Lower shock Mount Redesign, Paint, Engine In

A project log for SECApocalypse

Yamaha XJ750 "SECA" with adjustable Hossack front end and Mad Max style

sebwierssebwiers 01/27/2017 at 17:280 Comments

I discovered that the lower shock mount I'd built prevented the engine from fitting in the frame. I could have trimmed the aluminum plate down enough to make it fit, but felt it would give up to much strength. Instead I decided to redesign the lower shock mount. This required a bit of frame modification. During my strain testing, I'd also cracked the frame where there was a pre-existing nick, so I needed to repair that crack and decided to run some strengthening rails across the frame rails since they were not designed with supporting a loaded shock in mind.

This is the ugly little crack

This is the start of the new shock mount. The 3/8" engine mount bolts (bumped up from 10mm because I have better selection on threaded lengths and strength for SAE bolts) run into threads in the aluminum block, and half inch bolts run through the shock mount into threads in the aluminum block. This is structural but probably not strong enough for road use. Instead it serves to hold everything in place while I do frame modifications.

The finished shock mount adds a beefy rectangular box beam expanding the lower engine mount, which the shock mount bolts directly to (the box beam has a threaded weld insert). Speed holes in the box beam allow paint in, the ones on the shock mount are just there to look nice.

I then pulled everything apart and painted it. Rustoleum "rusty red" primer holds very well on the bare metal, not nearly as well over the existing powder coat, but will be the final paint color. One of the shop managers calls the color "hot rod red" or "prototype red" and that's exactly why I like it. As a kid I saw the same color covering the fenders and hoods on a lot of a lot of sweet 1960's muscle cars rolling around Detroit. You can also see the 3/4" x 1/8" bar welded to the front & bottom of the frame rails, adding some strength to the repair in the area that cracked.
The bare steel suspension components are coated with linseed oil, which is commonly used to protect metal sculptures and as an inexpensive underbody coating. I really wanted to preserve the raw metal look of the parts I had fabricated. The linseed oil takes a long time to harden when applied to metal but forms a nice solid film. Its not as durable as powder coat or properly applied paint, but it's very easy to touch up and (unlike paint) can be applied over oily & rusty surfaces.

The engine went back in pretty easily, the only tight spot being there's just 1/8" clearance from the bulge of the cam chain to the upper shock mount. As a big bonus, the new lower shock mount design allows the original exhaust to fit without any modification. The headers do run very close to the shock, and will be wrapped with insulation to keep heat away from the shock. They actually make exhaust pipe wrap specifically to keep heat away from other under-hood components, although on motorcycles its more often used for looks and safety (to prevent burns). I get a two-for-one deal, because I both need to keep the shock from getting hot, and get to hide my ugly old pipes.

I'm really exited to have the major fabrication work done and engine back in, but there's still a LONG way to go before I can ride it. :-P