Accuracy of Expandable Reamers in Brass

I am trying to repair a trumpet who's valves are shot ... ie oversize. Too much clearance. My plan of action is to add a few thou of nickel plate to the Monel pistons and to open up the mating brass cylinder. Apparently the ideal clearance between the parts is about half a thou, 3/10 's is perfect. I've considered honing the cylinders but can't find a hone to go down to the approximate 5/8's diameter. I'm thinking now that an expandable reamer might do the trick. I'm not sure how accurate the final size would be and I'd like to control the tolerance to +/- 0.0001. Finish is important, so I'd almost certainly hand lap the piston into the cylinder. Problem is of course, I need to accommodate the piston into the cylinder which means going over size in the first place but not more than say 0.0002. Just measuring a roughly 5/8's hole that is about 4 inches deep - through hole mind you, so it's 2 inches from either end - is a challenge. My telescoping gauges from China are junk and even with a good set, that transfer measurement ain't going to be +/- 0.0001 accurate. Any suggestions ?

C.M. said:

I've considered honing the cylinders but can't find a hone to go down to the approximate 5/8's diameter. I'm thinking now that an expandable reamer might do the trick. I'm not sure how accurate the final size would be and I'd like to control the tolerance to +/- 0.0001. Finish is important, so I'd almost certainly hand lap the piston into the cylinder.

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You can very easily lap a couple thousandths of nickle plate, the only problem is limiting the bell mouthing on the ends. Lapping that little is quite a bit like honing, really.

Problem is of course, I need to accommodate the piston into the cylinder which means going over size in the first place but not more than say 0.0002. Just measuring a roughly 5/8's hole that is about 4 inches deep - through hole mind you, so it's 2 inches from either end - is a challenge. My telescoping gauges from China are junk and even with a good set, that transfer measurement ain't going to be +/- 0.0001 accurate. Any suggestions ?

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Maybe a set of Deltronic gages over that range? You can get them in tenths around a nominal diameter.

My experience with expandable hand reamers is that you work your way up by trial and error. Be sure to ream a hole in a trial piece first (with a brand new reamer) just to get a feel for how it cuts and whether there is a wart on one of the blades that will produce a score mark if you pass the reamer right through the hole. You might want to carefully hone the exit corners of the blades to reduce any burring they might bring up.

I've had an older musician who used to be a machinist come in and borrow one of my reamers to do this sort of job.

Adjust the reamer initially so it cuts next to nothing as you make an initial pass through all the holes to even them up. Then adjust the blade nuts by 1/4 turn and cut your first pass on all the holes. Keep adjusting and cutting until all the holes are cleaned up all the way through. If you finish them all without adjusting the reamer in between, they should all be pretty close to identical. I like to finish ream from both ends just to make sure the bore is straight through, no funnel left on one end. Then machine a trial piston to test the fit. Then finish the real pistons at that diameter. I would avoid lapping just for the sake of knowing there is no bit of embedded grit lurking to spoil things.

Be very gentle with the reamer entry. Any bit of crookedness will have the reamer dig in. Just pull it straight out (don't turn it backwards), and adjust to a lower bite and try again.

Turning up a split lap is a simple matter. Aluminum will be fine for a few valves of a single trumpet.
Fine Clover compound or equal would do the cutting.

Size the lap after charging by running the process on a test piece of similar brass tubing. McMaster Car may supply both compound and brass tubing.

I hesitate to suggest an expansion reamer, or any reamer for the thin wall and soldered construction of a trumpet. One catch, and there will an irreparable blemish in the surface. And brass catches easily.

You can get ball hones way way smaller than 5/8... easy to use.

Look up "flex hone".

Lakeside53 said:

You can get ball hones way way smaller than 5/8... easy to use.

Look up "flex hone".

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How do they do in bores with sizeable crossholes in them?

Lakeside53 said:

You can get ball hones way way smaller than 5/8... easy to use.

Look up "flex hone".

Click to expand...

I'm pretty sure that he wants to make the sides of the hole parallel to one another when it's not starting that way. Ball hone won't do that.