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Old 02-12-2020, 03:42 PM   #1
Harvard 387
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OS Max FP carburetor screws


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OK. I have a number of OS Max 2-stroke engines and I am about fed up with the screws that hold the carb into the crankcase stripping out the screw head. I know that the Japanese Phillips head has a different angle and profile from the USA version and I have a PH2 screwdriver to work on them. This helps.

My problem is I HATE PHILLIPS SCREW HEADS! even when treated like velvet they are prone to stripping with the least bit of torque.

I have changed the muffler bolts on all of my OS motors to Allen head screws but I can't seem to identify the thread on the carb hold down screws to do the same thing.
They are 3.5mm diameter but I can't figure out the thread pitch.

Engine people, any ideas?
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Old 02-12-2020, 04:06 PM   #2
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Re: OS Max FP carburetor screws

You could tap it to something more common ,would a 4mm work, that is very common .the problem with going to cap head is it will be very easy to strip the threads once you get a good grip on the screw because the threads are so shallow . They may have done it that way to prevent you from stripping the more expensive part ..
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Old 02-12-2020, 04:20 PM   #3
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Re: OS Max FP carburetor screws

It should be a 3.5X5 not sure on the pitch. If your stripping it then your over tightening it.
Some Computer case screws are the same thread.
As soon as the tip of my Philips has any wear I pitch it. When I work on a engine I grind down all my Allen keys after I've used them before I work on the next engine.
You could cut a slot in the screw head also. Cheers Floyd

Last edited by floyd; 02-13-2020 at 12:22 AM.
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Old 02-12-2020, 05:06 PM   #4
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Re: OS Max FP carburetor screws

I hear you brother!
Philips isn't great, I had both screws fall out one time and the throttle servo just rotated the carb and wouldn't let me shut it off.
That was the only time I've ever had trouble getting a glow engine to quit!

I use Loctite 243 on them with the right bit and toss them if they get any worse than perfect.

I think mine were M3.5 x .6, it's not a common thread but I found some with a slot and bought a bunch.
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Old 02-12-2020, 06:38 PM   #5
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Re: OS Max FP carburetor screws

Tower Hobbies lists the screws as a 3mm x 5 mm.
https://www.towerhobbies.com/cgi-bin...p?I=LXCF07&P=X
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Old 02-12-2020, 07:51 PM   #6
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Re: OS Max FP carburetor screws

OS used two sizes on the FP and LA engines. So when he measured 3.5m its the older sized thread.
A 3m screw is too small to fit my own FP40. The link below shows the right size. Cheers Floyd

https://www.towerhobbies.com/cgi-bin...01p?I=OSMG2421

Last edited by floyd; 02-12-2020 at 08:18 PM.
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Old 02-12-2020, 08:19 PM   #7
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Re: OS Max FP carburetor screws

Good to know that information that they used 2 different sizes. Cause if it were me, I would be looking at buying the 3mm x 5 mm screws. Thanks Floyd for the heads up, and the knowledge.
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Old 02-12-2020, 08:29 PM   #8
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Re: OS Max FP carburetor screws

I have no problem over tightening the screws if I switch to Allen screws. Someone has stripped the Phillips head before I ever bought this engines.

There are 2 Phillips screw heads. One is the standard USA Phillips and the screwdriver blades are tapered to "cam out" when too much torque is applied. The other is the Japanese Industrial head. It has a shallower bottom and a shallow angle on the flutes. The screwdriver blades are not tapered so it stays locked into the head.

If you use a USA screwdriver in a Japanese Phillips screw the blade bottoms out before the flutes of the blade fully engage the sides of the screw head, this is why a standard Phillips screwdriver makes such a mess of Japanese screws.

I agree that the screw must be 3.5mm and .6 thread might be right as well. I have 3m x .5 screws and they don't quite work.
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Old 02-12-2020, 08:37 PM   #9
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Re: OS Max FP carburetor screws

I've seen lots of errors on OS parts over the years. My friend ordered a key way for his OS FS-1.20 engine that was the wrong size when he got it. Even though Horizon listed it as correct for that engine. Os used two different crank shafts on the FS-1.20 engines. Fortunately I stock the right key way and traded him. Cheers Floyd
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Old 02-12-2020, 08:37 PM   #10
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Re: OS Max FP carburetor screws

OK. Problem solved. Thanks Floyd.
I took screws out of my 40D carb on a newer motor and it matches the screw from the 3A carb I am trying to fix.

The screw size is 3.5m x 5m, same for both engines.

Now to find a source of new screws ...... Allen head preferably.

Thanks to all.
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Pull back on the stick .... trees get smaller
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