Choosing a oil pressure relief valve spring

Amongs the parts I forgot to order while rebuilding my IO-540-C4B5 was new oil pressure relief valve parts. This is comprised of a steel ball and a spring, and shim washers.

In the Lycoming IO-540 narrow deck IPC, it lists 5 part numbers for the spring (page 1-5.) It's bears an asterisk, for which the footnote says:

"Select spring to give minimum of three threads exposed on adjusting screw to allow field adjustment. Reference SSP-1776 for spring identification"

Ok, so referencing the service table of limits lists the dimension and other specs for the 5 springs. I have the old spring, and I think it's 68668 based solely on the free length (spec says 2.04, I measure 2.06.) It bears no color dye mark.

What really determines which one you need? Without all five at your disposal, how should you choose? Who knows if the core 540 I bought even had the original.

Thanks.

I can't help with specifics as far as part #'s go but if it were me, I would want to error towards the short side.

Even if using the adjustable housing, you could still add a shim washer or two if it ran out of adjustment range but there is no way to back it off more if the spring is too long.

Good thinking, Scott. Thanks. I dont have the adjustable housing. I'm thinking all adjustment is via washers.

You must be a Lenhardt resident?

Longest and shortest spring doesn't equal toughest and weakest springs. as you can see the toughest spring is the second to shortest. and that spring is a really, really, high tension spring compared to any of the others.

Highest tension to lowest are:
LW-18085 1.93 inches free length
LW-11716 2.12 "
61084 2.18 " (Most common spring)
68668 2.04 "
77467 1.90 "

Use the 61084 spring with an adjustable tower or a tall tower and a 68668 with a short tower. Use the 77467 in a short tower if you can't get the op low enough even without adjusting washers and using the spacer with two gaskets. That was my starting points and worked out really good 99% of the time. You should never need to use the top two springs in any normally aspirated engines.
Good Luck,
Mahlon

mahlon_r said:

Longest and shortest spring doesn't equal toughest and weakest springs. as you can see the toughest spring is the second to shortest. and that spring is a really, really, high tension spring compared to any of the others.

Highest tension to lowest are:
LW-18085 1.93 inches free length
LW-11716 2.12 "
61084 2.18 " (Most common spring)
68668 2.04 "
77467 1.90 "

Use the 61084 spring with an adjustable tower or a tall tower and a 68668 with a short tower. Use the 77467 in a short tower if you can't get the op low enough even without adjusting washers and using the spacer with two gaskets. That was my starting points and worked out really good 99% of the time. You should never need to use the top two springs in any normally aspirated engines.
Good Luck,
Mahlon

Click to expand...

I do have the short tower. I'll grab another 68668. Thanks Mahlon!

Just an idea.

I know we all want to save the cost where we can, but just an idea here.
If you are in the meddle of a rebuild, order a new adjustable valve an be done with it so you don't have to ever mess with the shims anymore.
Just an idea or thought for fodder. Yours, R.E.A. III # 80888

Robert Anglin said:

I know we all want to save the cost where we can, but just an idea here.
If you are in the meddle of a rebuild, order a new adjustable valve an be done with it so you don't have to ever mess with the shims anymore.
Just an idea or thought for fodder. Yours, R.E.A. III # 80888

Click to expand...

Probably a good idea. Looks like they're about $180 new, can be had cheaper on ebay.