Cowling Prop Spacer

I am getting ready to start the cowling on my 7A. I have read about and seen people use PVC spacers of 2 1/4" length for simulating the CS Hartzell prop distance from the starter gear to spinner backplate.

I just wanted to check and see if that has worked out well for those that used that method instead of just mounting the prop. Was 2 1/4" the right distance for a 1/4" final cowl to spinner clearance?

I'd like to keep my prop safe in its box until the final mounting if possible.

I wanted the same but needed to understand blade clearance with the composite prop, so I got mine out and fitted the spinner.

You might want to do the same as that will provide you with the proper dimensions to make the PVC spacers. For a FP prop it is pretty straight forward, for a CS not so much due to the mounting plane offset for the spinner plate from the crank flange.

I have heard of people using the wrong numbers given to them and it yielded an improper cowl/spinner gap.

I am not an expert, but have ben through this step. YMMV

I don't remember if 2 1/4" is the right number, but I did use this method to fit my cowl. It worked fine when fitting the cowl for the Hartzell constant speed prop.

Measure from the prop mounting flange to the hub surface where the spinner back plate spacers sit. Subtract the back plate spacer thickness and spinner back plate thickness (don't forget the back plate doubler). The result is the required length for the PVC spacers.

Once the spinner back plate is in the correct location in space, relative to the engine flywheel......then you can set the cowl to spinner gap to whatever distance you want. I used 1/4".

I used the spacers and it worked great.


I used some snap bushings to center the 7/16" bolts to the 5/8" holes on the spinner bulkhead.


I cut a round spacer out of 1/4" plywood, and drilled some holes in it large enough to slip over the PVC pipe spacers.


A couple of small bolts hold it up against the spinner bulkhead.

I had a fellow my the name of Ed Mc Gowin who come over with a hartzell prop hub... You night find a prop shop and be able to borrow an old hub. We inserted two bolts on the rear side to simulate a 3/16 spacing to the cowling.
It worked great.
Jack

Thanks for the advice guys.

I will try Steve's idea of calculating the spacer length from my hub measurements and see if it comes out to the 2 1/4" length. If it doesn't, I will mount the prop, remeasure everything and then make the spacers and take the prop off.

This is one area where a mistake could be very costly so I don't want to screw up. (not the first time)

Thanks for the pics Bruce. Once I get the spacers made, I will use your idea of a 1/4" plate behind the spinner to get the front of the cowl straight.

Thanks again for your replies.

I was wondering the same thing but with a long James cowl. It needs a prop extension. I am thinking I can just put the extension on behind the prop spacer. Does this sound correct?

This is an old thread, but it was one of the better ones that I found searching for the right dimensions for temporary spacers. Let me add one data point from my own prop, in hopes that it adds value to someone's build.

I already have my prop so don't actually need the spacers, but I thought I'd measure it to see how good that oft-quoted 2-1/4" spacer length was. My prop is a C2YR-1BFP/F74972 72" Hartzell constant speed, ordered directly from Vans.

Measured the horizontal distance between the edge of the flange that sits on the starter ring gear to the edge of the prop hub that the spinner bulkhead contacts. The distance on my prop was 2-1/2". Van's DWG C4 specifies the following stack-up against the prob hub:

• 1/4" S-604 spacer (came with the prop)
• 1/16" AN960-416 washer
• 1/16" S-602-1 spinner bulkhead
• 1/16" S-602B doubler ring

Temporary PVC spacers would go from the fwd edge of the starter ring gear to the aft edge of the spinner bulkhead (prior to trimming that bulkhead for the prop), so the correct size of the spacers should be the 2-1/2" distance, minus the S-604 spacer, minus the AN-960 washer, minus the thickness of the bulkhead = 2-1/8". I am not including the doubler ring thickness because the PVC spacers would not contact the doubler. This is 1/8" shorter than the 2-1/4" called out by Vans. Meaning if I positioned my cowling using 2-1/4" spacers, then installed the spinner on the actual prop, the gap between the spinner and the cowl would end up 1/8" too close. Glad I measured with the real prop.

At the end of the day, I think I will eventually just install the actual prop in order to properly position the front of the cowling, but hopefully this info helps someone. I'm glad I didn't just use 2-1/4" and cross my fingers.