reamed hole vs. drill rod size

[blockmanjohn]

Hi, I am attempting to fit a 5/32 1018 valve stem into a 5/32 reamed hole in a bronze valve cage. No go, so I have to adjust some thing. I would like a tight sliding fit for the engine that I am building (Jan Ridders Scuderi cycle engine). It is important that this fit is tight as this particular engine requires the intake valve guide and stem to resist the compression pressure for a certain amount of time. I thought about lapping the two together but wonder if there are any other options. Thanks, John.

 

[Brian Rupnow]

It is a strange thing, but an "on size" shaft will not fit into an "on size" reamed hole. Chuck the valve up in your lathe and with the lathe running at about 200 rpm, loop a piece of 200 grit carborundum sanding strip around it and gently work it back and forth on the valve stem. It doesn't take very much sanding, and don't get your fingers wound up in the lathe.

 

[kvom]

I have a set of under/over reamers that are .001" over and under. Over for a sliding fit and under for a press fit.

 

[Chris Murphy]

Micromark does a set of these 'over and under' reamers-look up item #84753 on their website....covers the range 1/8"-1/2" in 1/16" steps

 

[RonGinger]

Drill rod is not really round. Set up a piece to lap its od, blue the part with magic marker and take a light lap pass. You will see some linear bands along the rod. The way it finish ground causes these lobes. So you cannot expect it to be really round, like for a valve stem.

Give the stem a light lap pass to true it up.

 

[Rustkolector]

Drill rod is not really round. Set up a piece to lap its od, blue the part with magic marker and take a light lap pass. You will see some linear bands along the rod. The way it finish ground causes these lobes. So you cannot expect it to be really round, like for a valve stem.

Give the stem a light lap pass to true it up.

Ron is so right. Drill rod is not round. Some years ago Model Engine Builder magazine had an article on hones. The shaft hone looked simple enough to make. I made one and it does an excellent job at truing up ground shafting. If you need the tightest fit, a shaft hone will take off the shaft irregularities leaving a true round shaft and that is sometimes all you need to get a good fit. That said, when I fitted my first honed shaft to a reamed bronze bushing I was a bit disappointed to find that reamers also don't produce a true round hole.

Be careful lapping a bronze bushing. Clover type compounds will embed in the bronze. You can't remove it all and it keeps cutting. You need a non embedding compound like Timesaver which produces about .0015" clearance with the lap. Best fit might be to hone the shaft round and use use file backed fine abrasive paper and WD-40 on the valve stem and fit to a bored valve guide hole.
Jeff

 

[petertha]

Agree with above points. Been there done that.
The other thing I would add is bronze can be a grabby material, similar to brass. Regular drill tips can drift off just a bit especially on longer (depth:diameter) holes & deviation can feel like a slightly tighter fit to the length of a valve stem. A reamer will basically follow this pilot hole so you are chasing your tail. I found a short length, sharp carbide drill works good (in small diameter holes). If you get into larger diameter holes is best to dub the tip.

Also, on valve cages I found, if at all possible, drill the valve stem hole and counterbore all in one lathe setup vs reversing in a second operation. Even with a low TIR runout collet chuck its possible the 2 axis will not be co-linear to the degree valve seating is more problematic.

 

[peterl95124]

"200 grit", holy cow !!!, I use 600-grit wet-or-dry paper to get within 0.0005", then 1000-grit to take off tenths at a time to within 0.0001", and finally 1500 or 2000-grit for that last tenth and mirror polish. don't hold any of these by hand except for the last 2000-grit polish, or you'll get a tapered or worse stem, back it up with a hard steel plate. mic both ends and the middle to make sure you're applying even pressure to the plate/paper. YMMV, but I wouldn't even bring 200-grit into my shop.

One of the best investments I've made is a set of stub-length split-point cobalt drills, you don't need to spot drill with them and they don't drift. but of course for a bearing or a sleeve/guide you do want to ream. My experience is different brands of reamers have different cutting angles and make different sized holes, and also a given reamer will make different sized holes in different materials (EG aluminum-bronze is the worst, holes will always be undersize no matter what brand reamer you're using) so pick one for your engine and make them all the same so you can figure out what size to polish your stems down to and make them all the same also.

good luck. be safe. let us know what worked for you.

 

[petertha]

Some years ago Model Engine Builder magazine had an article on hones. The shaft hone looked simple enough to make. I made one and it does an excellent job at truing up ground shafting.

Can you post a picture of what you made and/or the specific MEB issue if you happen to know it?

After spending way too much time on seemingly 'simple' cylindrical things like valve shafts & wrist pins I'm ready to invest the time in a tool that works. These clamp styles come up in discussion often.
Series: External Hone « Tom's Maker Site

I've made some simple split laps by sandwiching two bars of metal, drill/ream a hole in the center. They work but seemingly not for long. Aluminum embeds lapping compound better but seem to wear out quickly. Steel & cast iron hold up longer but don't seem to embed as well. And then there is the rigmarole of how much oversize the lap hole should be. First it has to be large enough to accommodate a slightly elliptical section of drill rod. Then once the stock becomes truly circular, getting it down to the right diameter. I made some with multiple lap holes so it would give me a sequence & also by re-dressing the flat side on sandpaper makes a closer fit. In the end wet-o-dry backed aluminum stick was probably as effective.

My next step was to make a lapping tool that accommodates replaceable strips of wet/dry paper. In fine grit its actually quite consistent thickness across a sheet. When it wears, toss & replace. There would be a lot less mess than lapping compound.

 

[petertha]

One of the best investments I've made is a set of stub-length split-point cobalt drills, you don't need to spot drill with them and they don't drift. but of course for a bearing or a sleeve/guide you do want to ream.

I finally got a set of stub length drills out of necessity & also impressed with how they drilled. I don't think they are split point but I do know they are wider cone angle than typical 118-deg where one would use a 12-deg center. They might be like 140-deg or so? I hadn't realized this was the case until I got them. Turns out >= 140-deg center drills are not nearly as common & usually spendy. So while hunting around I've just been carefully spotting without. I'm not sure if its the shorter length rigidity or the point, but they seem better than typical jobber drills all things equal. I assumed all drills should be started with a center drill for best results & that these stubbies were just more forgiving?

The high flute angle, short length carbide drill I bought was amazing. It makes 2 perfect swirly chips & the hole looks as good if not better than the reamer in bronze. But I'm also not convinced this particular reamer is awesome. Bronze is a bit of a head scratcher at times.

 

[Rustkolector]

Petertha,
Gail Graham's simple but excellent external hone is shown on page 25 of MEB issue 25. The drawings are included in the same issue.
Jeff

 

[peterl95124]

BTW, if you're using drill-rod for the valve stem that sort of implies the valve head is separate ?
If so your real problems may all lie ahead of you. Unless the valve head and the stem are both cut
at the same time to ensure concentricity you may never get the valve to seal. Ditto for the valve
cage, the seat and guide have to be cut at the same time or again the valve may never seal.
No matter how much lapping compound or other techniques you use to try to get it to seal.

 

[blockmanjohn]

Hi all, thank you for all of the professional answers to my post. I think I will proceed as follows. Since it is critical that the intake valve stem be a tight sliding fit with the valve cage I am going to try 5/32 drill rod for the stem and a 5/32 reamed hole in the valve guide. I have some 1200 grit compound that I will load onto the stem and slowly work it into the guide. Does this sound reasonable? Also, is bronze better than brass for the cage?

Thanks, John.

 

[petertha]

Jeff, I'm still hunting for MEB back issue. By chance does the hone look like this? The description says "Small external hone made to Gail Graham's design from M.E.M. I made mine from steel instead of brass in his design."
https://www.model-engineer.co.uk/albums/member_photo.asp?a=46452&p=770267

 

[Rustkolector]

John,
Yes, phosphor bronze is much harder than brass. It holds a good seat and should wear better than brass. Machine the valve cages on one setup on the lathe to maintain concentricity. After drilling for the valve guide, and before reaming, make a light pass with a carbide boring cutter through the drilled hole to make sure it is straight and round. Set your compound at 45 degrees and cut the valve seat as smooth as you can. Make the cage valve seat narrow, only about .010" wide. Don't change the compound angle until you have finished making your two piece valves.

When using Clover type lapping compounds to open the reamed guide on brass or bronze the abrasive will embed in the guide. Much of it can be removed but some will remain embedded and continue to wear your precision valve stem. You would be better off to fit your valve stem material to your reamed holes without lapping compound. You can reduce your stem shaft material by honing or with file backed abrasive paper till you get the fit you need. Then make your two piece valves. Be sure to use a true running collet to hold you valves when you cut your valve seat angle to maintain the concentricity of the valve seat and stem. If you don't have a collet make a simple aluminum pot chuck.

To test your valves use a brake bleeder hand vacuum pump check your valves for sealing before you use any lapping compound on the seats. They may not need lapping.

 

[peterl95124]

blockmanjohn said "Does this sound reasonable", no you cannot use the parts that need to fit together to use as laps against each other, you will end up with an oversized sloppy fit. Get a high quality RH cut LH spiral reamer for your guide, they make the most consistent and nearly round as possible for a reamer hole. whatever size hole it makes, if you cut all the guides the same at least they will all be identical. Then use 1000-grit wet-or-dry paper on a stem to get it to a fit that you cannot detect any slop, measure it with your Starrett vernier mic good to 0.0001" and make as many as you need
that same size. they will probably not be exactly 0.1562" because the size hole a reamer actually makes depends on how it was sharpened and what type material it is cutting, and what type cutting fluid you're using, and at what speed you cut.

do not press fit this guide into the head, that will shrink the hole and the valve won't fit anymore (don't ask me how I know this!), instead loctite the guide in.

I have to ask one more time, since you say you're using 5/32 drill rod that sort of implies the valve head is separate from the valve stem, if so read my previous post.

Finally, brass is bad because it galls on steel, use CA544 leaded-phosphor-bronze or CA932 leaded-bearing-bronze.

 

[Rustkolector]

Petertha,
Yes, that is the hone I made and use.
Jeff

 

[blockmanjohn]

Yes, I plan to silver solder the stem to the valve head. I have never done this before but I think it will come out well.

 

[peterl95124]

Yes, I plan to silver solder the stem to the valve head. I have never done this before but I think it will come out well.

it might be worth asking folks for other opinions about this, maybe someone has actually done it, maybe those with lots of silver-soldering would have some good feedback

 

[Rustkolector]

Silver brazing SS head on the stem is fine. Turn a shoulder on the stem to keep the SS head square. Chamfer the stem hole in the head piece. Put a small dab of flux and small piece of braze wire in the chamfer. Stand the stem on end in a vise and heat from the stem side with a butane hand torch. No direct flame on the flux. Clean up in citric acid pickle solution.