Low compression, whats needed?

[William1275]

Hello all,
Over the weekend we did a compression test on my friends 345. It had a low of 70 and a high of 119. Not good at all from just basic thinking. So now he is looking at possibly pulling and rebuilding the engine with minimal parts needed to bring the compression back up.

Since I/ we don't know much about rebuilding engines, what parts would be needed to rebuild the engine with minimal parts to bring the engine back to within in compression specs? Camshaft? Rings? Valves? Lifters?

The basics of the engine that is know at this point. It is an IH 345. It has been bored over to .30. It was bought newly rebuilt about 15 years ago.

Thanks for any and all help.

[tnfalconer]

I would think the minimum would be head gaskets, piston rings, crank bearings, oil pan gaskets. Now having said that, it makes no sense to do just that. you are going to have to tear the engine down anyway and it makes more sense to have the block checked and cylinders honed before installing new rings. having no idea what is causing the problem you don't know what to fix. It could be anything from a cracked head to piston rings. None of them are obvious without the engine disassembled. Tear it down and build it. It will be much cheaper in the long run. I am speaking from personal experience.

[Jay Tabor]

best do a diagnostic and find out why the comnpression is low in a cyl.
once you have ascertained the problem -THEN you can get accurate costs for fixing it.

[robgimmel]

When you tested the compression did you do both a wet and a dry test? That can tell you a lot.

[Will Marsh]

I'll echo what Jay and Rob said.

119 is a respectable # for a dry compression test on an IH engine. 70 is very low. Without knowing the other 6 we can't even begin to evaluate how to approach it.

Start with a proper dry compression test. Remove ALL the spark plugs, block the carb wide open, and crank it 5 revolutions per cylinder test.

Then put a couple shots of oil in each cylinder through the plug hole and crank it 10 revolutions. Repeat the compression test.

Post the results and we can tell you how we'd approach it.

Will Marsh

[nctom]

X2. I use a little more oil and 5 revolutions. You dont want to wipe it too dry with the rings or it wont seal it up.

[Jay Tabor]

1 compression stroke is the only test that matters, cranking the engine 10 times or 50 or 100 doesnt mean a thing.
what?? your engine got variable ignition and valve timing?

the piston goes down and sucks air into the cyl, the piston goes up and the sparkplug fires. thats the way it works in the real world, believe it or not . .
so you place the piston at tdc of exhaust stroke and crank the engine, until you get one crank revolution and see the compression gauge needle move and stop.
thats the accurate cylinder compression test.
then you put the piston at tdc of compression stroke, apply compressed air to that cyl and watch and wait and see/hear where air is leaking from if any.
then youve done a test that means something.
low comp could be from a cracked head/cyl, valve seat, burnt valve/seat, worn guide, worn/broken rings, cracked/broken/burnt piston, springs that dont go sproing.broken valve/spring, bent pushrod, rocker arm/
bad head gasket,

[Will Marsh]

Originally Posted by Jay Tabor

1 compression stroke is the only test that matters, cranking the engine 10 times or 50 or 100 doesnt mean a thing.
what?? your engine got variable ignition and valve timing?

the piston goes down and sucks air into the cyl, the piston goes up and the sparkplug fires. thats the way it works in the real world, believe it or not . .
so you place the piston at tdc of exhaust stroke and crank the engine, until you get one crank revolution and see the compression gauge needle move and stop.
thats the accurate cylinder compression test.
then you put the piston at tdc of compression stroke, apply compressed air to that cyl and watch and wait and see/hear where air is leaking from if any.
then youve done a test that means something.
low comp could be from a cracked head/cyl, valve seat, burnt valve/seat, worn guide, worn/broken rings, cracked/broken/burnt piston, springs that dont go sproing.broken valve/spring, bent pushrod, rocker arm/
bad head gasket,

You're better than this, Jay. If what you are saying is true, I can hook up the compression gauge and turn it over by hand one revolution to get a good compression test?

The starter doesn't instantly turn the motor over at 300 rpm (or whatever the cranking speed actually is), it takes a revolution or 2 to get it up to speed. Thats why you turn it five, so you get at least one (and preferably 2 or 3) at speed for accurate and repeatable results. The gauge only reads the maximum one. You know that, but I don't know if the OP does, so that's why I posted it.

The leakdown test that you added is helpful to diagnose where/why there's low compression on a given cylinder, but we haven't gotten to that stage of the diagnosis yet.

Will Marsh

[Jay Tabor]

silly superstitions and old wives tale prevail here and always will. sad really is . .
if the starter takes a couple of engine revs to get ''up to speed' there are serious probs going on with that clunker. and a cyls compression psi is the least of them.
all that matters and is relevant is one complete intake and compression revolution of the crankshaft for that particular cylinder.
anything else is irrelevant and misleading.AND USELESS INFO.
cranking the cyl for 5 ,10-15-20-1000 compression strokes maybe keeps pushing the gauge needle up and up,
but the exhaust /intake valves are opening and closing each stroke, the cyls compressed air and gas escapes so its a meaningless test!
''get motor up to speed.'' ha ha ha!

''You're better than this, Jay. If what you are saying is true, I can hook up the compression gauge and turn it over by hand one revolution to get a good compression test?''

'''The starter doesn't instantly turn the motor over at 300 rpm (or whatever the cranking speed actually is), it takes a revolution or 2 to get it up to speed. Thats why you turn it five, so you get at least one (and preferably 2 or 3) at speed for accurate and repeatable results. The gauge only reads the maximum one. You know that, but I don't know if the OP does, so that's why I posted it.''

i dont know what type of 100 amp battery you have, maybe thats the problem, but any 1000cca batt will instantly engage the starter motor at its rated speed of turning!
and that little drive gear of ten teeth aughta be able to spin a 2 foot diameter flywheel with a hundred teeth at full speed upon initial engagement.
especially so with an engine with only 70 psi!!
if not its battery condition and battery cables . . VD
if not i suggest a hand crank- just poke a hole in the radiator and weld it to the harmonic balancer for those days when yer 100 amp jcp penny battery fails!
its obvious to any one that the test would entail using the starter to crank engine..
but that old hand crank maybe your speed,
if the starter takes a half hour to get up to speed, maybe thats why your engines take so long to start.!
mine starts before 1 revo.. and if your engine takes a few revolutions to get a fast enough speed to raise compression high enough to start it, what I want to know is does your intake and exhaust valves stay closed automatically for 5 revs to hold all that highly compressed gas in the cyl? so it goes bang when that coil fires!??
nope! it escapes after 1 revo. !!
so much for that old wives tale!

[kb'sforme]

i agree you need more testing. you may not have to rebuild it.
if you do rebuild it there is no way to plan on minimal parts. once inside and machine work is done (you have to have the machine work! otherwise your just wasting money) once the block is all cleaned up it you will be able to see what parts you need.
best case senareo, you can get what is know as a "rering kit" it wil include rings,bearings,and a full gasket set. this assumes all other parts are good. which brings us back to doing more testing.
x4 find out whats wrong first.