Having two OS Max S .35s, one I bought in High School, my curiosity was piqued by a reproduced Sig advertisement in the Mar/Apr 2013 Stunt News that said showed a young Dave Fitzgerald in 1976.  The ad said his Chipmunk, which Sig still sells the kit for, was powered by an Adamisin OS Max 35S.  I found Art's biography here on Stunthanger, but his biography didn't say what he did to the engines.
Did Art simply improve the reliability of the engine or did he increase its power?  Anyone know what he did to the engine? 

The reason I ask is that the Chipmunk has 570 square inches, and that is a little large from what people have told me is good range for the OS Max S .35 (510-540 square inches). 

Thanks,
Joe Ed Pederson

Get the Brian Gardner set up. Pulls like an Allis-Chalmers tractor!  TS

Now, THIS is an Allis-Chalmers tractor.  I haven't tested it to see how much it pulls....   26 HP diesel.

CT Schaeffer wrote:
Get the Brian Gardner set up. Pulls like an Allis-Chalmers tractor!  TS

I don't know who Brian Gardner is or how to contact him.  I've seen a Tom Lay ad in a 2013 issue of Stunt News, but I don't know if he's still in business or still with us.  I know I can get some parts for my OS Max S .35 from Jim Lee (I met Jim in May this year at the KCRC CL fly-in) and Super Tigre NVAs from RSM.

I'm a nube to CL other than the stumbling around with some friends in my youth (1960s to early 1970s).  I started CL with, of course, a Cox PT-19 and then a Carl Goldberg (CG) 1/2A P-40 (crashed it first flight, 5 days in a row before I got the hang of it).  I built and flew  practically all of the CG 1/2A kits, then a Ringmaster and a CG Shoestring.  I built a Junior Nobler but could never get the Mc Coy 19 to start.  I can't remember what happened to the Jr. Nobler (probably left at home when I joined the Navy).  I say stumbling around because we had no adult to guide us, we had no idea about the proper fuels for our .35s.  I'm not sure if we realized that unlike Cox .049s, we didn't need to run our .35s at max RPM's.  I do know our Mc Coy .35s were junk after less than 10 flights, which Jim Lee told me would require some form of abuse (fuel or running too hard).  So between possibly using inappropriate fuels and running our .35s too fast, I'm not sure my OS Max S .35 hasn't been abused.  Fortunately it still runs a steady four cycle and breaks to two cycle in maneuvers. 

I've been on Stunt Hanger and subscribed to PAMPA for a couple of months and am learning all kinds of things I wish I had known when I was a teenager.

My experience with an Allis-Chalmers was the old 1940s or 1950s tractor my Dad had and I learned to plow with when I was 12.  I had to brace my shoulder on the back of the metal seat and stretch straight out to put the clutch in.  But I almost couldn't kill it, no matter how indelicately I let the clutch out.

Now that I'm writing and can't see the previous posts, I wonder if the person who said the OS Max S .35 can pull a Sig Chipmunk meant a dead stock engine or one that is reworked/blueprinted.

Thanks for the replies,
Joe Ed Pederson

I have one still running just like it did in '77.  Getting a little cranky in it's old age.  Took 3 flips to start cold yesterday.  Best engine I have ever had but all I know that he did, since I have never taken it apart past the backplate, was to do something with the connecting rod and there is an insert in the venturi with a very small hole.  Keep in mind it is a 4-2 engine designed to run fast 4 with a break.  If you run it lean all the time it will overheat. If you can get your hands on one that has been treated with respect - do it. If it is in good shape it will easily pull the chipmunk.  Wish I could be more help, maybe Dennis will chime in.

Arrgh my ears are burning!  Don't know that I can really answer the original question but I thought I would just share what I remember of Dad's OS 35's.

Dad bought his first OS 35S in 1965 and put it in #2 son's 56 oz Nobler.  Part of the reason he bought it is that he was trying to bankroll (at that point in time) his own plus 3 son's stunt addiction, the OS was cheaper than a Fox, and the Foxes were so darned inconsistent as to be unusable.  At our first NATs that year the east coast guys all laughed at that "cheap Japanese junk engine" - a few noticed that it would eat a Fox for lunch.  The only real problem with that first engine was the venturi insert was loose fitting in the case (but it had a gasket) and the OS needles were prone to fail - the needle would come out of the carrier.  We were running lot of Rats, Goodyears and combat engines at the time. almost all Super Tigre, so swapping out the needle assy was a no-brainer, beside the Tigre needles used the tensioner on the needle and not those infuriating click springs!  The Tigre needles actually went where you wanted not springing all over the place.  I do not know how he decided it, but he made a new venturi insert that more closely fit the case and used a "K" drill (.281") for the opening, I think it wa slightly smaller than the venturi that came with the engine.  He had planned on experimenting - but did not have to.


Within a couple seasons we were all using OS 35's with K&B 100 fuel (I think it was 7% nitro and 18% castor) and they ran real well.  The east coasters by then had switched, and they were saying that the conrods were no good that they wore out in a dozen flights - things like that.  We NEVER wore out a rod but I will swag that sometime around then is when dad started filing an "X" on the front side of the bottom end of the rod to try to encourage a little more lubrication.  He worked on an engine for someone where the wrist pin had seriously scratched the cylinder - he started substituting Teflon end pads (just like he did in the Rat engines) in place of the brass.  We also ran some McCoys and he liked the little plastic button on the end of the crankpin that limited end play.  He started making some of those for the S35's only in Teflon and with a larger flatter end that was less likely to score the backplate.  Sometime later OS started installing steel washers between the crankcase and the drive washer; I think they actually did that for the RC folks who were using electric starters.  Again, the lessons from the Rat engines led to a few engines getting sandblasted exteriors (better cooling) and inside the bypass (better fuel vaporization).

Only other "mods" that come to mind.  Crankshafts with rolled threads usually are bent (than means everybody's) dad would chuck up the crank and put an indicator on it, then would deliver a carefully calculated whack with a hammer to get it straight again!  I know he also encountered a hand-full of engines with bad thrust washers that did not properly seat and would result in the prop drive not being perpendicular to the shaft (it wobbled).  He would take a stab at fixing these but more often than not he just replaced them.

Looking back I think a big part of the success was the fuel.  However when we started running mufflers the engines really started varnishing up.  Early synthetic oils (like K&B 100 with X2C oil) were not very good, but they ran cleaner.  Under absolutely NO circumstance would I have ever used 5% nitro/29% castor - to me that would be about the same as trying to run it on spar varnish.  I wish I had used a uniflow tank with muffler pressure and a tic more nitro (10%) to help them run a little cooler - yes you read that correctly.

Later when he started reworking FP 40's, dad recommended Cool Power all synthetic, but not a lot of stunt people went there...


Joe Ed, not sure if that is the answer you were looking for.  I guess the summary is that dad made sure there was nothing WRONG with the engine, then did a few little TLC touches that helped them run to their potential and last a little better...

(Clip)

.... good range for the OS Max S .35 (510-540 square inches). 

Thanks,
Joe Ed Pederson

Those are probably good numbers for a stock OS .35 that is well broken in.  A Tom Lay or Aldrich reworked OS .35 can do more.

However, I know that a stock OS .35S with a stock Jetstream muffler won the Walker Cup in a 47 oz, 610 sq in model in 1970.  That engine was good for 400 or 500 flights before the rod wore out. (To my knowledge, that is the only time the OS .35S has won the Nats.)

Dave Gierke had on OS .35S in his All American Eagle and did very well with that.  I think he used the OS in at least one of his Novi designs. I am not sure of the dimensions, but those airplanes were close to 600 sq in.

Keith