Tolarable compression psi on a 455 olds?
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Tolarable compression psi on a 455 olds?

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    Default Tolarable compression psi on a 455 olds?

    Hi guys-

    Still looking at this 72-73 Avenger with a olds 455 in it, can someone tell me what psi I should be getting in the cylinders? its been sitting for about 6 years, already pulled the plugs and sprayed the cylinders down, letting it soak.

    Thanks in advance!

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    Quote Originally Posted by Fabreezai View Post
    Hi guys-

    Still looking at this 72-73 Avenger with a olds 455 in it, can someone tell me what psi I should be getting in the cylinders? its been sitting for about 6 years, already pulled the plugs and sprayed the cylinders down, letting it soak.

    Thanks in advance!
    they tend to run around 125 to 150 (on average) look for "variation" of 15 to 20 psi more than anything , guages vary widely Tom

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    Quote Originally Posted by jetboatperformance View Post
    they tend to run around 125 to 150 (on average) look for "variation" of 15 to 20 psi more than anything , guages vary widely Tom
    Thanks for the reply, waiting for the battery to charge up, I have a brand new Snap-on gauge (he is kind of like a drug dealer ). I'll check and re-post.

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    Low compression Olds 455's should be about 130 PSI when new after broken in, high compression Olds 455's should be from 170 - 190 when new after broken in depending on year of the engine in question. Maybe this will help.
    http://www.oldsmobility.com/tuneup.htm

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    Thanks for the info Hotrod,

    The boat has been sitting for about 6 years, I did a compression check and came out as follows: #1=135, #2=140, #3=135, #4=100, #5=125, #6=100, #7=135, #8=75. Seems the right bank is where it should be but the left seems like #4,6,8 might have a sticky valves? would you agree? also looks like the heads have been changed. should I pull the valve cover? if so what should I look for?

    Thanks in advance.

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    Quote Originally Posted by Fabreezai View Post
    Thanks for the info Hotrod,

    The boat has been sitting for about 6 years, I did a compression check and came out as follows: #1=135, #2=140, #3=135, #4=100, #5=125, #6=100, #7=135, #8=75. Seems the right bank is where it should be but the left seems like #4,6,8 might have a sticky valves? would you agree? also looks like the heads have been changed. should I pull the valve cover? if so what should I look for?

    Thanks in advance.
    I know when doing a compression check you don't want to see more than a 10% difference in any of the cylinders.

    Going out on a limb, (probably incorrect here) I think you're supposed to squirt some oil in the cylinders that read low and redo the compression test, if the compression test readings go up then cylinders (cylinders themselves, pistons, rings, etc) are the probable cause, if it stays near the same then heads (valve guides, valves, etc) are the probable cause. Maybe someone will chime in and write the correct way to go about finding the problem without teardown. Regardless, some teardown will be required to fix it.

    But then there was that 248 GMC I had in high school that sat for decades and when I blew up the 216 Chevy the 248 went in, sh!tty compression test and all, and it ran for many years after.

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    Thanks again hotrod, the starter is acting up now so I have to pull it before I do another test. Anyone else with some info that would help?

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    Brushes in the starter are shot, had to order another. By the way that was real fun rubix cubeing it out of there! dang log exhausts!

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    I know what 'cher talking about. I had a hard time putting my starter on and the motor was hanging on the hoist!

    Have you had the engine running yet? I'd fire it and recheck the numbers if not. You may just have some rust in a valve seat or something. If it's still low you can do a leakdown test by blowing some air in the spark plug hole and listen where it's going.
    Malcolm

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    Malcolm- have not fired it up yet, has not been done for several years, as well I'm just looking at purchasing it at the moment so the more I find out about it (and the more I find wrong) the more the price goes down. already needs a lot of work, cosmetic wise, just doing all the engine check stuff before I fire it. I don't want to do any more damage to it if it's hurt already. If it was bad changing the starter on a hoist, imagine doing it mounted, have to use that third eye and all the double secret probation tools from Mr. Snap-On man. At least on this Avenger hull.

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    Next time put the starter bolts in and put two tiny rubber bands (or orings)around the bolt threads right next to the starter mount base this will hold the bolts while "postioning" and make the install simpler Tom

    pss an air ratchet makes this even better

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    Quote Originally Posted by jetboatperformance View Post
    Next time put the starter bolts in and put two tiny rubber bands (or orings)around the bolt threads right next to the starter mount base this will hold the bolts while "postioning" and make the install simpler Tom

    pss an air ratchet makes this even better
    That is genius, would have saved me many "choice" words and lots of skin on my fingers.

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    Thanks Tom!
    Good advice! It's those little things like that that make everything you do less of a project. kind of like using the right tool or the one that was designed for that purpose and not using "caveman tools". You can do it with them and take 1 hour, as Gregb said, loose a lot of skin, or do it it 10 min. with the right tool and simple logic. Love this site!

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    Tom-

    Was reading your tech tips on cooling the engine, it stated that all the water lines should be 5/8". On mine its 5/8" from the jet drive to the logs but everywhere else is 9/16" should these be changed? As well should I check/do/lube anything on the jet drive since it's been sitting for so long before I fire up the engine?

    Thanks in advance.

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