I have been a vintage BMW fan for over twenty years with three enjoyable Earles restorations behind me. Durning this time I have religiously kept my distance from old BMW motorcycles with polished engine components. Recently, my traditional restraint failed when I found a complete R51/2 in need of a restoration. Engine, transmission, valve covers and even the final drive have been painfully polished.
Is it possible to restore the factory surface texture? Will shot, bead or any other blasting do more than dull the polished surface? I am guessing that most of the original surface texture was a product of the casting process. Is the twin cam any more or less textured than an Earles casting?
Thanks in advance for this opportunity. I love old
BMW motorcycles and I would really like to restore this R51's original dignity.
Un-Polishing Castings
-
Mark Huggett
Re: Un-Polishing Castings
Hi Jeff
The R24 and R51/2 castings were very much rougher than their followers due to the conditions under which BMW worked in 1948 and 1949. The castings are full of quartz sand which one notices when fileing and polishing. Theoretically speaking, polishing strengthens the component as you are reducing the specific surface area. Thats why it all started in the racing scene. However, reduced surface area reduces heat dissipation and so the polished surface was sometime bead blasted and/or painted matt black. BMW works procedure was to blast the alu castings with stainless shot (up to 2mmØ) followed by blasting with aluminium shot. That it how one obtains that dull sheen on the castings that people always fail to achieve in their restorations. This procedure will help you to very slightly roughen up the surface, but the mattness achieved will be the main success in getting away form the highly polished surface. The other point to remember with blasting is to use high volumnes and low pressure.
Best regards,
Mark
Mark Huggett GmbH
The R24 and R51/2 castings were very much rougher than their followers due to the conditions under which BMW worked in 1948 and 1949. The castings are full of quartz sand which one notices when fileing and polishing. Theoretically speaking, polishing strengthens the component as you are reducing the specific surface area. Thats why it all started in the racing scene. However, reduced surface area reduces heat dissipation and so the polished surface was sometime bead blasted and/or painted matt black. BMW works procedure was to blast the alu castings with stainless shot (up to 2mmØ) followed by blasting with aluminium shot. That it how one obtains that dull sheen on the castings that people always fail to achieve in their restorations. This procedure will help you to very slightly roughen up the surface, but the mattness achieved will be the main success in getting away form the highly polished surface. The other point to remember with blasting is to use high volumnes and low pressure.
Best regards,
Mark
Mark Huggett GmbH
-
Dave Thomas
Re: Un-Polishing Castings
I recently restored an R26 and had a similar problem.
I first glass-bead-blasted the parts and then sprayed a
matte-finish clear on them, which has produced a finish
remarkably similar to the original.
The clear is an automotive acrylic mixed with a flattening
additive. The flattening additive is designed for use on
interior parts, body trim, etc., and can be mixed in
varying amounts to adjust the gloss level. The PPG/Delstar
product numbers are DAU75 for the clear and DX685 for the
flattening agent. Thinners are available in several
temperature ranges.
I've seen a method refered to as "vapor blasting"
in British publications, which also creates a similar finish.
I believe it's a combination of glass-bead and solvent- or
water-blasting. Perhaps a British enthusiast can enlighten us.
I first glass-bead-blasted the parts and then sprayed a
matte-finish clear on them, which has produced a finish
remarkably similar to the original.
The clear is an automotive acrylic mixed with a flattening
additive. The flattening additive is designed for use on
interior parts, body trim, etc., and can be mixed in
varying amounts to adjust the gloss level. The PPG/Delstar
product numbers are DAU75 for the clear and DX685 for the
flattening agent. Thinners are available in several
temperature ranges.
I've seen a method refered to as "vapor blasting"
in British publications, which also creates a similar finish.
I believe it's a combination of glass-bead and solvent- or
water-blasting. Perhaps a British enthusiast can enlighten us.