I have had the same issue with Aluminium bronze guides, the trick if you end up with a slip fit instead of a press fit in the housing is to copper plate the OD in a copper sulphate solution, varnish the surface you do not need plating. Reaming the ID is best done with a flex/ball hone as HSS reamer wears out before the first cut is finished and just rubs creating heat.
Reaming the ID is best done with a flex/ball hone as HSS reamer wears out before the first cut is finished and just rubs creating heat.
If I don't attack the bottom end of the engine in the right order (determine current balance factor, disassemble crank, remove and weigh small end reducing bushing to allow for a revised determination, etc.) it will mean extra work for me. Given that, my current plan(*) for the new small end bushing is to take into account Ampco's machining recommendations to turn the OD to the required oversize for a proper press fit, bore the ID to a to-be-determined size that is smaller than the final size (keeping in mind the ID will be reduced somewhat after the press fit), press it into place, and hone it to the final diameter to give the necessary clearance for the wrist pin. All the while insuring the bore of the bushing is precisely parallel to the crankshaft. Piece of cake, eh?...
Final honing to size may prove to be the most difficult part of this (although, not yet having machined the material, I may find problems there as well). For that my plan(*) is to use an appropriate Sunnen hone that I'll adapt to fit on my mill. Yes, I know I could hand it over to a machine shop to have this done for me but what's the fun in letting someone else screw it up when I can spend more money and time screwing it up myself?
(*)“No battle plan survives contact with the enemy.” Field Marshall Helmuth von Moltke
The bar of Ampco 18 aluminum bronze was delivered today but it will be a few days before I see how it machines. Not unlike that on Britbike, advice on machining forums ranges from excellent to... well, let's say, less than excellent. And, like Britbike, it's not possible to know whose advice to pay the most attention to without having spent time following other posts. This is relevant because advice on machining aluminum bronze ranges from saying it cuts 'almost like butter' to 'similar to tool steel', with a range of advice on the cutting edges to use. I think I've sorted all that out but I'll only know once I have it in the lathe.
McMaster-Carr, a favorite supplier of mine, gave a tolerance range on the diameter, and the rod I received came in slightly over the nominal 1-1/2" at 1.55" (also 1/8" longer than the 12" I paid for). However, there's a casting skin on it so I won't know its useful OD until I machine that off. In any case the OD is more than enough than needed to produce the final ~1.19" OD for the bush.
My initial plan is to modify the cutting edges of a sacrificial 25/32" (0.781") Silver & Demming drill to the geometry best for Al bronze that I found from a credible source. That will leave ~0.031 to remove using a boring bar with carbide insert to achieve the 13/16" of the gudgeon pin. I'll leave an appropriate amount of material (~0.001"; exact value to be determined) and sneak up on the final required ID, including the necessary clearance, with a Sunnen hone. Since its thermal expansion is similar to Al I'll allow it to cool before doing the final work on the OD and the ID. Anyway, as Gen. Custer said to Maj. Reno as he rode out that morning, "That's the plan..."
The honing unit arrived two days ago but I will need to design and make an appropriate holder for it. This unit only operates over the limited range 0.806"-0.837" (nb. 13/16" = 0.812") and is made to be used with a Sunnen honing machine having the necessary mechanism for the push/pull mechanism that operates on the stone to press it against the ID with adjustable pressure.
The stone is mounted on a holder with a 20o ramp so if pushed on by a 1/4-28 cap screw, a one-sixth turn would raise the stone by 0.002". I'll need to fabricate a holder with a bolt operating on it and a strong return spring that is suitable to mount in my lathe. With the small end of the rod in place, turning the cap screw until resistance is felt, plus a little, should be what it takes. I'll practice on a scrap piece of bronze first to get a feel for how fast material is removed.
The stone is mounted on a holder with a 20o ramp so if pushed on by a 1/4-28 cap screw, a one-sixth turn would raise the stone by 0.002". I'll need to fabricate a holder with a bolt operating on it and a strong return spring that is suitable to mount in my lathe. With the small end of the rod in place, turning the cap screw until resistance is felt, plus a little, should be what it takes.
An M5.5 x 0.5 cap screw should allow finer adjustment, if you can get hold of the appropriate screw and a tap to thread the holder.
An M5.5 x 0.5 cap screw should allow finer adjustment,
Thanks for pointing that out, but I hope finer adjustment won't be necessary. The mechanism I have in mind to make will push against the stone with a spring so it will be encouraged to advance into the bushing rather than being forced to. Unless the stone removes material very rapidly, even with the same setting of the adjustment it should take several sessions of cutting and measuring to approach the desired final ID value.