leak down test to test crank seal

[rusty nail]

I have an 787 carbed motor that i want to test for the crank seals before putting in a 98 speedster. I have heard of the leak down test but have never done it. Can someone describe the process and can it be bench tested? the motor it not in a position to run. I can bolt it down to my work space and run the starter if that is enough.

 

[IDoSeaDoo]

basically, you close off the exhaust port, the intake ports and spark plug hoes and pressurize the crank case. It should hold 10psi for some amount of time. You can spray soapy water on all the places where you suspect a leak. Only place you won't see it is the head gasket o-rings. Those will leak to the water jacket, but I've never seen them leak before. I made some block-off plates out of aluminum and used a thick sheet of neoprene gasket material to create gaskets to back to the aluminum to make a good seal. Most of my leaks were actually at the blockoff plates themselves, so the having a spray bottle of soapy water is key to this test. PM me if you need info on how to build a pressure test tool to slowly introduce pressurized air into the system.

 

[rusty nail]

do you send the pressure into the carb pulse line? And if the pressure simply holds at 10psi that tells you the internal crank seals are ok? Will i hear/feel/ or see air coming from the lower oil fitting under the starter if the internal crank seal is bad?

 

[IDoSeaDoo]

Yes, attach at pulse line.
Yes, if you don't cap those two oil circulation fittings and have an inner crank seal leak, you'll feel it come from there. You can get a water balloon and put it around one of the fittings and watch it blow up if you've got a leak.

 

[rusty nail]

what about the larger oil line under the starter? should i cap that or put a balloon around that as well?

This is awesome by the way

 

[IDoSeaDoo]

The two go to the same cavity. The rotation of the crank and gears creates circulation. Oil goes from the bottom (feed) out the top (exit) fitting. You'll have to plug one to test the seals. After you test the seals, it's usually good to test the the rotary shaft seal also by pressurizing that cavity and spraying/watching for bubbles around the seal.

 

[rusty nail]

do you think this would be good to apply the 10psi to the crankcase? 10 psi is not very much, does not seem to make sense to rig up something with a compressor.

https://www.canadiantire.ca/en/pdp/oemtools-automotive-tune-up-brake-bleed-kit-0250781p.html

 

[IDoSeaDoo]

yea, that'll work, but my way uses a compressor and allows for a slow, easy way to pressurize anything (carburetor, radiator, crank case etc). It might be cheaper to assemble too. I have an air compressor pressure regulator attached to a ball valve attached to a T, which has a gauge on top and a barb at the tip. You control how much pressure you want to let of the compressor with the regulator. Once you see the pressure start to build, you can close the valve and watch the gauge. I use this to pressure test fuel tanks which require a TON of air to pressurize. You'll be pumping all day to get any psi to build up in a big volume.

 

[rusty nail]

thanks for the drawing ....super helpful.