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Author Topic: OS Max H  (Read 1838 times)

Offline Joe Messinger

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OS Max H
« on: April 10, 2007, 09:30:27 PM »
Hello,

Thought some of you might be interested in the results of my OS Max H RC conversion project.

A friend who used to fly RC gave me a 1970's vintage OS Max H .40 RC engine that he no longer had any use for.  I removed the carburetor and made a venturi for it.  .265 opening.  I installed one of Randy Smith's PA needle valves.  Sent the head to Bob Zambelli who machined the dome of the head to reduce the compression.  One .010 head gasket.  Mounted the converted engine on one of the Fancherized Twisters I built. I used a Sullivan 4 oz. plastic tank with clunk. No pressure.  APC 10.5X4.5 prop.

The results were surprising.  A flight or two to get the needle adjustment right and the old engine performed wonderfully.  Plenty of power with a 2/4 break at just the right time.

I really didn't expect too much from the engine.  It made its power at high rpms as an RC and had a lot of hours on it.  Very good stunt engine now.  Maybe not a PA or RoJet but pretty darn impressive.

If I can find another Max H, I would definitely do the conversion again.

Joe
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Joe Messinger

Offline Ralph Wenzel (d)

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Re: OS Max H
« Reply #1 on: April 10, 2007, 11:42:40 PM »
Joe,
Got a friend who has a boxful of OS Max-H.40's; some bred for stunt, some for . . .   There were at least 4 variants. I'll ask him to email you direct.
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(Too many irons; not enough fire)

Ralph Wenzel
AMA 495785 League City, TX

Offline Joe Messinger

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Re: OS Max H
« Reply #2 on: April 11, 2007, 06:37:00 AM »
Thanks Ralph.  I'm quite happy with the performance of the modified old Max H and would like to have a few more in my inventory.

Regards,

Joe
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Joe Messinger

Offline Bob Zambelli

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Re: OS Max H
« Reply #3 on: April 11, 2007, 07:01:13 AM »
The Max-H case was used on a number of engines, including a Combat Special and a racing .29.

Regarding the .40 versions, the were at least three RC configurations, one RC ringed, one RC lapped, and one RC pylon. The pylon version has sleeve and crank timing designed for high RPM and I doubt that it would make a good stunt engine. It is identified by a small "P" on the bypass and the spacing between the head and case is .025 larger than on the regular RC .40s.

There is also the stunt .40H. It is identified by a small "S" on the bypass. It has a bushing at the front and the regular ball bearing (OS part # OS-30) at the rear. I can't remember if it's lapped or ringed but I've heard that it's an excellent stunt engine in stock form.

Bob Z.
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Offline phil c

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Re: OS Max H
« Reply #4 on: April 11, 2007, 05:12:57 PM »
Keith, in our club, showed up with a Max H with a little RR stamped under the H.  Presumably a rat race engine, with an iron piston.  It had a real bathtub venturi, about .45 in. diameter.

I cut him a .280 venturi which he installed with the OS needle valve(an oversize original, .155 diameter).  It ran great from the start.  First flight was just a bit lean, so it went at a fast two cycle, but the chicken hopper tank kept it from running away and overheating.  Second flight we richened it up quite a bit and it ran a steady 4 cycle, but way too sow.  Third flight was just about right on.  When the weather warms up we'll see how it does on larger props.  A 9/6 was just right for the plane he had it on.
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phil Cartier

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