OS Max-H .40 disassembly - - - piston pin stuck on connectin...

Stuka Stunt Main Forum · 18 of 18 known posts recovered

PeterH · Jul 31, 2001 10:50 PM

OS Max-H .40 disassembly - - - piston pin stuck on connecting rod#0 source

I'm stuck, and fresh out of ideas. My newly-acquired toy needs a complete stripdown so's the bearings and the ring can be swapped out - - the bearings are seriously rusty. All's well until I attempt to remove the piston, which means extracting the piston pin through the hole cast in the rear of the crankcase. No go. Piston pin aggressively seized on the connecting rod. Piston is floating on the pin, but pin is firmly stuck on the rod. Tried 20 minutes at 275 degrees, still stuck. Tried an easy-out to grab hold of the piston pin and drag the pin out - - pin is harder than my nice new screw extractor, its flutes show damage from teh piston pin. Right now the assembly has been given a bit of a flooding with penetrating oil, and I'll try something else tomorrow. This must not be a destructive disassembly, I need to use everything I find except for the bearings and the ring. Any ideas?

Second topic: some genius lost two of the backplate screws sometime during the engine's life and 'replaced' them with 4-40 cap screws with attendant thread damage. I am tempted to try to re-tap the injured threads and locate the right screws. Any idea what those threads might be?

Thanks, folks.

Peter Havriluk

Steve Helmick · Jul 31, 2001 11:25 PM

RE: OS Max-H .40 disassembly - - - piston pin stuck on connecting rod#1 source

I figure you turned the engine upside down and filled the piston with penetrating oil. That should work, if not, you can try the crock pot and anti-freeze trick. I would let the "Liquid Wrench" work a day, try it, another day, check again, etc. I think it will eventually come loose. Wrist pins are pretty hard stuff, probably approaching 60Rc.

As for the damaged threads, if you take one of the correct screws to a machine shop supply store (where they sell cutters and precision tools), they would measure it so they could sell you a tap. You probably should take the crankcase also, so they might look at the remaining hole and threads. My thought would be to buy a "roll tap", aka "form tap". These do not cut, they reform the material into the desired thread form and work excellent in aluminum as well as mild steel or alloy steels before heat treating, and 303 or 304 stainless. If you need anything beyond cranking a roll tap into the existing hole (with some STP for lube), I would suggest an experienced machinist do whatever is indicated...go to a bigger screw size, install a Heli-Coil or a Keen-Sert. Steve

ucflier · Aug 01, 2001 06:43 AM

RE: OS Max-H .40 disassembly - - - piston pin stuck on connecting rod#2 source

LAST EDITED ON Aug-01-01 AT 06:44 AM (EST)

Peter:

I think that George Aldrich told me to use a 5-40 tap on the OS screw holes. It will run in the original hole (which is a metric size, I think) and then 5-40 screws will make available replacements for the OS hardware.

LNeumann · Aug 01, 2001 07:53 AM

RE: OS Max-H .40 disassembly - - - piston pin stuck on connecting rod#3 source

On the 5-40 threads, this works with most of the OS engines of this size when the thread holes are stripped. The 40 (inch) thread size is very close to the original metric, and the 5-40 is just large enough to work in the oversized hole caused by a stripped out thread. However...

If your engine presently has several holes threaded with 4-40 bolts and they are holding, I think I would leave well enough alone. Sure, you could tap all four holes for 5-40 (or 4-40) to have a "matched" set, but I would leave any bolt in that works and not mess with the others unless absolutely necessary. You shouldn't have a need to disassemble the engine often anyway.

Leonard Neumann

De Hill · Aug 01, 2001 03:54 PM

RE: OS Max-H .40 disassembly - - - piston pin stuck on connecting rod#4 source

Take a small screw like you use to mount a nylon control horn and screw it into the wrist pin until it grabs.Then,heat the engine at 350 F for 10 minutes. remove from the oven, hold the engine down with a potholder, and turn the screw clockwise with a pair of pliers. At the same time, gently pull the wrist pin out. With any luck, it will come out.

PS: you will probably ruin the screw.

captcurt · Aug 02, 2001 06:24 AM

RE: OS Max-H .40 disassembly - - - piston pin stuck on connecting rod#5 source

Or a drywall screw works well also. THey are sharp AND hard.

But the heat really does it. I had one last fall stuck just like this--it was a real pain. Then you have to find a new wrist pin kit with the plastic pads.

PeterH · Aug 04, 2001 06:18 PM

RE: OS Max-H .40 disassembly - - - piston pin stuck on connecting rod#6 source

LAST EDITED ON Aug-04-01 AT 06:23 PM (EST)

Thanks for all the assembled comments, folks. The antifreeze trick worked on the conrod. Cooked the crankcase for a few days in the antifreeze and a half-hour of fussing later, it be apart. The drywall screws didn't make any impression on the piston pin, gut the easy-out eventually had enough 'tooth' to drag the pin out. So far so good. Now, another chapter to this minor tale: Any ideas about getting the rear bearing off the crankshaft? When the crankshaft departed, it came out with the rear bearing stuck in place. And they are real friendly. I cooked the pair at 350 degrees inna oven for an hour, no joy... I have a couple of flatbladed screwdrivers with funny tips now, but I also still have a rear bearing stuck on the crankshaft. Is it time to scrounge up a friendly machine shop to help? I think I want to salvage the rusty bearing for matching-up purposes if I can, when I chase the new-bearing windmill, else I'd cut the little beast off the crankshaft, but I don't have a new one in hand. I am not averse to paying for the job to be done, either, if anyone has encountered somebody who is willing to work patiently on a fussy little task for money (maybe I'd have to include this task as part of a reassembly deal).

Thanks again, folks. I very much appreciate the suggestions and hints.

Peter Havriluk

Steve Helmick · Aug 04, 2001 07:24 PM

RE: OS Max-H .40 disassembly - - - piston pin stuck on connecting rod#7 source

As these engines have a cutaway crank disc, I would make an alumium block with concentric holes for the front end of the crank. Ideally, it would be a slip fit on the main crank diameter and very loose on the reduced diameter sections at the nose. This block, I would put in a sturdy vise, get out a punch, hammer and propane torch. Heat the inner bearing race, trying to keep heat off the crank. The aluminum block will absorb heat from the crank if it fits well. With the crank vertical, you can tap on the inner race after applying heat. Some oil on the crank would help transfer heat from the crank to the block. As long as you can still read the numbers on the bearing or measure the OD and width, plus the crank itself, you can buy the bearings you need. Most engines use very standard size bearings.
I have done a little investigation on the 3mm metric screws. Our chart at work only shows a .5mm pitch for 3mm major diameter. Compared this to a 5-40, and they are quite different. The 40 tpi equates to .63mm pitch, so 3mm x .60 or .65 pitch would be indicated. Like most American machine shops, our info on metric stuff is limited by our continued use of the English system of measuring. The Metric system makes sense. We're the sole user of the English system in the entire World. It's stupid. Steve

PeterH · Aug 05, 2001 08:24 PM

RE: OS Max-H .40 disassembly - - - piston pin stuck on connecting rod#8 source

Well folks, success. I did not have the ready means to follow Steve's advice about the aluminum block, but I did take his advice about the torch and the inner bearing race. I have a little butane torch I use for soldering small stuff together, and I used that, directing the heat to the inner bearing race, and trying to avoid heating the crankshaft. Two bouts of heating the bearing and prying between the inner race and the crankweb produced results and the bearing popped off. Goody. Now I have an intact pair of bearings to go match up, and everything else has been disassembled and cleaned. I much appreciate the comments and tutorial help from everone who commented on this little project for me.

Peter Havriluk

LNeumann · Aug 05, 2001 10:34 PM

RE: OS Max-H .40 disassembly - - - piston pin stuck on connecting rod#9 source

If you do come up with new bearings for this engine, I would be interested in knowing where and what. I was under the impression that this particular set of bearings no longer exists. If you can't find new ones, used ones may be available from someone.

Leonard Neumann

The Kiwi · Aug 05, 2001 11:15 PM

RE: OS Max-H .40 availability of replacement bearings#10 source

>I was under the impression that this particular set of
>bearings no longer exists.
>
A couple of years ago, when I started building up my current over- abundance of OS Max engines, I acquired a Max-H 40 for a low auction price that had a click in a bearing. George put in a new one for me, and neither before starting or after finishing did he say anything about parts availability problems.

(What he wanted to do was make it even more of a 4-2-4 engine than it is, but my feeling is still that it's good enough already just the way it is!)

From that casual attitude, I can only assume it's a fairly standard size that doesn't present much problem in replacing (and let's not lose sight of the fact that probably 60%, maybe even a higher % than that, of Max-H 40's have a bushing in front and only a single rear bearing. The front bearing, on those that do have twinned BB's, may indeed be hard to find.

Somewhere, I read that the extra ounce of weight difference in the K&B 40's late model front end was a result of a discontinued supply of small front bearings, requiring a change in the entire front end to accommodate the replacement bearing.

Ty M. · Aug 06, 2001 12:22 AM

RE: OS Max-H .40 disassembly - - - piston pin stuck on connecting rod#12 source

Leonard, back in the 70's when I was in Pocatello, Idaho, we had a bearing dealer that stocked bearings for everything! In fact, we used to go to him and buy a dozen at a time for our ST .35s. It seems the Italian bearings were just fine, but the race and keeper came apart under high rpms like in combat! As to the Metric system, in 1776 we adopted it as official!! We just never used it except for our monetary system! Go figure. Most automotive mechanics today are used to it as virtually every car in the world went metric in the early 80's. As a mech I had to buy every metric tool out there including taps and dies. I still have them. Ace makes a small tap and die set that is metric and it has a gauge for determining various pitches of bolts and nuts. I believe you can get one similar at Sears or from a MATCO, MAC or Snap-On dealer. They will be more expensive than Sears though. Just some food for thought. Don't be stuck in the model plane world for tools.

Ty M. · Aug 06, 2001 12:14 AM

RE: OS Max-H .40 disassembly - - - piston pin stuck on connecting rod#11 source

Well it seems you have it under control. My advise is to get into a train hobby shop or catalog, that's right model trains. In the world of HO I use a tool called a "wheel puller" by North West Short Line up in the Seattle area. Just don't tell the tool what is's pulling.

They also make a press that is great for pulling or pushing bearings off shafts. These tools are for small items like our trains or model plane engines. Try them you'll like them.

Steve Helmick · Aug 06, 2001 12:48 AM

RE: OS Max-H .40 disassembly - - - piston pin stuck on connecting rod#13 source

LAST EDITED ON Aug-06-01 AT 00:49 AM (EST)

There's no reason to panic about the bearings until you get to a bearing shop, or you can check out Boca Bearing online, I think. They advertise in Model Aviation pretty much every month.
As for Kiwi's K&B .40 story, I once replaced the front bearing in a friend's K&B .40 FI that I reworked, '69 vintage, with a NIB sealed bearing from my junk parts bin. I peeled the seal off the inner side. It was NOT a model airplane engine bearing, but where it came from or what it was for is a mystery, but it fit and the owner tached it at 18k + on an 11-4 RevUp and 30% or 40% nitro. The guy said his K&B .40 schnuerles wouldn't tach that high on that prop, which was standard prop for C gas FF at the time for 'Duster 900's and Witch Doctor 850's. I think folks got tired of all the cranks that snapped off at the front bearing, and the factory finally did something about it because folks complained.
Steve

PeterH · Aug 06, 2001 08:10 AM

RE: OS Max-H .40 disassembly - - - piston pin stuck on connecting rod#14 source

The front bearing is the least of anybody's problem - - I looked in the scrapbox and found one that would match, in size, anyway, it was one I stripped from a Super Tiger. Different manufacturer, but numeric part number was even the same. I think the crankshaft is necked to a very standard size and the housing cast to conform to the OD of the stock bearing. The rear bearing, time will tell. The original part number starts with 'OS'! I'll pass along anything I find out.

Peter Havriluk

Bill Sawyer · Aug 06, 2001 08:59 PM

RE: OS Max-H .40 BEARINGS#15 source

I called Boca Bearings and told them what I wanted and the man there told me which one to use. He had them in stock and I got a set - Part numbers are

MR607-ZZ My cost back in Sept was $7.25
MR130-TH9/C3 My cost back in Sept was $19.95

Their phone is 561 998 0004
Email WWW.bocabearings.com

Bill Sawyer · Aug 06, 2001 09:15 PM

RE: OS Max-H .40 BEARINGS INSTALLING THEM#16 source

Preheat the engine to 350 degrees in the oven - Place the bearings in a freezer bag in the deepfreeze for a couple hours. This expands the case and shrinks the bearing for the easiest insertion you will get. DO NOT USE A HEAT GUN OR TORCH - YOU CAN WARP THE CASE. I install fiels in generators at work this way - still may be tight but it will work.

Steve Helmick · Aug 07, 2001 01:46 AM

RE: OS Max-H .40 BEARINGS INSTALLING THEM#17 source

I've never been an OS fan, so can't say how tight OS fits their bearings. Dozens of Super Tigre, Rossi, Cox Conquest, Taipan 2.5 and even most K&B's didn't need that much heat. Bill didn't go into a lot of detail about proceedure, but I always get the rear bearing on the crank first, then you can freeze the crank/bearing and heat the case up...200F to 250F should be enough. Once the rear brg. and crank are in, you can slide the front bearing on the crank snout and it aligns perfectly of course. IMHO, the rear bearing should be pretty tight in the case, and the front bearing a bit looser, just a hard push fit. This depends on the internal fits of the bearings used, and is why K&B fits theirs so tight...loose internal fits on the bearings. All work, just different philosophy, and maybe some cost adavantages. The "K&B Fit" DOES discourage disassembly. Steve