OS .35S/Jetstream muffler bolt size ???

[Bill Little]

Hi All,

On the older OS .35S engines the muffler mounts are threaded bosses in the ends of the exhaust stack.  I have a few Jetstream mufflers which attach this way.  I need to know the thread and length of the mounting bolts. 

Thanks!

BIG Bear
RNMM/AMM

[James C. Johnson]

Good luck...

They may be JIS like those used on Enyas.. I have one I use on my Dolphin .. I hogged out the guts and it is a good muffler but a bit of a PIA... The Phillips heads suck.. please heat the thing up before taking it apart and use a Phillips driver that fits perfectly... on several engines I drilled the holes out on the case and now use a tongue muffler.. the #703 strap muffler works well but is a tad heavy... last resort (which is probably the quickest way to go) hand tap to a size that is available. After attempting to look for JIS Threaded Cap Bolts for Enya 45s I was told to go to a 5/40 pitch on the re-tap.. the bolts on the Jetstream are in the 1 5/8" length range.

"The Jetstream mufflers had 4 components: an exhaust extension, an inner shell that bolted through to the engine lugs, two removable baffles, and an outer shell retained by 3 small Phillips screws. Regrettably, these screws would vibrate loose and the outer shell, screws and all, would fly off into the grass, being lost forever. This was really very sad, because in true OS fashion, these screws and the inner shell retaining screws were some exotic thread not available in Australia, then or now. Even to this day I have an OS Max III 35 and modified Jetstream muffler which I cannot use because of these mystery threads."

http://www.supercoolprops.com/articles/expansionchambersmufflers.php

[Trostle]

The bolt threads on those older OS engines and the Jetstream mufflers were something not readily available here.  The bolts are something like 2.5 mm or something even more different.

Some had some luck using 4-40 bolts.  It sometimes worked to tap for the 4-40 threads.  Or, has just been suggested, an even better solution would be to drill out and tap for 5-40 bolts.  Much less chance of stripping the threads and having even less utility for the engine/muffler combination.

I had some experience with these engines over a period of time and still prize some of the reworked and new ones I still have.  Same with the Jetstream mufflers.

Keith

[Dallas Hanna]

Nothing weird about the screws for the OS 35S Bill but it looks like many start looking for JIS when in reality, the Japs adopted the DIN (SI?) standard for threaded fasteners a century back. ( http://mdmetric.com/tech/tict.htm )  The screws which hold the jetstream muffler onto the stack are M2.6 x 0.45.  I think the ones which hold the halves together are M2.3 x 0.4.  I only say that as I only have one new Jetstream and remembering many years ago that the original cross heads were a PITA for either coming loose or having damaged heads as we all used Phillips drivers didn't we   !!  I changed the ones in my Jetstream to soc hd #4-40 with the heads turned to fit the counterbore.  The #4-40 screws I have use a 5/64" hex rather than the standard ? 3/32".

Taps and dies are readily available for both these sizes but for the M2.6 x 0.45 to get out of trouble you can get away with using the bolt which holds the OS PEST muffler (? didn't look at the 7xx number!) together.  It is a rolled thread M3 x 0.5 so cut the bolt off to the required length so you only have the plain shank which in 2.5mm diameter.  Open the M2.5 x 0.45 die out to max in the stock (MUST BE A SPLIT BUTTON DIE!!!!!) and run it down the bolt.  You should be able to get a good fitting M2.6 x 0.45 thread this way.  Don't be tempted to use a M2.5 as even though it's only 0.1mm difference it does make quite a difference to strength on the smaller pitches.  It very likely will strip the thread in the alloy case rather easily.

HH

[Dallas Hanna]

Good luck...

They may be JIS like those used on Enyas.. Even to this day I have an OS Max III 35 and modified Jetstream muffler which I cannot use because of these mystery threads."

http://www.supercoolprops.com/articles/expansionchambersmufflers.php

James, no mystery to many of us!  As explained to Bill in post above, most of the old DIN dia/pitch can be found OR we just make them.  Not sure now without searching but I seem to remember that OS did make one dodgy thread around M2.7 x 0.6 for one of their range of engines.  The conclusion we came to on that one was that OS was big enough to maybe roll their own threads in house.  Those screws can be made by purchasing standard (DIN) M3 x 0.6 screws and closing the M3 x 0.6 die down to skim them down to 2.7 dia.

See discussion here on Barton forum: http://controlline.org.uk/phpBB2/viewtopic.php?t=8097&highlight=

HH

[Geoff Goodworth]

I knew that Herb would come through with the good info but as a source, Micro Fasteners offer M2..6 x 0.45 in 6, 8 and 12mm lengths.

OS still uses M2.6 on a backplate for either the FP 20/25 or the LA25. Can't remember which one but I have some in my fastener stock.