Insane Hydraulics

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Manual Match-Lapping of Cylinder Blocks and Valve Plates of Hydraulic Pumps and Motors

A couple of tips on manual match-lapping of cylinder blocks and valve plates of hydraulic pumps and motors today. Nothing fancy, just a fellow self-taught tech sharing how he's been doing things.

So - the manual match-lapping is a (very slow) process of mating a barrel and a valve plate by rubbing one against the other with a lapping paste in between in hopes that the rolling and sliding action of the abrasive particles carves out the two perfectly matching surfaces, transforming the barrel/valve team from "leaky" to "good as new".

I never liked manual lapping and I never will, but it's a necessary evil because sometimes it simply makes sense - usually when it takes less than a couple of hours to recover a rotary group. Anything longer warrants parts replacement, unless, of course, it's one of those super-urgent repairs...

I did a lot of manual lapping back in the day when I spent all of my time in the shop, and as far I understand - manual lapping is like tensioning the chain of a chainsaw - if you talk to ten lumberjacks, you'll get ten different opinions, so the only thing that I can say "in my defense" is that I'll describe the way that has worked for me, but, as always, if you know a better one - I am all ears. And now - to the tips:

What is the first secret of manual match-lapping?

I would say that the first secret is knowing when to use it and when not to use it. For example - while you can lap surprisingly deep scores out of the bronze surface of a cylinder block, and it makes perfect sense to do so if the piston bores and the splines are in good condition, removing deep scores from the face of a hardened steel valve plate through manual lapping is a bad choice because you risk running out of bronze layer of the "poor" barrel before you work out all the deep scores. This is the case that I show in the video below, by the way - you can see that I "evacuated" a lot of bronze from the face of that cylinder block and still didn't remove all the scratches from the valve plate. In that case, it would have made much more sense to get a brand-new valve plate and then match-lap it to the cylinder block.

What is the second secret of manual match-lapping?

Applying small amounts of lapping paste frequently, with a thorough cleaning of the used paste in between the "charging cycles". Apply a tiny amount of paste, barely enough to cover the work, then add a little bit of oil, work it, then add a bit of oil one more time, and then stop as soon as you see the paste "wear out". It's hard to explain, but you will feel it because the paste becomes thinner and changes color due to the cut material getting mixed into it (which is especially visible when you lap bronze parts and the paste becomes brownish).

What is the third secret of manual lapping?

The third secret is pressure and motion. You should apply a fair amount of downward pressure on the part that you're holding (strong forearms required), and move the part in a circular pattern with increasing and decreasing radius, regularly alternating between circular and cycloidal motion and also changing the orientation of the valve plate. That's the best I can describe it. Watch the video below to see how I do it.

What should be on top - the valve plate or the barrel?

Good question. There are two "competitive" schools of manual match-lapping, I've tried both, and I prefer, hands down, the "valve plate on top", especially when you use a "motorized aid" (explained below).

Is a mirror surface finish necessary?

No, a mirror finish is not necessary. In fact - the dull finish that silicon carbide paste leaves is better for lubrication. So don't worry that your lapped valve plates don't "shine", and don't get "over-excited" if they do.

Do I need to remove all of the scores and scratches?

No, you don't. For example - if there's a deep circular score on the face of a valve plate, but it is very thin - you're better off leaving some of it, because, once again, you may "run out of barrel" before you lap a deep valve plate score out, and the rotary group can actually perform perfectly fine even with such a score.

How about them diamond lapping compounds?

They sound great, especially if you look at what companies like, for example, Kemet International, have to offer - but I, personally, have never tried them. I've used silicon carbide coarse and fine paste for many years - it's done the job. Why the silicon carbide? We bought tons of this stuff many years ago and are still using the same stock, so it's pure convenience in my case. Maybe I should evolve, but as I already said - I tend to avoid lapping parts altogether whenever I can.

How can you make it go faster?

It's called manual lapping, and it can be done 100% manually with the work fixed in a vise, but I strongly suggest that you do yourself a big favor and make a powered vertical chuck - it makes the manual lapping way faster and easier. It took me about half a day to build one - and all it took was a used orbital motor that I salvaged from scrap (one of the biggest perks of a hydraulic workshop is the unlimited access to hydraulic scrap), an old lathe chuck (also salvaged from scrap), and a hole in my mobile bench (and, of course, our test bench to drive the motor):

I am so bored and tired of manual lapping! What can I do?

Manual lapping is, indeed, a tedious exercise, which is why I recommend using the "ten fittings technique". Let me explain. Since the lapping paste wears out quickly and should be replaced at short intervals, manual lapping is, essentially, a series of consecutive "paste charging cycles". What I do is I put ten random hydraulic fittings in front of me, and move one fitting each time I re-charge the paste. Then - I evaluate the lapping progress only after I've done a ten-fitting cycle (which takes about 20-30 minutes). Or, sometimes I do a ten-fitting cycle - and then go do something else, and then I come back later and do another ten-fitting cycle. This is very helpful to people like me, who have an average attention span of about two seconds...

A couple of 10-fitting cycles should be enough to recover a bronze face. And 4 to 5 10-fitting cycles should be the biggest amount of time you spend lapping manually. If it takes more - you should have replaced the parts.

Here are some pictures of the parts that I lapped in the video. The barrel surface actually got perfectly lapped after about 15 charging cycles, and if you look carefully at the picture of the barrel face after "40 fittings" and compare it to the "before" pic - you'll see that the outer brim of the bronze surface got significantly thinner, which means that I removed a lot of the bronze layer, and that was really not necessary - the barrel should have been match-lapped with a new valve plate.

And now, finally the promised video:


Fun fact - some Eaton service manuals include the phrase "do not lap piston shoes" or "do not lap piston block". I would love to know why they say that!