New compression tester from Baileys.

[GrassGuerilla]

I'm pretty excited about getting my new compression tester. The automotive style tester I was using was giving me shaky results at best. Small displacement engines all tested rather low. In terms of quality the baileys (actually Trisco) is kinda cheesy. The dial glass is plastic, and seems to fall off, along with the bezel, pretty easy. It does however seem to work as it should. If I worked on small engines for a living, I'd want something better made. For a pro-Sumer hobbyist type, I'm hopeful it will be fine.

Now it's time for me to learn to better interpret the results. I was relieved that some recent aquisitions seem healthy. At least one, is more borderline. At 160+psi cold my Homelite 550 seems to be in fine shape. Similar results for my 046.

My 026 on the other hand is right about 140psi cold. Pulling the muffler reveals some slightly polished looking areas on the piston. Not really scratches, can't feel an edge at all. This saw was a recent find, and needs a carb kit at minimum. At 140 cold, should I put a carb kit in and run her, or re-ring it, or?

 

[cgraham1]

That's pretty low for that saw. Here's my MS260 that I just sold...

And here's the 026 that I got for my FIL...

 

[rmh3481]

Art,
Try a couple drops of oil or a shot of wd-40 down through the plug hole on your next test and see if things change.

 

[cgraham1]

I got mine for $5 at a yard sale. Never used, receipt in the box ($39.95 at local auto parts store).

I like to test all my saws...

036 Pro (Sold! )


Dolmar 7900...


Jonsered 2171 w/ new Meteor piston and base gasket removed...

 

[zogger]

Rings are cheap, if you feel like tearing it down.

 

[Chris-PA]

The issue of interpreting the results is why I've never bought one. I can see where it would be useful to judge condition if one kept track of the reading of a given saw. However, consider a cylinder with a small score or other defect that bleeds off 30psi at low cranking speeds. You might be able to pull it at 100rpm or so, so the piston spends about 200ms above the exhaust port.

Now consider that same engine at 10,000rpm. The piston now spends maybe 2ms above the exhaust port, but the combustion pressures are much higher. What does that same defect do now to the cylinder pressures?

It is not clear to me how to use the low speed measurement to figure out what happens at speed.

 

[MasterMech]

The issue of interpreting the results is why I've never bought one. I can see where it would be useful to judge condition if one kept track of the reading of a given saw. However, consider a cylinder with a small score or other defect that bleeds off 30psi at low cranking speeds. You might be able to pull it at 100rpm or so, so the piston spends about 200ms above the exhaust port.

Now consider that same engine at 10,000rpm. The piston now spends maybe 2ms above the exhaust port, but the combustion pressures are much higher. What does that same defect do now to the cylinder pressures?

It is not clear to me how to use the low speed measurement to figure out what happens at speed.

Compression testing is subjective. Some engines will run fine at 100-110 psi and others won't even cough with 120+. The only time a comp test provides definitive results is when an engine has a history recorded. If it blows 160 new and is in that ballpark until one day it won't start or runs poorly and blows 120psi, it's a pretty safe bet the problem is with the rings/piston/cylinder. With a ballpark figure, you know with a quick test wether the engine is worth further investigation or not. Too much weight is given to the individual test result however. Too many variables to say that a saw that blows 160 psi is better than one that blows 155. Barometric pressure, gauge differences, hose length, cranking speed, engine temp, oil presence, etc all affect that number.

It's not a useless tool and certainly worth having if you work on any type of engine. But much like vacuum gauges for cars back in the day, it's up to the user of the tool to correctly interpret the results. It's NOT the kind of tester/tool to light up a green or red light for "good" or "bad".

 

[Chris-PA]

Compression testing is subjective. Some engines will run fine at 100-110 psi and others won't even cough with 120+. The only time a comp test provides definitive results is when an engine has a history recorded. If it blows 160 new and is in that ballpark until one day it won't start or runs poorly and blows 120psi, it's a pretty safe bet the problem is with the rings/piston/cylinder. With a ballpark figure, you know with a quick test wether the engine is worth further investigation or not. Too much weight is given to the individual test result however. Too many variables to say that a saw that blows 160 psi is better than one that blows 155. Barometric pressure, gauge differences, hose length, cranking speed, engine temp, oil presence, etc all affect that number.

It's not a useless tool and certainly worth having if you work on any type of engine. But much like vacuum gauges for cars back in the day, it's up to the user of the tool to correctly interpret the results. It's NOT the kind of tester/tool to light up a green or red light for "good" or "bad".

Exactly!

 

[Coldfront]

It is great when a saw won't start, you check it and it is around 50 psi, a quick test to troubleshoot, anything at or above 110 psi with my comp. tester should start. I don't know if my tester is a cheap one or what but even saws that start good and run great I never seem to get it over 110 - 115 psi even after 10 or 12 hard pulls, and yes it has the scharder valave at the end of the hose where it screws into the spark plug hole. I would like to try it on a brand new saw sometime. I'm not sure how much it matters but the hose on my tester seems kind of long maybe a foot and a half long?

 

[MasterMech]

It is great when a saw won't start, you check it and it is around 50 psi, a quick test to troubleshoot, anything at or above 110 psi with my comp. tester should start. I don't know if my tester is a cheap one or what but even saws that start good and run great I never seem to get it over 110 - 115 psi even after 10 or 12 hard pulls, and yes it has the scharder valave at the end of the hose where it screws into the spark plug hole. I would like to try it on a brand new saw sometime. I'm not sure how much it matters but the hose on my tester seems kind of long maybe a foot and a half long?

I don't know about your specific gauge but if you have an air compressor, hook it up and compare the reading on the compressors gauge vs yours. Not exact since the compressors gauge is unlikely to be a calibrated gauge but it will give you some idea.

 

[Chris-PA]

I don't know about your specific gauge but if you have an air compressor, hook it up and compare the reading on the compressors gauge vs yours. Not exact since the compressors gauge is unlikely to be a calibrated gauge but it will give you some idea.

That will only tell you the basic accuracy of the gauge.

The cylinder has a finite (and small) volume. The volume of the gauge tube outside of the schrader valve gets added to the cylinder volume and decreases the effective compression ratio, which causes a lower reading. However, a compressor has basically in infinite volume, so the compression gauge and the compressor gauge will read the same (to within the accuracy of the gauge).

 

[CTYank]

That will only tell you the basic accuracy of the gauge.

The cylinder has a finite (and small) volume. The volume of the gauge tube outside of the schrader valve gets added to the cylinder volume and decreases the effective compression ratio, which causes a lower reading. However, a compressor has basically in infinite volume, so the compression gauge and the compressor gauge will read the same (to within the accuracy of the gauge).

Not really, being on the "other side" of the valve. The more volume in the hose, the more engine rotations it'll take to get up to a final reading. An asymptotic thing. Easy to verify that experimentally.

Big potential issue is the specs of the schrader valve in the tester tip. Typical valve has spring that's too stiff, so reading will be erroneous- low. Proper valve can be had as part of OTC comp. tester kit 5606 in repair parts kit #1000-5143. Used to be able to get such valves from Snap-On as repair parts. No mo.

Compression tester is a very useful piece of test equip. for the complaint "My saw just won't start." Won't help with seals or reed valve though. Experts say that reading above 90 psi is sufficient for starting.

 

[hamish]

That's pretty low for that saw. Here's my MS260 that I just sold...

And here's the 026 that I got for my FIL...
View attachment 358912

If they are all stock, time for a new gauge.

 

[MasterMech]

That will only tell you the basic accuracy of the gauge.

The cylinder has a finite (and small) volume. The volume of the gauge tube outside of the schrader valve gets added to the cylinder volume and decreases the effective compression ratio, which causes a lower reading. However, a compressor has basically in infinite volume, so the compression gauge and the compressor gauge will read the same (to within the accuracy of the gauge).

If you are doing the test correctly, cranking the engine until the needle stops climbing, be that 10 pulls or 50 (I'm exaggerating a bit....) then there should be no difference in ultimate pressure barring any leaks. If your air compressor can reach 125psi and has a 30 gallon tank, what pressure will it reach if you increase the tank volume to 60 gallons? The only factors that affect ultimate compression readings should be ring seal, cranking speed (which is related to ring seal) and external leaks.

 

[ramv102000]

I like this one , doesn't have a hose just a fitting about 3" long and screws directly into the spark plug hole. Shrader valve is at the end of the fitting . Bought it used on ebay for $10.00.

 

[Chris-PA]

If you are doing the test correctly, cranking the engine until the needle stops climbing, be that 10 pulls or 50 (I'm exaggerating a bit....) then there should be no difference in ultimate pressure barring any leaks. If your air compressor can reach 125psi and has a 30 gallon tank, what pressure will it reach if you increase the tank volume to 60 gallons? The only factors that affect ultimate compression readings should be ring seal, cranking speed (which is related to ring seal) and external leaks.

At one point I argued it the same way you are, but I don't believe that is correct. The set up in both cases is similar - generally a piston pump with an outlet check valve and an enclosed volume with a gauge. But there is one difference: We read the compressor's max pressure on the outlet of the check valve, which is presumably the pressure it will reach in the piston chamber. That is determined by it's compression ratio. If you increased the volume in the piston chamber of the compressor pump it would lower the compression ratio and the max pressure it could reach.

The same goes for the engine - we're all familiar with the fact that a larger volume combustion chamber will give a lower compression ratio and lower cylinder pressures (and any explanation has to explain that, as I'm pretty sure it is true). That's because it is inside of the check valve. When you put on a gauge, you end up adding a bit to the cylinder volume inside the schrader valve, which lowers the peak pressure. The volume on the outside of the check valve, which is what we read, will eventually stabilize at the max and then it will essentially be dead-headed and no longer exists as far as the engine goes.

So basically, the max pressure is always based on the mechanical compression ratio (relative volumes), and increasing the volume reduces it. Leaks of course will also reduce it.

 

[John from Cle Elum]

I believe the Schrader valve and relief valve are factors. I have two gages and there was a 30psi difference. I got suspicious (sp?) and traded the gages between the relief valve/flex tube/schrader assembly. Low and behold the readings were reversed. In other words, the gages were consistant when used with the same flex tube assembly. I suspect the higher one is correct. I believe the Schrader and the relief valve may be late to crack open.

Some one argue with me.

 

[blsnelling]

As with all of the other inexpensive compression gauges, my Baileys unit didn't last all that long. I've had a SnapOn for several years now and it's still going strong. They're worth the entry cost if you're going to use it much at all.

 

[MasterMech]

As with all of the other inexpensive compression gauges, my Baileys unit didn't last all that long. I've had a SnapOn for several years now and it's still going strong. They're worth the entry cost if you're going to use it much at all.

I have a Craftsman/MityVac compression tester set that has served me well for years and is still giving me good readings so far as I can tell. My only knock is that you must use an adapter for engines that use 10mm plug threads and that will lower your comp readings a fair amount.

I have thought about replacing this set just because and when I do, it will probably be the Snap-On.

 

[LegDeLimber]

Along with getting the correct valve core, You need to know that it's not leaking.
After pumping it up, I'd say the gauge should hold pressure a few days.
The one I'm using now will still have 2/3rds or so of it's initial reading after about 3 weeks.
Not perfect obviously, But I figure it's sufficient for the task.

http://www.arboristsite.com/community/attachments/comp-gauge-cores-napa-jpg.356474/
Well heck, i was trying to re-post the pic of the NAPA cores and their part #
phooey, been a long day and i'm tired.