smittythewelder
04-13-2007, 11:28 AM
A lot of old outboards, and some other engines, can get their bores cleaned up with no more than an aggressive honing job, but then the pistons are a little loose and rattly. Rather than bore such an engine and use up the diminishing supply of factory oversize pistons, you might be able to "oversize" your standard pistons (as long as the skirts haven't collapsed).
First get them cleaned. Hot-vapor degreasing is a good move, if you have access to that (big machine shops often do it). Salt-blasting the skirts after degreasing is a process you can do at home with a cheapo blaster. Don't glass-bead pistons (some of the glass gets permanently embedded).
Next take the pistons to any automotive machine shop that has been in business for a long time, and get the skirts knurled. This is a technique of metal displacement that will increase the O.D. of the skirts (it also raises a little metal right around the knurl, but the increased O.D. is the important effect, and you might want to lightly run a file over the knurl to take off the little high spots which will wear off anyway.
After knurling, go to a sprayed and baked molycoat on the skirts, another operation you can do yourself. Google KG Industries (formerly Kal-Gard); these guys sold a ton of molycoating supplies in the heyday of 2-stroke motorcycle racing in the Seventies, although they originally formulated it for gunsmiths. Find an old, cheap phonograph to set your pistons on while spraying on the coating with an airbrush.
Immediately before doing the coating, do a metal-prep of the pistons with "mag" wheel cleaner, a dilute phosphoric acid. Wash off the loosened oxides, then heat the piston with a propane torch, enough to drive off the moisture (i.e., just over 212F). LET THEM COOL to about 80F, then spray.
To get the thickness right, you need to practice, using scrap pistons which you can borrow from your auto machine shop. The great Zak, who molycoated all kinds of things, said he had successfully built up .004" on used pistons, although about .0008" will burnish off any buildup during break-in. After that, the coating holds up well, and of course you can re-coat in the future. Oh, I said don't glass-bead pistons, but if you know you are going to be molycoating the skirts you can probably glass-bead without ill-effects, since the coating will protect the cylinder from the glass embedded in the pistons.
So don't throw out those standard-bore pistons; there may be life in them yet! I first had this done in 1967, on the advice of a Boeing lubrication expert, and later did it myself with Kal-Gard.
First get them cleaned. Hot-vapor degreasing is a good move, if you have access to that (big machine shops often do it). Salt-blasting the skirts after degreasing is a process you can do at home with a cheapo blaster. Don't glass-bead pistons (some of the glass gets permanently embedded).
Next take the pistons to any automotive machine shop that has been in business for a long time, and get the skirts knurled. This is a technique of metal displacement that will increase the O.D. of the skirts (it also raises a little metal right around the knurl, but the increased O.D. is the important effect, and you might want to lightly run a file over the knurl to take off the little high spots which will wear off anyway.
After knurling, go to a sprayed and baked molycoat on the skirts, another operation you can do yourself. Google KG Industries (formerly Kal-Gard); these guys sold a ton of molycoating supplies in the heyday of 2-stroke motorcycle racing in the Seventies, although they originally formulated it for gunsmiths. Find an old, cheap phonograph to set your pistons on while spraying on the coating with an airbrush.
Immediately before doing the coating, do a metal-prep of the pistons with "mag" wheel cleaner, a dilute phosphoric acid. Wash off the loosened oxides, then heat the piston with a propane torch, enough to drive off the moisture (i.e., just over 212F). LET THEM COOL to about 80F, then spray.
To get the thickness right, you need to practice, using scrap pistons which you can borrow from your auto machine shop. The great Zak, who molycoated all kinds of things, said he had successfully built up .004" on used pistons, although about .0008" will burnish off any buildup during break-in. After that, the coating holds up well, and of course you can re-coat in the future. Oh, I said don't glass-bead pistons, but if you know you are going to be molycoating the skirts you can probably glass-bead without ill-effects, since the coating will protect the cylinder from the glass embedded in the pistons.
So don't throw out those standard-bore pistons; there may be life in them yet! I first had this done in 1967, on the advice of a Boeing lubrication expert, and later did it myself with Kal-Gard.