Proper Compression
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Joined: Oct 2001
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Proper Compression
I don't know what would be considered normal but 120 seems low, but for an old engine that might be ok. That 90 is really out of range. And sisnce it was the same both dry and wet i'd say you have a leaky or burned valve. Time for a rebuild.
-Jon
-Jon
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Proper Compression
Thanks for the info. My old 300-6 is around 150 psi so I thought these were low. I heard one fellow say his were at 190 psi per cyl. That sounds a bit high. I agree with the valve job. I will be doing the whole thing I guess. Thanks.
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Proper Compression
For the 75% rule, the lowest should be at least 75% of the highest. So if highest is 120, then 75% of 120 is 90. I would pull the valve cover on the head with the 90, crank it to get both valves on that cylinder closed, then try rotating each of the two pushrods by hand while watching for a hula dance out of either of them. It could be a very simple and cheap fix, like $1.99 for the pushrod and $10-15 for a rubber valve cover gasket set. Nothing to lose by looking.
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Proper Compression
Boy that would be good to find a worn or slightly bent pushrod or just a collapsed lifter. While I'm in there though I should get a rough idea of the shape of the cam lobes. Hopefully they are not too worn as well. Appreciate the info. Is there a way to get the valve cover off (passenger side) without removing the upper intake?
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Proper Compression
No don't think so. Add the upper intake gasket to the list. You can get a rough check of the lobe condition by unbolting the rocker and having a straight pushrod seated in the lifters seat. Put a small straight piece of something over the pushrod hole as a straightedge. Then mark a line on the pushrod while centering the pushrod in the hole. Then turn the crankshaft with a breaker bar to get to the maximum lift point, center and mark the pushrod again. Then pull out the pushrod and measure center to center on the marks. Remember this is just lobe lift, not multiplied by the rocker ratio, so it's only going to be something like .25 inch lift. Don't crank the engine by using the starter without pushrod and rocker in place bolted down, or else a lifter could pop up out of its bore and lay in the lifter valley, requiring a intake manifold pulloff. And all that gasket scraping and cleanup work. You probably know that already, but I wouldn't want anybody to get a nasty surprise and blame me!
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Proper Compression
If you have a pushrod that's bent or a lifter not pumping up,would'nt that keep the valve from opening as opposed to having it stuck open?I mean the spring pressure keeps the valve closed,the pushrod and lifter move the rocker to open the valve.If your compression is low I would think you would have a burned or worn valve that is not seating.Just thinking outloud.
CB
CB
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Proper Compression
Yeah, I think no matter what, I am going to pull the head and take a look at the whole thing. Ford could have made the process less complicated though in their manifold design. Thanks.
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Proper Compression
Rebuilt cylinder heads are not that expensive.However most of the time they replace whats bad and re-use valves and springs if they're ok.If you want to spend a little more you can send em out to a machine shop and have them replace the springs and at least the exhaust valves in addition to the normal valve giudes, seals etc.If you plan on keeping the truck a while I'd go that route.
CB
CB
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Proper Compression
An intake pushrod that is bent enough, but not enough to fall out, won't open the intake valve very far or for very many degrees. At the plugs out cranking speed, the cylinder will not fill completely on the intake stroke. So the peak compression reading will be low.
