Leaky Valves, Low Compression. Many Problems?

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bmarler
Posts: 110
Joined: 22 Dec 2008, 17:33

Re: Leaky Valves, Low Compression. Many Problems?

Unread post by bmarler »

i would closely check the bores while the pistons are out. check everything you can. look at surface finish, bore size, check for taper, consistent diameter, etc...
do i remember that low compression was a symptom this engine had before the decision was made to rebuild it?
Vive les Fulvia
Posts: 20
Joined: 18 May 2020, 10:47

Re: Leaky Valves, Low Compression. Many Problems?

Unread post by Vive les Fulvia »

The were other reasons but the final trigger was low compression. All cylinders were less that 90psi on a warm engine. Lower when cold.

The bores for cylinders #2, #3 & #4 were measured prior to honing and determined to still be within tolerance so were kept. The bore of cylinder #1 was corroded so a new liner was inserted. Considering cylinder #1 was the lowest compression post build did not make sense because this was a "new" cylinder.

The bores still appear to be in a similar condition post honing. The cross hatch is still visibly pronounced. I'll will double check before putting everything together. Do you have an tips/recommendation of the piston ring gap setting/procedure?
bmarler
Posts: 110
Joined: 22 Dec 2008, 17:33

Re: Leaky Valves, Low Compression. Many Problems?

Unread post by bmarler »

i always check the ring gap by inserting the top compression rings in the bore and check with feeler gauges. check the top, mid-stroke, and bottom of stroke. the bottom of the stroke is probably the most important place to make any adjustments. as it's usually the tightest. if too tight, you can carefully file off the ends, and carefully de-burr so you don't scratch the bore. there should be a ring gap specification in the ring manufacturers documentation, but i use the formula of .004 inches times the diameter of bore as the target. i would make a plastic slug to match the bore and use that to push down on the ring so you know it's square in the bore.
i would borrow or buy a bore gauge and check the bores carefully to be sure the machine work is done correctly. check the pistons too, so you can be sure their clearance is ok. when the ring is in the bore, i usually shine a small torch on it's backside and look for light escaping to be sure the bore is truly round.
you probably already know, but be careful to load the rings on the pistons so that the gaps are staggered.
read this article, it's got some good information: http://blog.wiseco.com/everything-you-n ... t-ring-gap

i am thinking though, it's rare for the new rings to be the problem of low compression, that's why i say again, check the bores carefully.
Vive les Fulvia
Posts: 20
Joined: 18 May 2020, 10:47

Re: Leaky Valves, Low Compression. Many Problems?

Unread post by Vive les Fulvia »

Thanks for the information. The low compression is more than likely due to the gaps being too large, over 3x too large in some instances. This is confirmed when the rings were inserted into the bores and the gaps measured. The roundness and taper were measured by the machine shop prior to honing and were still within the limits stated in the Tech Data.
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