Fix for OS-91 SZH Fan Run Out on Stratus

[Wayne]

On one of my new Stratus kits I could not get the run-out below 0.005" on the fan. After some time, I measure the crank, and found the run-out where the top collett fits was 0.0025". The run-out where the keyway was cut was almost 0.0"

I wrongly assumed the crankshaft was bad.

As some of you may have seen, the top collets shipped with the kits were very tight or maybe even did not fit at all over the the stepped portion of the crankshaft on the new OS-91 SZ-H engines. I think OS may have increased the size slightly where the upper collett fits.

My solution was to hone the inside of the top collect until it fit snuggly over the crankshaft.

That was wrong. The correct solution is to hone the inside of the top collett so it does not fit tightly on the crankshaft. If it fits tightly, the collett will be slightly off since that part of the crank may be off. That collett will have the same run-out as that part of the crankshaft.

I honed mine out slightly more and loosened it up. I spent 5 hours Sunday trying to get it right and could not. Tonight I spent 10 minutes and got it less than 0.001" after making it slightly looser.

How do you hone the collett? I took some 400 grit sand paper, and wrapped one or two turns around a drill bit, and put that in my drill press. Then I held onto the collett and honed away. Don't hold onto the collett with pliers or you will damage it. I used my fingers.

The goal is not to go nuts and make this too loose. But, as you know the top collett has an inside lip, and that lip need to sit sqaurely on the crankshaft where the step is. That part is where the run-out is zero.

My guess is this is almost like a thrust bearing, and if the outer race is too tight, it will never line up correctly or move right. With that top collett to tight, instead of the lower part of the crankshaft determining the run-out, it is determined by the part between the threads and where the keyway is cut, and that is where the run-out is.

I hope this makes sense and helps. Thanks to Tim Schoonard, Dino, Jerry and Eric for helping make sense of this.

Wayne
www.flymha.com

[WillJames]

Thanks for the heads-up Wayne! Good information here, thank you!!

[Gary O]

I just finished a Stratus kit with the SZ engine and I had to go with a thinner shim under the base collet since when I assembled the frame together there was no gap between the clutch and the fan. I'm wondering now, after reading this, if it's not a collet issue on the shaft not seating correctly instead of a spacing problem.

[EricLarson]

*** the following will be posted on my web site ***

Ok, there seems to be a lot of confusion installing the new OS SZ motor in the Stratus. First here is something to be aware of. The parts listed as OS for the Stratus where designated OS as there was only one case config that was being used. That was the OS C-spec and SX. That case config is different than the new OS SZ.

With the C-spec motor = follow the instructions in the Stratus manual. If you do not have a fan to clutch driver gap of .020 - .030 you did something wrong.

With the new SZ motor, here is the deal. The case to front bearing is now 3.275” and the C-spec was 3.235”. That difference is .040”. We need to adjust for that. DO NOT USE the small round space that is labeled OS on it. That is for the C-spec motor when you don’t use the PS mount backplate. THat spacer is .060" in thickness and is .020" more than we need. If you install that spacer, you will have NO gap.

***** This is the big confustion. The OS round spacer is ONLY for installing the OS C-spec motor without the PS backplate pump mount. This is only used on a NON-PS motor and only with the C-spec! *****

The best idea is to call MinAir and get a new bottom plate for the SZ. You then will just bolt the motor to the new plate (NO SPACER) and you will be good to go. This also assumes you use the Stock OS drive washer between the fan and motor. The new plate is the right thickness, relief cut for Hatori pipes, and has 3mm mount holes in it. It is the correct engine mount for the OS SZ motor.

If you use the old C-spec bottom plate = you need to put some bushing in there to allow for the difference in the 3mm vs 3.5 mm bolts used on the C-spec motor. You will need to get or make bushings 3mm id, 3.5mm od, and .100 tall. After that you still need to deal with the fact the vertical spacing is different. DO NOT USE the round spacer labeled OS! Just bolt the motor to the bottom plate with the bushings installed. Next you will need to compensate for the vertical difference by installing an additional .020” thick shim or washer between the motor and the fan. You will not have the OS drive washer (about .040”) and the additional washer/shim (about .020”) for a total space of .060”. This will allow your fan to clutch drive gap to be .020” to .030”.

If by some chance your gap is less than .015” you need to deal with that. Just remove or change the thickness of the washers between the fan and the motor front bearing to get the right fan to driver gap. If your gap is over .035” you should also adjust that gap by installing additional washers or shims between the fan and motor front bearings.

There will be some variance due to how the clutch driver was assembled and some washer tolerances. No big deal. Just get it close to the .020” - .030” area and you are fine.

Also MAKE SURE YOU DIAL INDICATE THOSE FANS :D We want a nice smooth model!

[Gary O]

A fix for the collet issue on the 91 SZ/Stratus that I tried Friday seemed to work well. Since the OS collet that comes in the MA kits is slightly too small for the new crank shaft in the SZ you have to make the bore bigger to get the collet to seat down on the shaft properly. I took an 8mm drill and drilled the collet out and it fit perfectly, still snug but went all the way on and seated in alignment. The runout on that fan <.001. I'm curious if MA is going to put another collet in the kits now for the SZ engine.

[EricLarson]

They will get the Collets fixed ASAP. The issue was not discovered until the the SZ hit the market... As I have been told it is already fixed on all future shipments.

Gary you did the right thing though. Not too big of a deal, but it is nice to have it right in the kits. Sorry you had to deal with that. I am glad the correct info got your there though and you got it dialed in great.

[Wayne]

Gary: I went back and did my second SZ-H engine this weekend. I opened the top collett up some, and it dialed to less than 0.001 easily.

My clutch/fan gap was around 26 mils or so. Beware on how you measure that. If you just bolt the top assembly to the lower assembly, the gap reading could be way off since the clutch may not be seated in the fan dampeners all the way. I make sure my glow plug is out, and spind the engine with the starter. The clutch/dampener then moves into the position it will be in when it is running. I think then any engine alligning will be more accurate also.

take care

Wayne
www.flymha.com

[wattybotts]

I, too just got the SZ with my new Stratus and ran into this problem. I took alittle bit off of the top collet and got my runout to 1/2 thou. My upper collet is just barely snug on the crankshaft.

This is how I was told to set the gap for the Fan Driver on the Stratus:

Loosen all 8 bolts that connect the upper and lower frames together.

Cut up an old credit card (which in my case was 21 thou each). Loosen all 4 bolts on the Mother of God Plate (the 5mm Base plate) so that the motor can move around alittle bit.

Slide the credit card in both the front and back of the fan gap, and with one hand on the top of the frame near the gyro mount, and the other on the bottom of the bottom plate, squeeze the two frames together.

Then, tighten up the 4 bolts on the Mother of God plate and tighten up the 8 bolts that connect the lower/upper frames.

Remove the credit card, and check the fan gap. It should read exactly 20 thou (or however big the card was) around, on all 4 sides.

I then spun the starter, just to make sure that everything set correctly, and the gap remained the same.

This is how I did mine.... We will see how it goes!!

[Gary O]

Wayne,

When I built my first Stratus with the SZ 91, I knew something wasn't right when I bolted the halves together and the gap closed completely when I just turned the motor over by hand a few times. After that is when I started looking for the problem. This is where I think some people give MA a bad rap. They can't help the fact that manufactuers make changes that affect parts in an existing kit. As Eric stated, the kits that ship from now on will have the correct collet and bottom plate so this won't happen. It's going to take something a lot bigger than a collet problem to fly anything but MA.

[saary]

i just got my kit. how can i define if i have the new collet for the sz or the standard collet? incase that i have the standard collet, how deep should i drell the collet to set correctly on the crank shaft?

[Wayne]

The lower collet needs to fit all the way down the crankshaft. It should be a firm fit, not loose and not so tight you can't push it on by hand. The upper collet needs to fit SLIGHTY looser. We're not talking falling off loose, but it needs a slight amount of wiggle rooom. That top collet, without the fan in place, should easily slip onto the crank all the way to where the collet has the internal collar.

Wayne

[Aviation addict]

I so wish I'd read all this last night. I've just spent the whole day cursing the fact that I couldn't get anything better than 0.002. Most of the time I had 5 thou +. My Kit was shipped to Ricks 2 weeks ago then straight to me but I seem to have the old OS collet. I'm going to have another go now. What a day!

[Aviation addict]

The best idea is to call MinAir and get a new bottom plate for the SZ. You then will just bolt the motor to the new plate (NO SPACER) and you will be good to go. This also assumes you use the Stock OS drive washer between the fan and motor. The new plate is the right thickness, relief cut for Hatori pipes, and has 3mm mount holes in it. It is the correct engine mount for the OS SZ motor.

How do I tell if my base plate is the SZ one or C Spec? Anyone got a picture of the SZ plate please? This is my first MA model and not having the right parts for my engine of choice will be a bit disappointing if that becomes the case.

The (SZ related) top collet already caught me out, is it just the base plate aside from that to look out for when using the SZ or is there more....? Cheers.

[DavidH]

http://www.miniatureaircraftusa.com/Store/proddetail.asp?prod=126%2D32

126-32 SZ Motor Plate.

The ones I have seen have SZ engraved into them. The lighter color looking area is where the engine will mount. It has been machined down to fit the SZ.

David

[Aviation addict]

Thanks David. My plate looks just like the one in the link. It doesn't have anything engraved on it but hopefully its the right one. It has the lighter grey machined area.

[saary]

The goal is not to go nuts and make this too loose. But, as you know the top collett has an inside lip, and that lip need to sit sqaurely on the crankshaft where the step is. That part is where the run-out is zero.



should the lip sit on the curved area in the crankshaft where the shaft sloops from 10mm to 8mm?

[Wayne]

Yes, when you put the collet on the crank without the fan it needs to sit on that lip.

Wayne

[saary]

where do you indicate the fan? from front surface or inside or must be both :bomb:

[Alfred]

Dial indidcate it here:

On the Stratus, that would be the Fan hub

http://www.helifreak.com/attachment.php?attachmentid=7550&d=1153733070

[saary]

i have 001 runout on the outside of the fan hub and 001 in the hub surface and 002 in the inside of the fan hub. will this be fine. this is the closest i can get :arggg:

[Alfred]

It's the inside where the ball goes that is the most importened. The others are partially compensated by the 2 dampers and the Fan/clutch gap.
Do it just like in the photo and go for maximum accuracy you can get.
By the way, a metric Dial Indicator will give you a higher resolution.

[saary]

what is the maximum acceptable runout i can have inside the fan hub?

[Mercuriell]

I've just assembled and dialled in (http://video.helifreak.com/?subpath=mercuriell&filename=Stratusdialhub.wmv)two fan hubs on BW Special Stratus' - no way would it dial in less than 2.5 thou unless I drilled out the outer recessed collet to 8mm to make it looser on the threaded crankshaft. I also burnished the smooth first bit of crankshaft so that the inside collet was a firm but sliding fit - doing this the hub was adjustable during nut tightening and I got the run out down to less than 0.8 thou.

[Ed W]

The easy fix to the fan/collet problem is to buy the Maryland Machine fan and spend your valuable time doing something other than tinkering with the fan.
Ed