Compression tester question.

[kevino]

 

My compression tester won't fit in the sparkplug hole.  It's the right size but unless I tear the motor apart I don't have enough  elbow room to thread it in.    I ordered an adapter to go with this 30 yr old tester.    It cost $8.    Napa had dozens of testers  on display but I saw nothing under $100. Any suggestions?  

TIA,

kevino

[GrumpyUnk]

I ordered a gauge kit from ebay/'zon(I forget). Somewhere less than $20 as I recall. It came with multple adapters, and one fit properly. I screwed the adapter onto the hose, and screwed it into the spark plug hole.
I then cranked the engine with the throttle fully open. The gauge would not register as the cylinder volume was not enough to work the gauge. May have been missing a check valve somewhere(internal to the gauge, not down at the adapter). The reading was way off. I had to take pics and send to vendor. They gave a refund. Adapters were fine, but the gauge too crappy.
I used my OLD(50+) gauge from J.C.Whitney, which had a real pressure gauge, and it all worked.
So, you can get a full kit from the web, non-name brand, and if you have a good gauge, it will let you do a test. Unless you spring for a good gauge, don't count on them working with a 50cc engine. The compressed air just goes back and forth inside the rubber hose and ends up not moving the gauge needle. In my case.
Old style gauges that have a rubber cone adapter that must be held in place manually may work better as there is a LOT less volume enclosed leading to the actual gauge mechanism. You will need two people or a remote starter switch.
tom

[jackrides]

A lot of the damn new gauges do not have the one way (Shrader) valve at the end of the spark plug hole adapter. They willalways read low in everything. Great for shops to unnecessary engine work, kind of like the brand of engine code readers that are designed to sell parts with an incomplete diagnosis. Thanks A.Z.

[FrankenMech]

Watch out for those 'battery' or 'charging system' testers too... They leave them set for a 1000A battery etc and everything tests 'bad' but then the computerized digital equipment never lies. Anyone with a digital meter is always an instant electrical engineer becuz the meter says so.

Also, with a small displacement engine it takes more revolutions to fill the hose and gauge volume with air to reach maximum pressure even with a good check valve. Those push-in testers you have to hold do not work well with high compression engines. It is a function of spark plug hole area, available hand strength, and compression ratio.

If the engine shroud and frame does not allow easy access then you just have to work harder and move around more parts to gain clear access.

[GrumpyUnk]

Ayup. I do think the push-on testers will likely be better than a set with adaptes and rubber hose on small engines as the volume is a lot less within the push-on, and the have a schraeder to capture the air(mandatory as you cannot remove for reading???). The el-cheapo appear to have missed the boat about the need for a check valve. Maybe I should look closer at the one on the shelf that did not work, but did have the size needed for a 139QMB sized spark plug. It just kind of huffed in and out, but did hold pressure at a lot lower level than the old one.
About the battery condition testers, I thought they used the internal resistance along with the battery voltage and did some mumbo-jumbo based on that and the 'rated' CCA to determine what percent of capacity was left.
Wouldn't that be a pretty simple measurement to do? The one I purchased seems to be 'close' if not accurate.
tom

[FrankenMech]

The old style resistance/voltmeter testers worked fairly well as long as you know how to use one. I have had one for over 40 years. The new style digital testers used by the Zone and other shops don't work worth a damn without training and operators that are not paid to sell repairs and parts. They are used to hoodwink the average customer.

The check valve is required in ANY compression tester.

[GrumpyUnk]

I think it might be worth checking to see if a check valve can be screwed into the fittings or hose immediately close to the working end. My impression is there was too much working room(volume of air) internally for the low buck gauge to work on a small displacement test. I have never looked to see if there are threads so the valve can be relocated. Somehow the old gauge gave resonable readings, a LOT higher than the new one. I put it off to having a better mechanism. The name 'Bourdon tube' comes to mind, but I would not bet much on it being correct.
tom

[FrankenMech]

The bourdon tube is what is inside a pressure or vacuum gauge that expands and contracts with pressure differences moving the needle. Just remember to capture a lot of pressure pulses with small displacement engines in order to get a proper reading. Inspect the check valve to make sure it is tight and does not leak. Most are made like a Schrader valve for a tire and can be removed or tightened with ordinary bike tire tools. The check valve should be as close to the combustion chamber as possible.

I have tools to make the threads for the original Schrader valve, but not the Chinese copy metric bastardized 'international standard' threads they use now but it is close enough. I think manufacturers altered the thread specs to get around the Schrader patent. Patents are mostly useless today.

[snaker]

I've done compression tests on engines and haven't noticed any unusual problems with the small volumes.
That is the role of the one way valve.
It allows a sort of a pump jack building up of the pressure at the gauge a little at a time.
It may take more strokes to build up to max pressure, but it still should get there.
Just keep going till the pressure reading peaks.

Another thing the one way does is dampen the needle oscillations and keep any vacuum from getting to the gauge.
These gauges have a mechanical stop at zero and the engines intake vacuum along with any violent oscillations would damage the gauge needle.

[snaker]

Sept 20, 2023 7:32:44 GMT -5 GrumpyUnk said:

I think it might be worth checking to see if a check valve can be screwed into the fittings or hose immediately close to the working end. My impression is there was too much working room(volume of air) internally for the low buck gauge to work on a small displacement test. I have never looked to see if there are threads so the valve can be relocated. Somehow the old gauge gave resonable readings, a LOT higher than the new one. I put it off to having a better mechanism. The name 'Bourdon tube' comes to mind, but I would not bet much on it being correct.
tom

Probably wouldn't hurt.

[GrumpyUnk]

Logic says, in a perfect world, the final compression pressure reached could be displayed on the gauge if you used enough compression strokes. In the real world, it did not work out that way. Perhaps the cranking rpm was not enough to overcome the slight leakage(undetectable by me) that may have occurred. Pretty sure the check valve(near the gauge, after the closest fitting) was working as it would keep the highest pressure reached.
I used a better quality gauge, with the check valve 'near' the gauge, and the fitting from the low-buck gauge set that fit the scooter threads. It pumped up and displayed numbers that were reasonable and significantly higher than the China built gauge.
My conclusion was that the China gauge mechanism was too coarse to get a decent reading that was at least in the ballpark. The old(close to 50 years) gauge, made in USA, with a name I recognized back when, worked and registered more accurately.
I do not think I will bother fiddling with the China gauge. The hoses & fittings will work with my old one. Only reason to buy a new set was to get the proper sized threaded adapter, which I now have.
If I had no other gauge, I would send the China gauge back as defective. You get what you pay for. In some instances it makes a big difference in performance. As far as China scooter parts, mostly, they come off the same production line, slap on a different label, and send it. Of course there are multiple assembly lines and quality varies, but to 'meet the standards' set by the bosses, they all will cut the same corner. I honestly cannot state that one supplier is better than the others. Some do have pride in their work, and some have pride in packing for shipment, and I have given appropriate feedback and purchased accordingly. One cylinder package was so nicely done, I did not want to open it. I ordered from them again, even when their price increased.
Some who have advertised as having a specific feature, when it is disclosed that is not true, will try to refund partial, such as $5. I have gotten full refunds, and of course pushback. When something was late, I got a refund, and then paid them when the item arrived. I will not be a cheat like some of them. Wayoff topic.
tom

[jackrides]

Where to get a tap to match a Shrader valve or the newer equivilent?

[FrankenMech]

Amazon 5/16-32 UNEF

www.amazon.com/gp/product/B07VCSW1W4

I bought one of these years ago. This Amazon seller has supplied good equipment in the past. BEWARE -you will need the proper specified drill or reamer for this tap. The UNEF threads are very easy to mess up with the wrong tap drill size. This tap is not suitable for hand tapping or threading in a drill motor. It is just for use in a lathe, milling machine, or industrial tool-room drill press.

I believe from examination of the original patent this is the size that was used. It will work with the modern equipment using the metric bastardized semi-equivalent specs.

The check valve needs to be as close to the combustion chamber as possible otherwise the increased volume in fittings, tubing, etc. will decrease the pressure reading. The pressure release valve can be anywhere between the gauge and check valve. The pressure on the gauge should show steady without dropping until the pressure release is operated. I had one compression tester with a faulty pressure release valve that could not be fixed, lowest bidder China equipment. I tossed it in the trash but kept an adapter.

[jackrides]

Frankenmech, that is a seriously good tool site! Maybe I'm misunderstanding the pics of Shrader valves, but a simple tap won't provide the smooth taper for the rubber seal on the valve to snug up against. What am I missing? Thanks!

[FrankenMech]

The tapered nose on the tap works OK. If you are careful the pilot drill leaves a nice chamfer also.