GT 750 Engine Polishing

[WindyDucat]

I spent most of the summer disassembling my 1973 GT 750 for a full cosmetic and mechanical restoration. (The bike has only 5,000 miles, however the Minnesota weather has been hard on the metals and rubber parts.)

The engine is in great mechanical shape and no need to "bust it apart", however the casings need to be cleaned and polished. The clutch and alternator covers have been removed and polished to a mirror finish.

My question is what is the best method of polishing the engine which is mostly rough cast? I was considering brass wire wheels - not trying to get a mirror polish, just bring back the original luster. I considered soda blasting, but decided against it because of the mess, and chance for soda to get inside the engine. What brass wire wheels are the best?

Thanks! Cal

[Macdesmo]

Cal,
Put the cases back on, plug the engine openings up and soda blast. If a bit of soda gets in it doesn't harm the engine. This issue is currently on the Laverdaforum in the technical section. There you will see two soda blasted MV engines that I have posted as part of the discussion.
Ian

[WindyDucat]

Ian, the results on the Laverda look great, thanks. Tonight I'll try soda blasting one of the valve covers which is off to check the results. If it works, I can easily block off all the openings. Did you use a professional grade soda blaster? I have a simple one that I bought at Northern Tool.

Cal

[lior_baruch]

Hey Ian,

Did you polish it as well after the soda blasting?

Thanks,

Lior.

[WindyDucat]

I test some engine parts with the soda blaster last night and here are the results.

First, it appears that there are two different aluminum alloys on the engine. The main engine components like the cylinders which are a rough cast on the outside and do not scratch easily, and the clutch and alternator covers which are smooth cast and seem softer and scratch easily.

On the rough cast engine parts, the soda blaster easily cut through all the surface build up of oil, corrosion, etc and got into places that would be near impossible. They came out very clean and presentable but did not have that "new" look, and did expose imperfections in the castings. (I'll post some pic later). I ran the blasted parts under the buffing wheel with mixed results... the high spots of the rough cast shined while the low spots collect the rouge, which can be cleaned out with window cleaner, and did not polish. I need to experiment with different rouges. Overall, I am really excited at how well the soda blaster thoroughly clean the engine, especially in all the little nooks and crannies, and how fast and easy it is. Just need to figure out how to do the final polish.

On the softer clutch cover it removed all the surface junk and left a satin finish which was easily turned into a mirror finish with the buffing wheel. Because the covers are softer they nick and scratch easily. I had previously removed the scratch with several steps of wet sanding finishing with a 2000 grit. The steps for the softer parts appears to be 1) wet sand to cut scratches, 2) soda blast to remove all stains and give a consistent satin finish, 3) buff for final polish and mirror finish.

Also, I tested the soda blaster on some of the hardware like the engine mount bolts and it worked like a charm cleaning them up to a like new finish. I am going to blast all the hardware before reassembly.

Next step, seal up all engine openings, get a lot of soda and clean the engine.
I will post some pic when I get back in town on Monday

Cal

[DesmoDog]

For the case halves I relied on aluminum wheel cleaner. Cleaners actually, I bought a couple different kinds with mixed results, depending on the ingredients. Spray it on, hit the castings with a wire brush (by hand) and rinse. Repeat as needed. Carb cleaner can come in handy here too.





I tried using a brass brush on aluminum once, it turned the valve cover a different color. I ended up using stainless steel brushes.

[Macdesmo]

Lior,
I am not sure which post you looked at for the soda blast reference on the Laverdaforum. I did 2 MV Agustas with soda blasting but didn't have to polish any cases as there aren't any that can be polished. You might have seen a Laverda triple somewhere that I did some years ago and had it posted up. If that's what you found then it was a crank out rebuild, bead blasted and I polished the cases.

Desmo D,
A great piece of work!

Ian

[RockingM]

Originally Posted by DesmoDog

For the case halves I relied on aluminum wheel cleaner. Cleaners actually, I bought a couple different kinds with mixed results, depending on the ingredients. Spray it on, hit the castings with a wire brush (by hand) and rinse. Repeat as needed. Carb cleaner can come in handy here too.





I tried using a brass brush on aluminum once, it turned the valve cover a different color. I ended up using stainless steel brushes.

Looks real nice DD. Which brand of wheel cleaner did you find worked the best. I've some wheel cleaner (acid) that isn't doing the job..

[DesmoDog]

Originally Posted by RockingM

Looks real nice DD. Which brand of wheel cleaner did you find worked the best. I've some wheel cleaner (acid) that isn't doing the job..

Thanks. I'm afraid I don't remember which brand worked best, I ended up with two and I switched back and forth. They had different types of acid in them and each one worked better on different situation but it's been about six years since I did it so the details are foggy... which reminds me - good ventilation is essential if you go this route. I did it in a small closed room once and it wasn't a great experience! I made sure the window was open all the time after that.

Read the labels and get the one with the most warnings on it, that should be a good start.

[RockingM]

Originally Posted by DesmoDog

Read the labels and get the one with the most warnings on it, that should be a good start.

ah should have known that.