magnum 120 FS repairs

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peterl95124

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I have a Magnum (chinese made from japanese OS molds) 120 four stroke that needs some work,
I thought being a model engine builder this should not be too hard,
only to become extremely disappointed with the Magnum internals once I got it apart,

the valves need to be replaced because they're steel rather than stainless and in a methanol
engine that doesn't last, mine are so rusty the stems looks like they are about to break

the bearings need to be replaced because they're steel rather than stainless and in a methanol
engine that doesn't last, mine are so rusty they don't turn smooth even after cleaning and oiling

I can make my own replacement valves from stainless,
and I can buy replacement bearings in stainless,

but firstly I can't get the bottom camshaft bearing out, its at the bottom of a blind hole,
and its in there tight, no amount of heat and whacking it on a wood block will get it out.

I also read about hydraulically forcing it out by pouring molten wax
down the bearing ID and then pounding a close fit rod down the hole, but my bearings aren't
sealed so the wax, after cooling and hardening, just oozes out past the retainer.

design wise the camshaft bore needs to be open at both ends and capped at both ends so
the bearings can be removed, but that's 20/20 hindsight and doesn't apply to an already built engine

has anyone else solved this problem, if so how ?

TIA,
Peter A Lawrence.
 
I found a small ID bearing puller on the web several years ago. I was working on a OS 91 FS cam bearing. Tomorrow I will post a picture. It is very easy to make.
 
I've also made a small bearing puller for exactly the same purpose.
Turn the end of a convenient piece of steel bar to a close fit in the bearing bore and drill through to a convenient tapping size.
Tap with a taper tap from the other end of the bar, so that the point of the tap just arrives at the turned end.
Slit the turned end, e.g. with a junior hacksaw, far enough that a bolt screwed into the thread appears in the slit.
Insert into the bearing, tighten the bolt and pull.
 
the pouring wax into the hole and pounding it out almost sounds like the old trick of using plain white sandwich bread and stuffing it in a pilot bearing hole to get the the pilot bearing out of a truck when replacing the clutch and such. i couldnt believe that actually works (and it did on an old 77 chevy half ton truck i used to have). idk if it will work on something that small but maybe bread instead of wax?
 
Hi Peter195124

Go to RC Groups: OS FS 91 Surpass cam bearing removal - RC Groups.
Post #8 has pictures of factory tool.

Here are pictures of the tool i made.

20231010_081601.jpg







20231010_081436.jpg





20231010_081518.jpg


Sending a PM
 
Copy moulds, I am 99.5% sure that OS did not authorise the copy.
Yes, for sure. The Op seems disillusioned also on materials that "should be" used in the valves and the bearings.
Never seen stainless used yet in any commercial model engine, OS included and Magnum or any other spin-off, highly unlikely as well.
One thing to watch is C4 bearings (if I remember correctly) are a must....higher clearance.
 
Where do you think that they were running C4 clearance? It is quite easy to check those clearances assuming the bearing is not shot away.
I'd really like to see you be able to measure that.........
 
I'd really like to see you be able to measure that.........
No problem at all.

The radial plays are well quoted for the various different fits and these are easily detectable measurements with a decent micrometer, you just take all the slack up in one direction and take a measurement then do the same measurement with the slack taken up in the other direction.

Do you want me to take a few photos of this being done?

Axial play is easier still.

Where were Magnum (Sanye Precision) using the C4's on the 120?
 
Where were Magnum (Sanye Precision) using the C4's on the 120?
Google is great for those that have no idea isn't it???
Look it up, seeing you seem to know it all.

Either way, doesn't cahnge the fact that "stainless" were never used, in bearings or the valves or any other part of the engines.
Magnum being an elcheapo knock off, surprised it has any usable metal at all.
 
I have seen references to 'stainless' valves within engine reviews by some of the longstanding engine review guys who at times had direct engagement with factory on certain engines. I just pulled a review from an OS 5-cyl radial which I had handy. Sometimes OS shared common parts across their other engines depending on size & era. Now what 'stainless' actually means in terms of alloy composition is probably factory confidential.

This link has lots of good data & reviews, source of my attachment.
http://sceptreflight.com/
 

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many thanks to all who replied, I finally succeeded by combining all the ideas,

first I made an expanding mandrel like Peter Twissell suggested, but it kept slipping out, even when I used pliers to turn the allen key to tighten the expanding screw

so then I added Gary Davidsons suggestion and turned a lip at the end of the mandrel to catch on the chamfer on the bore of the inner bearing race, which I didn't know was there, I was just hoping there was some clearance behind the bearing that the lip could fit into (as there normally would be to avoid friction)

that finally worked, but it was still hard to remove, really stuck in there

in the end not sure I saved anything buying a used engine,
but did learn some great tricks from you guys

(PS, still want to try silly-putty in place of wax, has some unusual properties, maybe if I'm foolish to buy another used engine)
 
Magnum being an elcheapo knock off, surprised it has any usable metal at all.
Magnum, ASP, SC and one other brand were all products of Sanye Precision, all their engines were knock off copies of OS engines with the occasional bit of originality.

The engines were pretty well made and the folk at the factory certainly knew about engineering, read the following to support that.

At some stage between around 2010 and 2016 Sanye Precision changed hands and became part of AECC (Aero Engine Corporation China) which employed about 37,000 people building full size aircraft engines, I believe both GT and ICE types. The model engine business at the same time was slowing down and eventually the model engine business was shelved.

I have it on good authority that the model engine business side of Sanye is starting up again, I am not sure if it is with the same people running it though.

Don't think that I am any fan of knock off Chinese goods but the Sanye engines were not bad products.
 
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I have seen references to 'stainless' valves within engine reviews by some of the longstanding engine review guys who at times had direct engagement with factory on certain engines.
Well Clarence Lee was one of the most respected engine reviewers of his era, so I am inclined to believe him.

I don't know if Clarance had deep communication with the OS factory but the English reviewer Peter Chinn certainly had close links with Ogawa and OS. see photo below from the OS 50th anniversary book which was produced in very limited numbers for distributers and reviewers around the World, my copy was given to me by a major distributor.
20231012_082919.jpg
 
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Not to belabor valve materials & not actually sure who wrote this particular review article. I remember doing a similar search of commercial engines in the past out of curiosity when I was deciding on my own valves. I wish I was more organized & kept the screen grabs at the time. I seem to recall I found about a handful mentioning stainless. Most articles didn't mention material at all. Some like this article have a sense of being fed some factory info & appears to be a treatment over & above bar stock SS. For sure more common stainless alloys features in many successful shop made engines, but the question was more about commercial engines. All I can say from my own experience is they were not 'corrosion proof' that's for sure. Methanol fuel can be a killer on internals if stored improperly.
 

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