Use of a Standardized Engine Leak-Down Tester

[Cdn850T5NT]

Folks, for any of you who have used an engine leak-down tester (i.e. the FAA standardized one, with a 0.040" dia. x 0.250" length orifice for a less-than-5" bore cylinder, and with standard 80 psi upstream pressure) - have any of you considered doing this not just at TDC, but perhaps with lower than 80 psi... from near BDC all the way up to TDC?  Or mebe at the full 80 psi?

Using this device at TDC of course tells you whether the head gasket is sealing, or whether the intake or exhaust valve is sealing (and allows you also to infer what the rings are doing versus the cylinder).  But if it were possible to do it at say various points from BDC to TDC - it might tell you if a particular flaw or score mark is in place along the cylinder wall.

Now there is risk associated with this, and maybe it is totally impractical to do it (though you would put the bike in 5th or 6th gear... and the hold-back force likely is considerable and can be calculated).  Maybe the wheel would have to be blocked against rotation.  Highly likely.  Mebe the brakes alone would be OK?

Interested in hearing folks' thoughts.

Three other things:

i) I totally support, also, the use of a compression tester.  Said device gives a dynamic indication of compresssion, and allows a person to infer whether the cam profile is not worn-down (very) significantly.  Interestingly, you can have an engine that shows great compression (with a tester) - but not-so-great values of leakdown (i.e. higher than 5%) with the leakdown tester.  Just so folks know, a 5% leakdown is:  80psi on the upstream gauge, and then 76psi on the downstream gauge... i.e. a 4 psi (drop) on 80 psi... is 76 / 80.  I BELIEVE I am right, here.;

ii) view the two photos of my extremely ugly (but fully functional) home-made leakdown tester (sorry if one pix is on its side).  Pls, no disparaging comments  .  Oh, and BTW, all of my downstream-of-the-tester fittings are of the wide-open type... the quick connect is a pressure-washer type, not a compressed-air type quick-connect; and

iii) we have the Aviation industry to thank for this wonderful device... around the time of the Second World War (I think Eastern Technologies developed the device as an engine health check during the war effort).  It is only in the Aviation industry that standardized pressures, orifice sizes, and instructions for doing the test are available.  The funny / stupid thing is that outside of Aviation, the size of the orifice... a critical component.... is not even SPECIFIED.  Crazy!

[acogoff]

    I feel you are overthinking the use of a simple tool. Things I have found using this tester in the conventional way are loose valve guides( caused  valve not seat properly or intermitantly). Loose exhaust seat (pretty much no compression on that cylinder accompanied by a clanking noise)Yikes!. Broken piston rings. This job is best performed with two people, one running the pressure up and one holding the prop at TDC. I have done it alone, but great care must be taken to stay clear of the prop arc.
     It is just a basic test to tell you the parts are all in there and somewhat functional. For more thorough examination, removing the valve covers for a valve stem wiggle and a bore scope would be the tool of choice for cylinder examination. But the basic differential compression tester has it's place. 

[Chuck in Indiana]

I feel you are overthinking the use of a simple tool.

Agreed. 

[Fontain25]

Leak down tester and bore scope will get you all the diagnosis you will need. If you let the piston travel to BDC one of the valves is going to open, just depends on which direction the motor "feels" like turning when the air pressure is applied. If you wanted to remove rockers before test then you could get a cylinder wall leakage value, don't know if percentages would still be the same as for TDC or not. If you do overthink it and test this way let us know what happens.

[Cdn850T5NT]

Thx for comments. Obviously there  are some veteran piston engine aircraft mechanics out there... Thx 

Availability of inexpensive borescopes these days makes direct bore examination pretty easy. Also, with the oil pan off you can see some bottom-bore locations quite well on the Guzzi.  I may try the BDC-transitioning-to-TDC (on the compression stroke) just to see what it shows... just for kicks.

[Kiwi_Roy]

I have never used a Leak Down Tester, are you looking for the pressure difference across the orifice i.e, flow rate?
You would need some form of standard orifice otherwise how could you quantify the leakage between one rig and another?

Prior to seeing the apparatus I had assumed you were looking for the time it takes the pressure to decay.

What is a typical pressure drop for a cylinder in Good, Average, Poor condition?

[Old Jock]

Roy as far as I know you are looking at pressure difference

The supply is giving a cosntant stream of air at a fixed prssure.

If there is a leak then downstream of the small orifice the pressure will be lower due to the leaking air, the orifice acts as a restictor to allow the pressure downstream to fall and give you a measurement.

If you had no leak then the pressure both sides of the orifice would be the same as pressure downstream would build and with nowhere to go (no flow) would build until it was the same as upstream

It's similar to a leakdown test on pipework except in that case you look for pressure decay over a given length of time but it's essentially doing the same thing.

Chuck and others are the experts but I believe you can also do a wet leakdown where you put some oil (I think or maybe it's gas) this stops leakage from the rings so can help you find if the problem is cylinder head or rings/barrel wear

[Cdn850T5NT]

Attaching some docs that some folks may want as a reference.

Snaps of the FAA document that provides guidance.  The snap-pages are numbered...  but I incorrectly show those numbers as "section 8.6".  That should be "section 8.14" instead.

Who sells a nifty cylinder differential testing rig:  search for "Eastern Technologies".

Teledyne Continental document that details it, as well, for Continental Aircraft engines: 

https://m.aircraftspruce.com/catalog/pdf/servicebulletin.pdf

Other article:  https://www.aopa.org/news-and-media/all-news/2017/may/pilot/savvy-maintenance-borescope

I cannot guarantee that ANY of these docs are the most up-to-date ones.

'Way too much, I know... but for you Techies... mebe right up your alley.

Hopefully these docs stay in order.  Otherwise, view the titles to see the order.

I defer to those savy Aircraft Mechanics in our midst...