Odd and unexpected cylinder leak down test results

[creia]

This is the same engine from my other post on 4/24/23 with the mag side bushing question...
M-130202
T- 0135-07
C- 77110710
Prior to disassembly this engine had decent compression (75 PSI) and good leak down tests results (17%) . All internals were inspected and carefully measured and ALL were well within specs. (For example, the bore only had .0005-.001" wear and no scoring whatsoever and you could still see the original crosshatching in many areas). All things considered I would say that this engine had the least wear of any engine I have ever taken into inventory- it must have had very little use. All I did was a light lapping on the valves then put everything back together, using new gaskets and seals throughout. So far, so good- right? Well, not really...

Imagine my surprise when I performed another leak down test and it was now 60% and compression dropped to 68 PSI!! I performed numerous tests using 2 different testers and rotated the piston a cycle or 2 between tests a few times. Apparently, air is getting past the rings as almost all of the noted leakage is coming from the sump oil fill hole (cap removed) with a little getting past the intake valve. (Guess I'll need to lap again) I am kind of stumped here
Does anybody have any ideas before I take it all apart again? Maybe a broken compression ring when I reinserted the piston? When reassembling, If the rings were not staggered (gap) in the exact same position on the piston as they were when I removed them could that have caused this? That is to say were they worn/seated in a certain position in the bore and by me changing that they are no longer seated/sealing? It is quite rare that I reinstall the same rings on the engines I work on. Typically, due to the bore/ring wear they either get new chrome rings or a new O/S piston ring set.
Michael

[bgsengine]

Tried running it yet? Rings - whether new or old, do need to be run in a little bit to actually "seat" so a change in your readings before and after may be understandable. Leakdown (and compression) tests are more of a DIAGNOSTIC indicator to be used in troubleshooting , rather than proof of a rebuild - at least until after a couple hours or so of run time, so you may likely see variations between engines (or even same engine on different days or temperatures or atmospheric pressures) which is generally why a *range* is given for test result numbers as indicators of condition. I'd fix the intake valve leakdown issue (and after lapping, smear a super thin layer of grease over the face and "slap" it in lightly then pull it back out without allowing valve to rotate , inspect the seat for shinier areas where grease transferred vs where it didn't - may give an indicator of valve not being perfectly square to the seat or slightly warped seats, etc.) and then give it an hour or two of run time (run it through from idle to top no load a few times, get it to a good steady operating temperature and then let it cool down) and then try your tests again.

[creia]

After reading your reply (Thank you ) I went ahead and installed the carb/tank, added oil and fuel. It starts, idles, and revs VERY nicely!
I'll do what you suggested and run it some more then revisit the cylinder leakdown and compression tests.
Michael

[creia]

UPDATE:
After relapping the leaking intake valve and running the engine about 1/2 hour I tried again on the leakdown and compression testing. Here are the results:
Cylinder Leak Down = 15% leakage
Compression = 68 PSI

While the leakdown test results improved significantly, the compression remained the same. I noticed that there was still a little bit of leakage at the intake valve, however, I have a theory...
I noticed that the valve guide bushing on this valve is worn with more side to side wiggle than I would like, even though it did not come close to failing the Briggs valve guide "Plug Gauge" insertion tool (Are they actually any good?). My theory is that due to the worn valve guide (and resulting valve stem movement) maybe the valve is not seating squarely at the block seat? What do you think?
Michael

[KE4AVB]

Personally I have replace valve guides that caused problems even though the go/no go gauge pass as okay. May be just me but the gauge is worthless.

[bgsengine]

I noticed that the valve guide bushing on this valve is worn with more side to side wiggle than I would like, even though it did not come close to failing the Briggs valve guide "Plug Gauge" insertion tool (Are they actually any good?). My theory is that due to the worn valve guide (and resulting valve stem movement) maybe the valve is not seating squarely at the block seat? What do you think?
Michael

Briggs valve guide plug gauge is worthless - it ONLY gives you "out of spec" tolerance, but that's way too much IMHO.

Did you do the "slap test" as I described earlier? Even if you lap the valve , if you did not cut the seat also (need a seat cutter set from Neway for that) you can lap in a nice even visible seating surface on the valve, (and not notice it really on the seat) but it still may not seal completely if the valve seat is not perfect (maybe square enough but worn more in one spot than in others, sort of a wavy seat surface) it can still have "spots" where the valve is not seating. a thin (very thin) smear of grease on valve face (or seat, optionally) and the mating surface clean and dry, then lightly slap in the valve to the seat (a bit more force than just dropping it in) and then being careful not to rotate the valve at all until you get it off the seat, pop the valve out and inspect the previously clean and dry surface for trace of grease transfer - if your seat is not perfect, it'll show up here.

[creia]

I tried the "slap test" as you described, however, I could not get any results that really told me anything. I could not see any grease on the seat anywhere, which does not seem right. I am assuming it was my (bad) technique Maybe I was not putting on enough grease? As far as cutting the seat- I did not do that. (I have never done that on any engine and do not have the cutters) I only lap the valves. Do I understand correctly that I should be cutting the seat on every engine that I lap the valves?
Michael

[bgsengine]

I tried the "slap test" as you described, however, I could not get any results that really told me anything. I could not see any grease on the seat anywhere, which does not seem right. I am assuming it was my (bad) technique Maybe I was not putting on enough grease? As far as cutting the seat- I did not do that. (I have never done that on any engine and do not have the cutters) I only lap the valves. Do I understand correctly that I should be cutting the seat on every engine that I lap the valves?
Michael

1) Try a thicker coat of grease (smear on with finger) and a strong raking light (light coming from the side, try various angles.. you also have to change your angle of looking at it.. often need to look opposite from the direction your light is coming in from, but not direct into the light of course... lots of head movements sideways, up, down, etc... ) to examine for the telltale grease spot(s) after the test (It is also possible to slap it in TOO hard so you won't see much except a faint trace of "moisture" in a strong raking light)

2) Not necessarily need to recut valve seats every time, though it is generally recommended after replacing seats (Since they rarely ever would be perfectly square) I got into the habit of recutting on every rebuild I did just because it was easier to cut seats before assembling engine, while block was still bare - easier to clean away any machining bits & pieces.. but still always did the slap test to be sure my lapped valve was actually seating all the way around. In lieu of a seat cutter, I HAVE been known to lap in valves *excessively* with a power drill (bolt soldered to the head of the valve for L-Head, or chuck valve stem in drill for OHV) to grind down the seats but it takes a lot fo time and isn't fool proof, and often need to replace the valve once done and lap THAT in to the seat.