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Checking compression on sitting engine


BrianR

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Hey guys, I have an engine that has been sitting for a few years without any oil. My question is: do I just check the compression as is or do I squirt some oil in to wet the cylinders for a proper seal? What will give me the most accurate/ true compression reading? I don't want to falsly increase the pressure with the oil but running it dry may show a lower reading. What should I do?

Thanks

Brian

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fully charged battery

starter motor installed

no oil in the cylinders - 'dry' first

then after the 'dry' cranking - add 2 to 3 onces

of MARVEL MYSTERY oil thru the spark plug

holes and re test. WATCH OUT for oil

spraying out of the plug holes !!

if intake/carb is fitted - hold WIDE OPEN

'86 R65 650cc #6128390 22,000m
'64 R27 250cc #383851 18,000m
'11 FORD Transit #T058971 28,000m "Truckette"
'13 500 ABARTH #DT600282 6,666m "TAZIO"

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Hi--

Another confounding situation: the best compression readings to compare with ratings or other engines are taken on a warm engine. I would continue with your effort, but when the results are odd you should consider how a warm engine could have tighter tolerances and potentially better (higher and/or more uniform) readings.

I realize that getting your engine warm may not be very practical, but it may well affect your results.

Larry

Larry Ayers

~~~~~~~~~~~~~~

’73 Malaga— first car, now gone

'74tii Malaga

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Guest Anonymous

Just remember compression readings are not for comparison between engine and engine. People "think" that a 125psi motor is worse than a 150psi motor but infact they're not comparable. It's not uncommon to have a 304 cam 10:1 motor compression test low due to cam overlap. The book's compression readings are for rough guesstimate only.

If you're going to get a reading on this motor, first I'd pull the plugs and rotate the motor by hand with oil in the cylinders and in the motor. Several times. Last thing you want is to have a little rust in one cylinder and then slam a piston into it with a starter. Once the motor is lubed up, hit it with the starter, no carb is fine. You're mostly looking for numbers above 100-110 and even across the board. Thats kinda the limits of the test. A leak down test might be better use and does not require a starter to be hooked up.

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And on top of all this, it only takes a fine haze of rust on a valve seat

(or a small bit of crud)

to give you a low compression cylinder.

All that said, the best, most solid engines I've had leaked down

and compressed within just a few psi, hot or cold.

The more tired ones, not so much.

Oh, and lash your valves, too- as Oldguy says, anything that affects

valve duration will change a compression reading a lot.

And I'd oil the cam manually just to give it a fighting chance...

t

"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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Guest Anonymous
And on top of all this, it only takes a fine haze of rust on a valve seat

(or a small bit of crud)

to give you a low compression cylinder.

Leakdown would find this which is why I recommend it more than "compression".

At the end of the day. They're just tests. I've had pistons with broken upper ringlands still compression test at 125.

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Yeah, agreed- a leakdown's a good way to know what the valves are up

to, but it won't necessarily find a ring or

bore problem... compression 'can', but like you, I had

a broken compresion ring come up OUT of its groove and beat hell out of everything-

and even after all that, it wasn't really down much on compression

OR leakdown, as the #2 ring and bore were ok.

They're just tests.

t

"I learn best through painful, expensive experience, so I feel like I've gotten my money's worth." MattL

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