Edwin

My Feedback: (2)

I'll be sending an OS.91fs head to Clarence Lee for and exhaust thread insert, but was wondering, would a magnum .91 head fit an OS.91fs head?
Edwin

w8ye

My Feedback: (16)

The intake is different if it is a Suppass II

Edwin

My Feedback: (2)

This one is just surpass. Definately not surpass 2. I was thinking it would be good to have a couple of backup heads.
Edwin

w8ye

My Feedback: (16)

Some of the Magnum/ASP engines are so much like their OS counter part that the major components will interchange. Then again you get into situations where some specific parts will interchange and others will not.

One thing for sure is that quality control is superior at OS. Also the casting metal is stronger in a OS than a Magnum.

However, there is no reason you cannot have a Magnum engine last you the rest of your life.

Some interchange experiences - OS 48/52 Surpass four stroke carbs will interchange with the Magnum 52 and 61 four strokes.

I've sold my OS 91 Surpass I and also my Magnum 91 four stroke engines some years ago and cannot remember the specifics of just what parts will interchange on those two. But I'm thinking that the Magnum is somewhat a cross between the design of the OS Surpass I & II engines in the head/intake manifold area and you get into a incompatibility problem even when you add on some more parts to try to get the swap to work. A Magnum 91 valve cover will fit on the OS 91 Surpass I head but the OS valve cover will not clear the rocker arms on a magnum. I know the crankshaft bearings are different between the Magnum and OS engines and the front prop drive washer is keyed on the OS and has a tapered collet on the Magnum. So there is actually nothing about the crankcase, crankshaft, bearings about the two that will interchange.

My experience also is that OS valve springs are twice as strong as corresponding Magnum valve springs.

A big part of Sanye/Magnum/ASP quality control is at the engine assembly point when there is obviously something wrong with some part and it is thrown away. When you buy Magnum spare parts, you may get some unusable part as there was never any quality control? The parts are made by CNC processes and there should not be anything wrong with them??? (But sometimes there is) This is from personal experience and has taught me to not bother with reworking someone else's Magnums and I sold all of them that I owned (6).. Exchanges of bad parts for good ones gets messy and time consuming.

TFF

http://www.seniorpattern.com/newslet...RAPR2007NL.pdf this news letter has the differences.

w8ye

My Feedback: (16)

It is on page 6 of that PDF file.

There are more differences in the two engines than described in the article but it gives you the idea and doesn't say anything wrong.

One thing I noticed comparing the Magnum, Surpass I, & the Surpass II is that the Surpass II has the plug recessed down in the head to where the shoulder part of the plug is completely exposed inside the combustion chamber. This difference alone is worth about 400 rpm which is considerable.

cahusker

My Feedback: (50)

I just happen to have both on my bench at the moment for repairs. The answer is no an OS head has a different bolt pattern than the Magnum head on the 91's. The big difference is on the intake side where the OS has the bolt further to the rear nearer the intake manifold by some 4-5mm. I agree with w8ye that the Magnums are not nearly as good as the OS engines are and while I have gotten reasonably good service out of the Magnums they do not produce the same power and with the same reliablilty as OS, my two cents worth.

Edwin

My Feedback: (2)

No argument here on quality difference. OS heads are pretty hard to find. I have 4 OS.91 surpass engines and really like them, but their age is making it harder to get the major parts. The one with the problem head was a purchase from a friend that bought the plane at an estate sale but didnt want the engine. I got the engine for $40. The head has damaged threads on the exhaust port and is on its way to Clarence Lee for his insert repair. You cant beat his price and workmanship for something like this. I was just hoping for a shot in the dark match up. But looks like a no go. Many years back my goal was to have four OS.91 surpass for a Don Smith B-17 project when I retire, still some years away. Looks like I'll need to rethink my engine choice and see what comes up in the future. Who knows, maybe a gasser that size might turn up.
Edwin