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Lower than expected cold crank compression

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  #21  
Old 03-17-2017, 10:09 PM
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Originally Posted by SgtMic
New cylinders came in today so I'm rebuilding. Hope to have it done tonight and hit the road tomorrow for the break in.

Pistons and cylinders were matched so I shouldn't have any ring set issues.
Did you check the ring gaps?
 
  #22  
Old 03-18-2017, 11:21 AM
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Originally Posted by SgtMic
So getting oil past the rings isn't an issue?
Go troll elsewhere please.
I didn't see anything about oil. I was talking about compression. I repair car engines, I never did bike engine, but engine is engine, doesn't matter if it is lawn mower or Corvette. They are pretty much the same.
I wouldn't worry about your compression. If you don't like my opinion, fine with me, I don't care.
 
  #23  
Old 03-18-2017, 12:06 PM
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Originally Posted by 13RoadKing
I didn't see anything about oil. I was talking about compression. I repair car engines, I never did bike engine, but engine is engine, doesn't matter if it is lawn mower or Corvette. They are pretty much the same.
I wouldn't worry about your compression. If you don't like my opinion, fine with me, I don't care.
You rebuild Corvette motors for a living?
 
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Old 03-18-2017, 12:14 PM
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Originally Posted by Max Headflow
You rebuild Corvette motors for a living?
I am a car mechanic (kind of) and I do everything, including transmissions and paint, but my main specialty is engines repair. I never repaired Corvette engine but they are old style pushrod engines, not much changed in last 50 years.
 
  #25  
Old 03-18-2017, 12:47 PM
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Originally Posted by 13RoadKing
I am a car mechanic (kind of) and I do everything, including transmissions and paint, but my main specialty is engines repair. I never repaired Corvette engine but they are old style pushrod engines, not much changed in last 50 years.
So higher compression, direct fuel injection, variable cam timing, cylinder finish / ring stack, balancing and I sure there are others changes don't make any difference?

Do you bore cylinders, replace valve seats, do valve jobs, mill heads? Any performance work? Degree cams? CC Heads / pistons other blue printing stuff like balance the crank? Do you tune fuel injection? Tune carbs even? What kind of performance work do you do?

It's not hard to bring a production motor back to spec tho you still may need to cut valve/seats bore cylinders, etc. When you want to squeeze a little extra out you can start changing parts but the process can quickly become more than just buy parts and replace what you want. Clearances need checking, cranking pressures set, intake and exhaust flow set.. Etc. Still you can run into cranks that twist, rings that don't seat, issues with poorly designed after market pistons, improper cylinder finish etc.
 
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Old 03-18-2017, 12:56 PM
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Originally Posted by Max Headflow
Did you check the ring gaps?
Everything checked out good. Got 20 miles in today for some break in time.
 
  #27  
Old 03-18-2017, 01:05 PM
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Originally Posted by SgtMic
Everything checked out good. Got 20 miles in today for some break in time.


A lot of times the rings will need filing.. Especially on the after market stuff..
 
  #28  
Old 03-19-2017, 05:33 PM
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Got 70 miles on the new rebuild. Been watching the timing gauges on the PV and it was pulling 5-7 around 5k on some very hard back road wot pulls. Looks like it should hold up on a dyno now.
 
  #29  
Old 03-20-2017, 02:14 AM
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Originally Posted by SgtMic
It's possible that I just didn't get a great break in for the rings.
considering your engine guy didn't find anything wrong with it, you did work for nothing.
It happens, a few months ago I removed/installed Toyota Sequoia V8 engine 3 times.
Compression has to be checked on hot engine. Manuals usually specify not more than 10% compression difference between cylinders. I wouldn't do anything if compression were 20% different. It wouldn't be noticeable.
 
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