Leak Down Test on Engine Before Installing Pushrods?

5498 Views 7 Replies 5 Participants Last post by CNC BLOCKS NE, Oct 15, 2009

crazy8s

Discussion Starter · Oct 13, 2009

This may be a dumb idea (if it is, I cannot figure out why it would be) so I turn to the collective wisdom here for answers. I am assembling a BBC. The engine is together but pushrods/rockers are not yet installed. Is there any reason why I could not perform a leak down test with the engine in this state? Since the valves will be closed, it should accurately simulate TDC of the compression stroke regardless of where the piston is in the bore and the actual cam position. IMHO, this would greatly simplify the leak down test because there would be no need to be precise with crankshaft/timing position because all 16 valves would be closed.

Please tell me my instinct is correct, or why this theory is full of holes.

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boatbob2

· #2 · Oct 13, 2009

Leak down test........

Hi,sounds like a plan,,,,,,,EXCEPT for one thing,the rings are not seated yet !! so,any reading you get on the compression guage will be inaccurate,by how much.i dont know.why not do your first test,then after the rings are seated,take another test,then post the results. i think all of us would like to know the difference.

:welcome:

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crazy8s

Discussion Starter · #3 · Oct 13, 2009

boatbob2 said:
Hi,sounds like a plan,,,,,,,EXCEPT for one thing,the rings are not seated yet !! so,any reading you get on the compression guage will be inaccurate,by how much.i dont know.why not do your first test,then after the rings are seated,take another test,then post the results. i think all of us would like to know the difference. :welcome:

Good call Bob. I will do just that!

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BogiesAnnex1

· #4 · Oct 13, 2009

crazy8s said:
This may be a dumb idea (if it is, I cannot figure out why it would be) so I turn to the collective wisdom here for answers. I am assembling a BBC. The engine is together but pushrods/rockers are not yet installed. Is there any reason why I could not perform a leak down test with the engine in this state? Since the valves will be closed, it should accurately simulate TDC of the compression stroke regardless of where the piston is in the bore and the actual cam position. IMHO, this would greatly simplify the leak down test because there would be no need to be precise with crankshaft/timing position because all 16 valves would be closed.

Please tell me my instinct is correct, or why this theory is full of holes.

Given the valves have to open and shut to take a breath which you will measure the leakdown rate from, just how are you going to get there without pushrods?

Bogie

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crazy8s

Discussion Starter · #5 · Oct 14, 2009

oldbogie said:
Given the valves have to open and shut to take a breath which you will measure the leakdown rate from, just how are you going to get there without pushrods?

Why would the engine need to take a breath? The difference between the compressed air being fed into the the cylinder and what is leaking out is what is being tested, isn't it? Or am I misunderstanding the process altogether?

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k-star

· #6 · Oct 14, 2009

leak-down

You can do it with out the pushrods, I do it all the time. Also the ring set is not an issue.

I leak every engine I build before I start it, and after it's run and there is never more then a 1% difference.

Make sure the cylinder you are testing is about 1 or 2 degrees before TDC. that will place the rings on the bottom of the groove, like they would be in the "as run" condition. Again i have tested in many positions and there is very little difference.

You would need pushrods to do a compression test.

Keith

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BogiesAnnex1

· #7 · Oct 14, 2009

crazy8s said:
Why would the engine need to take a breath? The difference between the compressed air being fed into the the cylinder and what is leaking out is what is being tested, isn't it? Or am I misunderstanding the process altogether?

I'm sorry I got lost between compression testing and leak down testing. Your right, since you're feeding it compressed air and measuring the time it holds up the pressure, the valves do not have to function beyond hopefully holding ir pressure.

Bogie

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CNC BLOCKS NE

· #8 · Oct 15, 2009

crazy8s said:
Why would the engine need to take a breath? The difference between the compressed air being fed into the the cylinder and what is leaking out is what is being tested, isn't it? Or am I misunderstanding the process altogether?

We just went through a 327 that the customer was complaining his oil was getting dirty in less then a 100 miles and he did a compression test and it was 170 to 200

When we leaked it down @ TDC it leaked down between 10% the best and worst was around 35% the average was about 20% and this engine had 400 miles on it.

When it was tore down you could see where the ring we touching and where they were not as it was not plate hone and when we put the torque plate on the block the owner checked it wit the bore gauge and could see why the rings were not sealing

Here are the threads about this issue.
http://forums.corvetteforum.com/c1-and-c2-corvettes/2289212-oil-pressure-problem-saga.html

http://forums.corvetteforum.com/c1-...-engine-rebuild-oil-pressure-saga-update.html

We see more distortion on a BBC theb a SBC accept a 400 block as on the BBC the bolt holes are further from the cylinders causing more leverage.

Here is another link on a 454 that came in using oil after 2000 miles and was down on power.
http://www.camaros.net/forums/showthread.php?t=58964

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