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Old 12-12-2011, 18:21   #1
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Compression 4-108?

Thanks to a San Francisco boatyard that will remain nameless, we are spending our vacation getting our Perkins 4-108 rebuild in Ensenada Mexico.

Does anyone know what the compression should be in one of these engine after a rebuild? The gauge that the mechanics here are using say “250” but it doesn’t have say units…

Thanks in advance for any help.

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Old 12-12-2011, 18:57   #2
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Re: Compression 4-108?

Compression Ratio is 22:1

22 x 14.7 = 323 psi




A 1:1 ratio is equal to 0 PSI. 14.7 PSI is equal to a 2:1 ratio. Just multiply your ratio by 14.7 to get PSI, or divide PSI by 14.7 to get ratio.




Read more: How do you convert compression ratio and psi
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Old 12-12-2011, 18:58   #3
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Re: Compression 4-108?

Tha gauge for compression checking should be at Min. 500psi scale. Mine is 1000PSI
(70 BAR)

Normal gas engines compression gauges have a scale max around 250 PSI.

A diesel compression should be over 400 PSI if it cranks good so an automotive one will not show much.
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Old 12-12-2011, 21:15   #4
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Re: Compression 4-108?

Nemos...Can you tell us the exact problem you're having with the engine?
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Old 13-12-2011, 09:17   #5
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Re: Compression 4-108?

It’s quite a story…

We are supposed to be on our trip of a lifetime, but we are stuck in Ensenada with a broken engine…was supposed to be surveyed back in S.F. – but that’s another story.

Although running well, engine started to leak from front and rear seals. Didn’t want it to crap out in Honduras so agreed to a rebuild.

Engine is now rebuilt and we are getting completing different opinions as to what the compressions should be…

All the experts back in the States agree that all 4 cylinders should be within 10%.

But one renowned company says the pressure should be above 350 PSI and another says anything above 250 PSI is O.K.

Confused….

Nemos
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Old 13-12-2011, 09:24   #6
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Re: Compression 4-108?

The 4108 is notorious for seal leakage (also the oil pan gasket), but that is no reason to tear the whole engine apart--replacing the seals is a 1 day job.
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Old 13-12-2011, 10:09   #7
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Re: Compression 4-108?

I chase oil leaks on my Westerbeke 4-107 (Perkins) for 2 years and finally just put a pan underneath it. I would loose about a thimble full after 4 hours of running. In my earlier days, I was a Triumph motorcycle mechanic. So it was no surprise to me that a British engine leaked. Personally, if the engine starts easy and runs smoothly, just go with it. Just replace the seals and be sure that the engine's crankcase is vented properly.
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Old 13-12-2011, 11:07   #8
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Re: Compression 4-108?

Why did the British never make a nuclear power plant?

Answer – because they couldn’t figure out how to make it leak oil.

We had babied this Perkins for years with its oil leaks. But we had been told that once the rear seal goes, it can fail completely. If that happens in a far-flung location – big problem.
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Old 13-12-2011, 11:14   #9
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Re: Compression 4-108?

Quote:
Originally Posted by Celestialsailor View Post
I chase oil leaks on my Westerbeke 4-107 (Perkins) for 2 years and finally just put a pan underneath it. I would loose about a thimble full after 4 hours of running. In my earlier days, I was a Triumph motorcycle mechanic. So it was no surprise to me that a British engine leaked. Personally, if the engine starts easy and runs smoothly, just go with it. Just replace the seals and be sure that the engine's crankcase is vented properly.
Perkins and Ford 427 side-oilers.
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Old 13-12-2011, 17:36   #10
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Re: Compression 4-108?

I hatew to say this, but you don't need to rebuild an engine to replace a rear oil seal on a Perkins
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Old 13-12-2011, 19:08   #11
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Re: Compression 4-108?

I agree with all the above !! a Perk leaks about half of it's life if you don't keep up on them. if you have the room you can change the pan gasket and the seals in a couple of days easy. but it's done now and you were wondering about the compression, it depends on cranking speed, lube, run it a while and have it cked warm if it pegs a 250 gauge quick its good LOL you really can't tell much that way run it and watch the oil pressure and the temp if all ok and the leaks are stopped your good to go Bob and Connie
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