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Q&As since 1989


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Finishing of model aircraft engines

1998

I play with model aircraft engines. Design,build and race them. The latest technology has a alum piston running in a hard chrome plated alum cylinder.Some years ago I did plate my own cylinders using the double zincate method, it worked after a fashion, when I did not get blisters.

BTW the alum contained 12% Si which did not make life any easier.In the meanwhile I have had a commercial outfit do the chroming for me but delivery is getting a problem,12-18 weeks. I would like to try again, I still have my notes from before but I would like you to tell me if there has been any developments in the intervening years.

I have a recipe from the Russians which in addition to the Cr0₃ uses Na₂W0₄ and H₃P0₄ ? i.e., plating tungsten chrome, do you have any comments.

Thank you.

Brian Fairey
- Canada

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Not a lot has changed. You left some steps out of your process, or you could expect problems. It is not cost effective to maintain a mini plating line for one or two now and then. If you are really hung up on it, I will look over your process and make comments.

Have you tried low phosphorous electroless nickel. Contact Dave Groves thru Enthone-Omi. He races model boats very competitively and you might be able to trade information and work. He is a salesman based out of TN.

James Watts
- Navarre, Florida
1997

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1998

I HAVE BEEN CHROMING CYLINDERS FOR A WHILE NOW AND HAVE THE PROCESS SETUP DOWN PAT.

First: Let's look at a brass cylinder to be chromed --

You will need a power supply with adjustable voltage capable of up to 10A supply. clean the part with rubber rubber gloves [on eBay or Amazon] on, using washing up liquid. Rinse. Mix up about 1 litre of water with .5-1 oz of Drano [affil links] (Sodium Hydroxide). suspend the cylinder in a stainless steel container and fill with this mix until the part is submerged. connect the positive cable to the container and the negative to the part. wind up the power until the part is free gassing (this should require .1 - 1A do not over power as this will cause local burning) for 2 minutes. remove and rinse. mix 100-1 mix of Cr0₃ to distilled water exactly. in a plastic container fill with Cr0₃ mix and heat with fish tank heater to 45-50 °C.

Make anodes out of lead, at least 50% larger than the surface area of the part. Space Anodes equally around part. connect neg. to part and positive to anodes(connect anodes together electrically if more than one) and pass .75A per sq/inch of cylinder area and leave in bath for 3-4 hours. Make sure gentle stream of bubbles agitates the part in the bath from underneath using a fish tank air pump with a restricter valve attached.

Lap chrome using alloy piston and wooden handle, lubricate with WD-40 [on eBay or Amazon] or penetrating oil.

There you are, go to it!.

Steve Buckpitt

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I'm modeller and collectioner of vintage airplane model engine. I have two old engine that I want to save. Only possibility is hard chrome plating of pistons (diameter 3/4 of inch, high 3/4 of inch, very small). I did try a company, but the problem is that they asked for plating so small piece $60 american dollars. It is to much for me. Do you know perhaps someone that can do that work for more reasonable price or gratis?

Thanks,

Kazimierz W [last name deleted for privacy by Editor]
Ascot, Canada
2000

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Hello, Mr. Wirpszo.

Usually, electroplating is not the kind of work that can be done "pro bono". Chrome plating shops must monitor and control their emissions, do medical surveillance exams and blood tests on their employes, treat their water, manifest their waste shipments, be responsible for the waste for eternity, submit lengthly reports to the government virtually every week, and admit regulators whenever they show up & then pay for that regulator's time on-the-spot.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2000

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Thanks for your answer. I don't understand the remark about a price. The price 60 dollars seems to you low or high? If that price is low I'm really surprised.

You take in account the dimension of this piston? The price depends on size or per piece disregarding the dimension? Thanks once time more.

Kazimierz W [last name deleted for privacy by Editor], Senior Electronic Engineer
Ascot, Canada
2000

If you were ordering thousands of these the price is very high, Kaz. But when you are ordering 1 or 2, the price is low.

Can you get your mechanic or plumber to do anything for less than $60? It does not surprise me that a plating shop cannot inspect this part, strip the old coating, preclean it, mask it, work out how to fixture it with custom rack and anodes, neutralize and rinse it, unmask it, and re-inspect it in less time than a plumber can change a washer. My roofer wants $125 to look at my roof leak, not to fix it :-)

Good luck!

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2000

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I have been restoring worn model engine parts by hard chroming for many years with excellent results. Made up my first chroming rig while in high school. It consists of simply a 500 millilitre beaker [beakers on eBay or Amazon] as the tank, a hotplate, thermometer, 6V battery, some stainless steel wire for use as a variable resistor for controlling the current, 16 gauge wire, and anodes made from lead flashing.

The solution is chromium trioxide, about 40 oz. weight per gallon dissolved in distilled water, plus about 0.4 ounces concentrated sulfuric acid.

Optimum operating temperature is about 140 °F. with about 3.5 volts across the part.

Plating worn cylinder sleeves requires the use of an internal anode down the centre of the sleeve separated by plastic spacers.

A typical worn ABC sleeve requires less than five minutes plating time, since you usually need to reduce the inside diameter by about 0.0001 inch. The bath plates at about 0.001 inch per hour.

Carl Risteen
Engineering consulting - Fredericton New Brunswick Canada
December 9, 2009

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December 10, 2009

Hi. I won't go so far as to say you folks should not be doing chrome plating, because I don't know your situations -- but the non-chemists reading this and considering doing it should know that "chromium trioxide" is chromic acid or very highly concentrated hexavalent chromium, which is extremely toxic.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey