Caution against adding oil to cylinder for compression testing

[GeoHorn]

I’m not a highly-experienced “diesel” mechanic… got tons of professional experience on gassers…but not diesels..

One of the common methods to test piston-ring health in gasoline engines is to add some oil to the cylinder to help “seal” the rings to observe if there’s a significant “rise” in compression reading.

I’ve read on these forums similar advice given to use this technique on diesels. I’ve been informed anecdotally by experienced diesel techs that this can be very damaging due to the higher-compressions formed by smaller clearances within the piston-to-head interface of diesels. I.E.-if you put oil in a diesel and then crank the engine …you might bend a connecting rod or hole a piston.

I was reading thru the WSM on my Kubota diesel (M4700) and it points out that when working on this engine …. when removing the head… DO NOT remove the old head-gasket. I found this curious until I read further… where they advise to place “fuse” material on the piston crown, reinstall the head, then rotate the piston beyond TDC …then remove the head AGAIN….to measure the “fuse” to see if it was compressed.

(after reading thru this, I realized that “fuse” refers to a product similar to “plastigage” products)

If the clearance between piston and head is adequate…then the instructions have you replace the head gasket with a NEW gasket during reassembly.

This educated me on two points:
1- The clearance between a piston and a head is LESS THAN .030” at TDC. …. So injecting oil into a cylinder to test for compressions would indeed be subjecting the engine to possible SERIOUS damage.

2- Leaving the old head gasket in-place for this test is intended to provide a ”used” / “compressed” gasket for the test…instead of a new gasket which might be somewhat thicker.

I hope this info is helpful to others. Here’s a screen-shot of the applicable page in the M4700 WSM (F-series engine)

 

[kubotafreak]

Soft solder is used many times. You need to know the crush range to know which thickness to use. I just did this on my idi kubota engine I'm building. If you do it with a roll of solder you can bend a section of it and do it through the injector port. Will allow you to save torques on the gasket/bolts, and labor. You do need to be cognizant of valve placement.
I suggest a thin spray lube vs engine oil if you must wet test. It will fog the cylinder and wick where it needs to be. It also would take a huge amount to affect cylinder volume. You are absolutely right because oil doesn't go where it needs too. To do it on a diesel you would need to inject the oil, and then let it free cycle(open port: injector or glow plug)to release the extra out of the cylinder. Problem is even this method also changes chamber cc dramatically. Which will greatly skew compression numbers.
Probably should also disclose how bad starting fluid is on a diesel. It is not a soft ignition front, and can easily crack pistons/rings.

 

[bird dogger]

This educated me on two points:
1- The clearance between a piston and a head is LESS THAN View attachment 74396 .003” at TDC. …. So injecting oil into a cylinder to test for compressions would indeed be subjecting the engine to possible SERIOUS damage.

Maybe it was just a typo, but shouldn't you have said..... Less than .030" (thirty thou?) .0217 to .0276 inches.

 

[GeoHorn]

Maybe it was just a typo, but shouldn't you have said..... Less than .030" (thirty thou?) .0217 to .0276 inches.

Thanks for the catch. (edited 1st post to correct the error)

 

[bird dogger]

Thanks for the catch. (edited 1st post to correct the error)

I thought .003" was unrealistic. But even at .030" or just under.....that's not a lot of clearance and drives home the point you illustrated. I hadn't realized the TDC clearance was that minimal. Thanks for the post
Geo!!

 

[Trkr]

I have some antique Sheppard Diesel tractors, that have 22 to 1 compression ratio. I just learned the hard way there is no room in there. Part of the cold start involves a hand pump hooked to a fuel atomizer in the intake. The atomizer apparently wasn't working, and I pumped liquid fuel into it, got too much in the cylinders, and pushed the head gasket out. Haven't got it apart yet, hope that's the only damage.
When checking piston to valve clearance in my race engines, I use modeling clay.

 

[Henro]

When checking piston to valve clearance in my race engines, I use modeling clay.

Interesting. Please tell us more...trying to figure out how one would reliably use a mic on modelling clay after pealing it off...naturally there are other possibilities ...

 

[kubotafreak]

Interesting. Please tell us more...trying to figure out how one would reliably use a mic on modelling clay after pealing it off...naturally there are other possibilities ...

The clay would make more sense if it were used to check for safe clearance on valves, for high lift applications(relief cut pistons). In this scenario you need a whole playing field of view...

 

[Trkr]

Lightly oil the piston-valve, install clay. Run test , remove head and carefully remove clay. Cut in half, and mic.

 

[Trkr]

The clay would make more sense if it were used to check for safe clearance on valves, for high lift applications(relief cut pistons). In this scenario you need a whole playing field of view...

That's what I'm checking, Big Block Chevy with high lift solid roller cam, 13 to 1 compression.. Installing thinner head gasket, checking to see if I have at least .100 on the exhaust valve, little less on intake

 

[Henro]

Lightly oil the piston-valve, install clay. Run test , remove head and carefully remove clay. Cut in half, and mic.

LOL. Maybe just a visual inspection would tell me personally more.

Just call me "Heavy Handed Mike" I guess...or maybe should I change my new nickname to Heavy Handed Mic?

 

[Mark_BX25D]

Lightly oil the piston-valve, install clay. Run test , remove head and carefully remove clay. Cut in half, and mic.

I would trust that method about as far as I can throw a pig by the eyebrows.

 

[kubotafreak]

I would trust that method about as far as I can throw a pig by the eyebrows.

I can understand the skepticism. However, when it comes to final build stackup of engine parts, and custom milling of piston crowns, and valve depth there is not many other alternatives. No matter how good the blueprinting, a sliver of solder or plastigauge wont give the amount of impression you would need in a valve relief case. The only other real alternative is a different impression medium. This method has been around long before anyone on this forum was born. Granted it takes skill to pull off with precision.
Now if one is just needing flat top piston to head clearance, then the solder/plastigauge method is plenty sufficient.

 

[lugbolt]

the other potential issue in doing this is that oil is also a combustible. Meaning, if even a tiny amount of oil is artificially introduced into the cylinder, it can act as a fuel, and the engine attempt to start/run.

I have done this, and it will quickly get your attention.

for that reason alone, I am not a fan of recirculating crankcase vapor back to the diesel engine. A rollover can (and has) resulted in a runaway. It's supposed to stop the engine in a rollover event but oil running back into the intake (without a separator system) can negate any safety switches. I've also seen this happen.

yes 0.030" on that engine. On smaller engines it's considerably less space between the crown of the piston and the bottom of the head. Such little clearance but it's gotta be that way. Tiny diesels need all the cranking compression that they can get-D782's are spec'd around 600 psi aren't they? To make that the clearances have to be very tight.

 

[Trkr]

Well it looks like this site

I would trust that method about as far as I can throw a pig by the eyebrows.

Well it looks like this site has gone to hell with a some know it all argumentative assholes.
I'm 69 years young, been using that method on my engines forever. So much for trying to contribute anything here, done with this place.

 

[Mark_BX25D]

I'm 69 years young, been using that method on my engines forever. So much for trying to contribute anything here, done with this place.

If you can't take disagreement, the Internet is not a good place to be.

Come to think of it, that doesn't work very well anywhere than I can think of.

 

[Mark_BX25D]

I can understand the skepticism. However, when it comes to final build stackup of engine parts, and custom milling of piston crowns, and valve depth there is not many other alternatives. No matter how good the blueprinting, a sliver of solder or plastigauge wont give the amount of impression you would need in a valve relief case. The only other real alternative is a different impression medium. This method has been around long before anyone on this forum was born. Granted it takes skill to pull off with precision.

Thank you for that informative post. I done been edumacated.

I can imagine that it gets done with less than the required skill at times.

 

[lugbolt]

Been doing race engines for my entire life

everyone has their own way of doing things

the myth .100/.080 is just that a myth. A "rule of thumb", so to speak. Mine are .037 and .045 (I/E). You have to KNOW your engine, and it's application.

oil on the valve makes the clay stick to it, gives you a sort of false reading

also clay will compress, then it will expand as the valve moves away from the piston, which gives you a false reading (somewhat). As the piston and valve fall away from each other the clay often gets "pulled". It will only get you close, to give you an idea. If you're gonna run tight like I am, you want to KNOW for sure, not guess. And I do things differently than many do but again if you run tight you need to know exactly! When I was young and did things the 'normal' way, yes I used clay or play-doh, but rather than oil, I used flour so that the play-doh was easier to remove and it had a lot less tendency to stick.

a big block chevy at 13:1 has huge domes (usually), most folks use steel rods, they are relatively short rods in comparison to some other engine designs (assuming 454/496/etc) and "generally" speaking lower RPM than what I am used to, so the combination of that can run quite a bit closer piston to valve clearance than "the rule of thumb". The better the parts, the closer you can run. H-beams, Ti valves, good (expensive) springs, keep the RPM in check, etc.

Kubota's are almost always sub 3000 RPM (BX's and G/GR series are some exceptions) and they need all the cylinder pressure that they can generate, so they run them VERY close, as mentioned above. That is piston crown to cylinder head clearance, not piston to valve clearance, and it is SO close, that the measurements need to be precise. Clay isn't precise enough. Diesel engines in general are a different ballgame altogether in that sense. There are very good engine assemblers out there, and there are guys who know what they are doing.