Shimming engine mount forward

I ran into an issue swapping C/S propellers on my RV-7A...now I don't have sufficient clearance (or any) from the nose of the cowling to the spinner backplate. Assuming I have the spinner backplate mounted as close to the prop hub as possible I thought I would consult the collective brain trust for other options. I posted in the RV-7 specific section and another RV-er suggested shimming with extra washers at the engine mount to engine interface, that got me to thinking about shimming at the engine mount to firewall location. I understand it would move my nose gear forward a commensurate amount and also shift my cg forward but that is actually a good thing in my case. Also possibly introduce some new cowling to engine component interference in the forward cowling area. Would placing a few large area washers between all of the mount points and firewall have any other implications? Any ideas on a better way out of this situation? The only other solution I can think of is to rebuild the forward end of the cowling and I hate to mess up my pretty paint job!

Bruce RV-7A (purchased)
2021 dues gladly paid

Assuming the blades have good clearance, make the large spinner bulkhead a 2 piece design. Make the center piece almost as large in diameter as the bulkhead, using the original bulkhead as a drilling template. Cut the inside of the original bulk out large enough that it can slide forward relateive to the cowl. The inside cutout likely needs to grow in various areas to clear the hub. Cut/make as many shims as necessay to get clearance and rivet it all back together. You will need to be carefull in your jigging to eliminate runout in the diameter. This all assumes you have enough clearance between the blade roots and the bulkhead to move it back. This part gets HEAVY dynamic loading, so use LOTS of rivets and space them strategically to eliminate balance issues.

I would also call Sam at Saber to see if he can make you a 1/4" steel prop shim and custom length prop lugs. I could make lugs like this, but for a critical application, I would want an expert to do it and he does this for a living.

I had exactly the same problem on my RV-6 when I installed a Hartzell prop to replace a Sensenich. I just shimmed my mount away from the firewall with AN-970 washers and used longer bolts. Mine was close to being okay so two washers at each location gave me the clearance I needed. Once you've done that be sure to check to make sure everything still fits properly under the cowl, especially the baffling seals. This worked fine for me.

I'd touch base with Van's before shimming an engine mount. You're changing the joint from having the bolts directly in shear to a joint where there is also bending. If you don't need more than a washer or two, it'll probably work.

But the downside of failing those bolts is pretty serious, so a call to Van's is in your best interest.

If anything, I would say maybe a small hub extension if one exist. But the right way to do it would be to rework the cowl. At least you are shortening it which is a lot easier than extending it out.

Thanks to all for the input!

I think I have come around to believing the right way to do this is to shorten the cowling. Not having built one of these that didn't even occur to me but since that is what is done during the initial fit of the cowling it only makes sense to trim a bit more off the aft end of the cowling to get the required clearance. It will mean filling in all of the existing fastener holes and repainting everything firewall forward but it seems like the right thing to do...at least for today! Time to "bite the bullet" and develop some fiberglass skills...

Bruce