PA-12 O-320 STC Long/Short Mount?

What's the difference between the Univair STC which he says is the old Stoddards STC and the Mackenzie Flying Service STC? I have the original mount which had an 0290 on it. I also have the install instructions for the Makenzie STC. The makenzie dosent say anything about beefing up the mount just remove the o235 and bolt on the 0320. I assume this is considered the long mount. Is Univairs the short mount? I guess I want the long mount to avoid cutting the cowls. Just want to make sure I get the correct STC.

Thanks in advance!

Univair and Crosswinds STOL both use the short mount. Kenmore and McKenzie use the long mount. The Kenmore mount is supposed to have the lower tubes reinforced. I don't know about the McKenzie. I have spoken to the new owner of the McKenzie STC once. he was supposed to send me information regarding his STC but never did. Everytime I tried to call him back, he never returned my calls.

I did the Kenmore STC about 25 years ago for that exact reason (to avoid cutting the cowls and changing the engine mount). From time to time I wish I had gone with a short mount STC. PA 12's have a tendency to be nose heavy, a short mount would help that condition.
That being said, it is not that big a deal, just a little heavy in the flare. Also, great access to the accessory case, and the mags are not up against the firewall.

there's actually 3 mount lengths....(that I have seen..)

original long
short
and crosswinds short, that is about 1" shorter than above and swings sideways like a cub.....

like every thing on a -12 ain't 2 of them the same!

I have the Crosswinds short, and kinda think I'd maybe stick with the original long if doing it again. The first reason is purely aesthetic - I think the plane looks better with the longer cowl. The second reason is that it's REALLY tight behind the engine, which requires rather sharp bends in mixture and throttle cables. The last reason is CG - I ended up with the empty CG at 12.9", and empty range per the TCDS is 9.5" - 13.3". So mine is pretty far aft. I used light alternator and starter, which moved the CG aft a bit farther, but used Sutton exhaust which moved it back forward slightly. I flew the plane for a long time with the Kenmore STC, reinforced long mount, and it was nose heavy, as noted. However it didn't have extended baggage at that time, and with that survival gear could go in the back to adjust CG. Just some thoughts - -

Pluses and minuses with either. I have flown both. Mine has the Kenmore original long mount reinforced and I really like it. I was a bit concerned on the first flight and questioned my much experienced A&I as I use a bunch of nose up trim solo. His reply was a slight chuckle in his voice, "think about it, dummy---anything you put in the plane is behind you" Have to admit; that was 11 years ago and no problem ---so far:roll:

After flying both, with and without extended bagage, I would cut the cowling and go with the short mount, and not everything you put in a 12 is behind you (your fuel is forward), and so would be your skis. Maybe with small tanks, small tires, and a small prop you might be better off. Just my two cents. Besides I like to see over the cowling with 3"gear and 35" tires.

Mike is correct that there are three different lengths of mounts for the 12. Kenmore/Mckinzie (stock length), Univair/Stoddard (-3 inches), and Crosswinds (-6 inches). I have measured all three.

I went with the Univair because I liked the look of the longer nose but wanted to move the CG aft somewhat. The mags can be removed with this mount. Beware that the instructions are to cut the stock cowling to fit, but my measurements put one of the cuts on the side cowling doors right through one of the stock latch holes.

I built all of the cowling from scratch and it took a long time. Also, the angle that the bottom cowl intersects the nosebowl with a shortened mount is so extreem because the 12 firewall is quite a bit taller than the 18. I built the lower cowl with a streached angle to fit the nosebowl and transitioned to the slope of the lower cowl. The attached images are of the cowling during fabrication and the final product.

In closing, the nice thing about the stock length is that the stock cowling fits. The nice thing about the Crosswinds mount is that the top and bottom cowling from an 18 fits ( I would suspect that the lower cowling to nosebowl transition is going to be a problem) but new side doors will have to be fabricated. The Univair mount, everything has to be fabricated or a highly modified stock cowling.

Hope that this helps.

Jody

N5126H said:

After flying both, with and without extended bagage, I would cut the cowling and go with the short mount, and not everything you put in a 12 is behind you (your fuel is forward), and so would be your skis. Maybe with small tanks, small tires, and a small prop you might be better off. Just my two cents. Besides I like to see over the cowling with 3"gear and 35" tires.

Click to expand...

The TCDS lists the pilot seat as 6" aft of datum, and fuel 23" aft of datum.

Geezer:

When pointed downhill or on the brakes there is more fuel in the front of the tanks than in the rear. My experience is that lots of fuel and little weight in the rear makes for a nose heavy 12.
Bill

Bill -
Agreed - fuel wt will result in a more fwd cg than the same weight in the rear seat or baggage. And of course it will slosh fwd when braking, but it won't get more fwd than 6" aft of the leading edge (datum). So the original statement that everything is behind the pilot is still correct. For example, in my 12, 6 gal of fuel, 180 lb in the front seat, and nothing else in the plane puts the cg at 12.2". Same configuration except with 30 gal of fuel puts the cg at 13.2". Operating CG range per the TCDS is 9.0" - 18.6".

I also agree that my 12 felt pretty nose-heavy before - can't wait to see what it feels like now given that the cg is moved aft some (dunno how much, cuz I don't have the old cg data nearby right now). First flight should be in about a week, depending on my IA's schedule and assuming he doesn't see anything he doesn't like!

How about a few pictures, Gordon ????

I like the long mount. My empty weight CG is at the very forward limit. However, I agree everything I put in it is behind the empty CG, so it moves rearward. I have stock 19 gallon tanks, and 3" extended gear with 31" BW, 160hp, borer prop.. I don't find it nose heavy, and I can lock brakes then lift the tail at a standstill and go in big rocks without the tail coming down if I do not have passengers in the rear seat which is a real advantage. I never brake real hard (why?) so fear of tipping over from hitting brakes is not too much of an issue for me. Just the way I roll, your mileage and preferences will vary.

I recently picked up a PA12 engine mount for my 2+2 project. How can I tell if it is a long or short mount? It's not a swing out mount if that helps identify it. Anything that I can easily measure?
Thanks,
Marty57

Marty,

An original -12 mount is 13"-14" front to back. The lowers are about 13" and the uppers are about 14".

Wayne

Thanks Wayne; I'll measure mine and see what I have.
Marty57