|Mick Bailey||22/11/2021 18:07:22|
|25 forum posts|
This winter I intend to do a top-end rebuild on an old industrial petrol engine. The bronze valve guides are 4.45mm bore and 42m long overall. The problem I've previously had on small guides was getting the bore concentric. I've had to resort to finishing the bore but leaving the guide oversized, then mounting the guide between centres to finish turn.
I saw small guides being commercially CNC machined and the outside was finished first, then just drilled (using a solid carbide drill), reamed and parted off. I've been unable to drill through this length accurately enough, though I was using HSS drills. The drill always wanders and subsequent finishing follows the original bore
What's the best technique/procedure to use?
|John ATTLEE||22/11/2021 18:31:30|
|21 forum posts|
I have made valve guides out of PB using your original technique with poor equipment and my limited skills and it was fine. If you did the ID first and then had the arbour running true, it is hard to see what could go wrong.
|Roger B||22/11/2021 20:28:04|
186 forum posts
I think that the answer depends on how you cut the valve seat. If it is cut from the valve guide bore the concentricity of the guide doesn't matter. if it was cut from the the bore in the cylinder head then concentricity is important.
|Oily Rag||22/11/2021 21:16:13|
540 forum posts
Why not 'gun drill' the bores with a single lip cutter made from drill stock? Guaranteed size and roundness as well as concentric.
I think this has been covered before, but more to do with automotive size valve guides, in that thread I replied the production method I most favoured was the broached finish with 'ballising'
Here is the picture I posted from that thread of the Triumph Meriden tool used for producing the PB guides in their engines:-
|Clive Brown 1||22/11/2021 21:25:50|
|806 forum posts|
I'd consider making a silver steel D-bit to your final diameter. After pilot drilling undersize,this can be started accurately centred in the job and should cut true, correcting any minor wandering of the pilot hole.
|Mick B1||22/11/2021 21:54:21|
|2153 forum posts|
I've come to the view that wandering often occurs if you drive the drill hard and don't peck often enough. If swarf gets trapped between the cylindrical land and the wall of the hole it'll deflect the drill point and one lip will cut deeper, diverting the hole slightly, and going deeper will only exaggerate the effect.
|John ATTLEE||23/11/2021 06:47:57|
|21 forum posts|
Surely, the seat will have to re-cut the seat in any case. I use an engine machine shop because it is so cheap it is not worth doing myself. I recently had a problem with a side valve continental engine in a Hyster forklift. It was not worth taking the engine out to take it to the machine shop. Fortunately after the valve guides were replaced, and after a quick grind, the valve faces and seats were fine.
|John MC||23/11/2021 08:27:30|
373 forum posts
I think Mick B is doing it exactly right, a bit of a faff making a mandrel, but concentricity is important and thats the way to do it.
Incidentally, phosphor bronze is not the best material for valve guides, it doesn't like the heat, especially around the exhaust. Aluminium bronze is much better, better still is Colsibro, a free cutting version of aluminium bronze. Cast iron is also worth considering in preference to PB.
|Mick Bailey||23/11/2021 08:52:09|
|25 forum posts|
Did the machine shop make the guides? No parts availability for my engine so everything needs to be made or adapted. I did think of getting spiral inserts fitted, though. Which machine shop did you use?
I reference the seat off the guide bore but like to achieve a concentric guide to minimise material removal from the seat. My valve refacer finishes the valve head to within 0.01mm concentricity to the stem and I like guides to have no more runout than this. This makes for a seat that only needs a light dressing to restore the angles and seat width and no need to grind in the valves.
My ML7 has a lever tailstock feed, which makes for rapid and frequent pecking to clear chips, but even with premium, new drills the hole will wander slightly with smaller holes. Last winter I checked my lathe alignment as I had a batch of guides to make in a larger size (6.98mm bore) and these turned out perfectly. I was able to use a solid carbide boring tool and then finish ream with a LH spiral carbide reamer and machine the outside and bore without removing the guide from the chuck for the main finishing. They only needed re-chucking to machine the oil seal register. I'll try different techniques and take a look at 'gun drilling' and D-bit finishing - I'd like to improve my methods of drilling deeper, small diameter holes accurately.
|John ATTLEE||23/11/2021 09:18:47|
|21 forum posts|
The last guides I made was for a JAP engine. It was easier to make new guides than mess about trying to find NOS ones. I used PB because it was what I had in stock! The engine would only be running very few hours so life was not an issue. So useful to learn of better materials on a forum.
|Nigel McBurney 1||23/11/2021 09:34:50|
999 forum posts
I would use cast iron,I have restored engines built between 1905 and 1950 and they have all had the valves guides made from cast iron, also most of them had the bores for the valve stems direct in the head. Also on the older slow running engines the valves had steel stems with cast iron heads.Petrol engines of this period also relied on valve stem lubrication from the occsional squirt from the oil can, egines using coal gas,paraffin or heavier oils which did not totally vapourise relied on lubrication from the oily fuel entering the engine and the exhaust valves lubricated from the unburnt fuel carried over in the exhaust gas.Very often there is little wear in the valve guide, replacement or boring out the existing hole and fitting a thin sleeve is due to old engines left in the open and the stem rusts solid in the head and damage can occurr removing the old valve.
|Mick Bailey||23/11/2021 10:34:08|
|25 forum posts|
My preference where bronze guides were fitted originally is to use aluminium bronze, or cast iron where this was used. The nice thing about cast iron guides is they can be finish lapped before fitting and the bore doesn't shrink down and hour-glass like a bronze guide heat-shrunk into an alloy head, so no need to re-hone after fitting.
My local stockist has Colisbro but the size range starts at 1/2", so plenty of expensive swarf. I've also used PB104, but this can be difficult to get hold of. Last year I was looking around for some C63000 bronze, which is popular in the USA for making guides but isn't easily found over here in the UK. The same with some of the manganese bronzes that are suitable for making guides - special order only and in larger quantities than I could use. The cast iron bar in smaller diameters seems to have disappeared. I used to buy down to 3/8" but 3/4" or even 1" is getting more common as the smallest stocked size.
Please login to post a reply.
Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!
You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.
Click THIS LINK for full contact details.
For subscription issues please see THIS LINK.