Last season it seemed that my throttle was leaking. I made a fixture that would allow it to be put under air pressure on the bench, and it leaked down pretty quickly. I started lapping it, and could see irregularities in the color of the seat. I continued to lap until the seat showed as a nice clean bronze-colored circle, of consistent width all the way around.

It still leaks air, coming out the throttle exit to the superheater. Should I assume that my fixture is unable to simulate the closing force on the valve stem in real life? Plus, that the valve will somehow seat better when hot and with 500+ PSI of steam pushing it closed? I can't think of anything else to do to improve the mating between the plug and the seat.

Thanks...

Dear Kelly, You are doing your lapping correctly. Finishing your lapping with the "fine" lapping compound yielding a continuous lapped seat is the best that one can do. I re-lap my throttle seats about once every two years. They hold steam tight with steam on them. I have never tested them with air pressure. I once did have a throttle body that had a crack through the throttle's seat face. The crack could only be seen with a magnifying glass though. It took a while to find it, and a replacement throttle was the only remedy for that problem. Another important thing to check is to make sure that your throttle stem is not bent. If the stem is bent, your throttle will not always re-seat in the same seated position that it was lapped in at.

I have found on [gas] car valves ,,,It is essential to lift and turn in the lapping process,,
lest you get concentric circles that will NOT seal,,and cannot be made to seal ,,
even by endless further lapping,,
IF you are starting from scratch,,new or freshly machined,,a narrow seat edge is best,,
Higher pressure on the seat is not likely to effect things,,
Lift an' turn,,,lift an'turn,,,,boring,,,,,not too much pressure ,,lift an' turn,,
What compound are you using???,,,
What words of wisdom do you guys have in this topic,,,
I have run into a few stubborn ones over the years,,,,,,Ben

The throttle seat does not rotate but returns to the same position each time.
Some times just filing some off the end of the rod so when the valve end is on the rod in a different position helps. Another thing you can do is alien the three dots or alignment holes on the shaft in the correct position and give the valve seat a good rape.
One thing to remember is to clean the parts very well to remove the compound before reassembling. The steam will push any lapping compound left right into your engine.
Rolly

I had used the only thing I could find at any local auto parts store - Permatex Valve Grinding Compound. Could find no other grades or anything that used the word "lapping". It was pretty gritty, and I feared that I had scored the seat. Casting about for other potential lighter abrasives, I picked up some DuPont 7 rubbing compound. I could put my ear next to the outlet and hear it cutting just a little bit - seemed like a good thing.

Thinking about the non-rotating alignment of the stem controlled by the punch marks, as Rolly described, and thinking about Ben's lift-turn-drop, I settled on an alternating lift-turn drop pattern that only went 10 or 15 degrees on either side of where the plug will be forever seating, rather than trying to do some kind of full circles. Slow work, as the abrasive wore away after 2 or 3 twists each time.

After a good bit of that, it leaked a lot less. And I got a beautifully polished contact circle on the plug, so I could see where it was contacting the seat. I even switched to Kit Scratch-Out to get an even smoother surface. Still a very small bit of air leakage would push on my thumb when I held it over the outlet.

It seemed like it might be possible to get to zero leakage this way, but... The contact circle on the tapered plug is almost 2/32 wide, and it runs right out to the outer circumference of the plug. This makes me think that the plug is now starting to bore its way into the metal of the throttle body.

Expert Stanley tech Bill Barnes reminded me that the contact circle should be much closer to a thin line than a 2/32 strip. And, he has a tool to cut back the metal to achieve that, and he's not far from me. So hopefully by Tuesday evening it will be sealing 100%.

I got a machinist friend to put the stem assembly on a comparator, and we determined that the metering barrel is pretty much perfectly concentric to the angled plug face. Wish I would have thought to have him lay it on the stem and check for straightness there.

Lapping is nothing more then fine grinding away of something. To cut or ware away something you have to use the same material or something harder. Most grinding compounds are crushed rock or mineral. Pumice and rottenstone are a good material for fine lapping they come in many grit sizes and can be bought by the pound in good wood workers supply house. You use to be able to get then in a drug store. Diamond grinding should only be used on very hard metals. I use fine pumice and linseed oil to polish oil finishes on wood then switch to rottenstone for the finish rubbing. But for nonferrous metal good old tooth past works well. (Pumice) same thing your dentist uses to clean your teeth. Another material most have right in the house is calcium carbonate (Ajax) works great. But any polishing or grinding some of the stone of cutting material ends up in the surface your grinding and very good cleaning is need to remove it or the parts just ware away faster when in use. An ultra sonic vibrater tub with freeon is one of the best ways to remove embedded partials. But you can also boil the parts in clean water.

Rolly



Edited 1 time(s). Last edit at 06/10/2013 12:31PM by Rolly.

Another methiod to "read" the surface contact,,,,
Put blue dykum [sp?] on one surface and touch together,,
with or without turning,,to get a reading,,
this is like greese or iodine [paint it on ,let dry] 2products,,
one should,,,,,,,should///aughtta work,,,
Another ,,is to chalk one surface,,rub together,,and see where it touches
This is not so easy to read but it works,,Practice,,,Patience,,and so on,,
[into the night]] haha,,Ben
OOOps,,,on that 1/8" band,,,look for a shiny concentric line around,,,Overlapping is O-K,,,a gap is not O-K,,The shiny band may or may not be present on this job,,CB

Hi Kelley,,,Just curious if you had success with the throttle,,
and what you observed,,Cheers,,Ben