one cylinder

  • Thread starter Thread starter PDHSB
  • Start date Start date Apr 9, 2021

PDHSB

Member
Jun 11, 2020
37
5
18
VA
Apr 9, 2021
#1
  • Apr 9, 2021
  • #1
I just got a junkyard engine, compression tested it out of the car. Results are 180psi on all cylinders but #4, sitting at about 150. Is it fine to just bolt on my heads to this short block or should I just rebuild this one?
 

nickyb

I gotta say i never painted my nuts, Never Ever
15 Year Member
Apr 3, 2009
2,979
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nevada
Apr 9, 2021
#2
  • Apr 9, 2021
  • #2
Let's see,it's out , it needs some help, yes I vote yes rebuild her. We can rebuild her better then before,she'll run better,faster , longer.
Parts list, new pistons( yourchoice), rings, rod bolts, rod,main,cam bearings, oil pump( standard press.) , cam,lifters,gasket kit.Oh a good machine shop.
This is my very own opinion , I'm sure you'll get more. Happy motoring.
 

7991LXnSHO

wanna catch the space herp
10 Year Member
Sep 1, 2010
7,387
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Kearney, NE
Apr 9, 2021
#3
  • Apr 9, 2021
  • #3
On another thread I was outvoted about someone running a wet compression test and doing a leak down test with similar numbers. It was running fine in the car, but the owner was just testing for due diligence.
I still say finish the tests to see why the compression is down (is it leaking out the valves, past the rings, or past a gasket?), then also tell us your intended use, how long you want it to run and your budget.
 
Reactions: stormsedge

PDHSB

Member
Jun 11, 2020
37
5
18
VA
Apr 9, 2021
#4
  • Apr 9, 2021
  • #4
7991LXnSHO said:
On another thread I was outvoted about someone running a wet compression test and doing a leak down test with similar numbers. It was running fine in the car, but the owner was just testing for due diligence.
I still say finish the tests to see why the compression is down (is it leaking out the valves, past the rings, or past a gasket?), then also tell us your intended use, how long you want it to run and your budget.
Click to expand...
I intend to run it like this just for the Summer, I should have enough saved for a forged rotating assembly by then so it's inevitably coming back apart.

I dropped maybe 1.5 teaspoons of oil in the cylinder, compression went up to 185ish. So in thinking rings. I just don't want to spend an extra month without my car, I'm anxious to finally bolt on my ported heads and enjoy it for once. If it's bad enough I'll rebuild it, but if it'll get me through the summer I'm fine as long as it'll let me get over that 300hp I've been aiming for.
 

7991LXnSHO

wanna catch the space herp
10 Year Member
Sep 1, 2010
7,387
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Kearney, NE
Apr 9, 2021
#5
  • Apr 9, 2021
  • #5
PDHSB said:
I intend to run it like this just for the Summer, I should have enough saved for a forged rotating assembly by then so it's inevitably coming back apart.

I dropped maybe 1.5 teaspoons of oil in the cylinder, compression went up to 185ish. So in thinking rings. I just don't want to spend an extra month without my car, I'm anxious to finally bolt on my ported heads and enjoy it for once. If it's bad enough I'll rebuild it, but if it'll get me through the summer I'm fine as long as it'll let me get over that 300hp I've been aiming for.
Click to expand...
Run it for now - if it has oil pressure.
 

PDHSB

Member
Jun 11, 2020
37
5
18
VA
Apr 9, 2021
#6
  • Apr 9, 2021
  • #6
7991LXnSHO said:
Run it for now - if it has oil pressure.
Click to expand...
Everything but the shortblock is getting torn down to be replaced with my ported heads and cams, so that won't be too big a concern.
 

7991LXnSHO

wanna catch the space herp
10 Year Member
Sep 1, 2010
7,387
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194
Kearney, NE
Apr 9, 2021
#7
  • Apr 9, 2021
  • #7
PDHSB said:
Everything but the shortblock is getting torn down to be replaced with my ported heads and cams, so that won't be too big a concern.
Click to expand...
That leaves your rod and main bearings, and they are inspectable before you make the final decision.
 

stormsedge

5 Year Member
Jun 17, 2018
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Apr 11, 2021
#8
  • Apr 11, 2021
  • #8
Or, as an alternative short term "fix" (since you intend a full rebuild shortly): while the engine is out, pull the pan and re-ring #4. You can take a look at the rod bearing at the same time just to see what you have. That way, you may gain some longevity if the rebuild is pushed to the right. Cheers.
 
Reactions: 7991LXnSHO and Potomus Pete

PDHSB

Member
Jun 11, 2020
37
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VA
Apr 11, 2021
#9
  • Apr 11, 2021
  • #9
stormsedge said:
Or, as an alternative short term "fix" (since you intend a full rebuild shortly): while the engine is out, pull the pan and re-ring #4. You can take a look at the rod bearing at the same time just to see what you have. That way, you may gain some longevity if the rebuild is pushed to the right. Cheers.
Click to expand...
Will the rings seat right if the cylinder is worn though? I was told this thing was at 130k when pulled. I might re ring it if it'll give me a good temporary increase.
 

7991LXnSHO

wanna catch the space herp
10 Year Member
Sep 1, 2010
7,387
2,745
194
Kearney, NE
Apr 11, 2021
#10
  • Apr 11, 2021
  • #10
PDHSB said:
Will the rings seat right if the cylinder is worn though? I was told this thing was at 130k when pulled. I might re ring it if it'll give me a good temporary increase.
Click to expand...
If the crosshatch pattern is still there and there is no ridge at the top of the bore, it should work. Search “Flex Hone cylinder ball hone” for the right tool to deglaze the cylinder walls. That’s the right way to do a redneck rebuild that your rings will seat in. I’ve done it with small engines, and know others who do it with car engines.
In the first episode of Roadkill, they do this in a Cleveland for a Cougar, and although they show the process briefly, it is not like their ensuing level of Hackery.

Ball Hones for Cylinder Wall Deglazing and Engine Rebuilds

Flexible honing tools from Brush Research Manufacturing (BRM) have a few nicknames. Although we market our surface finishing solutio...
www.flexhoneblog.com
 
Last edited: Apr 11, 2021
Reactions: stormsedge

HemiRick

I'd be looking at jacking under the house
Jun 28, 2020
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Apr 11, 2021
#11
  • Apr 11, 2021
  • #11
@ 130K I wouldn't touch it, its probably fine.....
 
Reactions: 7991LXnSHO

stormsedge

5 Year Member
Jun 17, 2018
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Apr 11, 2021
#12
  • Apr 11, 2021
  • #12
7991LXnSHO said:
If the crosshatch pattern is still there and there is no ridge at the top of the bore, it should work. Search “Flex Hone cylinder ball hone” for the right tool to deglaze the cylinder walls. That’s the right way to do a redneck rebuild that your rings will seat in. I’ve done it with small engines, and know others who do it with car engines.
In the first episode of Roadkill, they do this in a Cleveland for a Cougar, and although they show the process briefly, it is not like their ensuing level of Hackery.

Ball Hones for Cylinder Wall Deglazing and Engine Rebuilds

Flexible honing tools from Brush Research Manufacturing (BRM) have a few nicknames. Although we market our surface finishing solutio...
www.flexhoneblog.com
Click to expand...
^this^...and maybe--it depends on the wear. It just seems counterintuitive to slap your nice hot rod heads and cams on a used short block that is not up to snuff. Hope it does what you want.
 
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