1969 Chris Craft Roamer 46 Refit: Cummins Engine Install — Spacers

Having finally discovered that the mounts that came with my Cummins engines would not work on the Roamer without modification, I took them to a local machine shop and had some work done. Meanwhile, back in February 2014, I had a fancy schmancy idea for engine beds. Unfortunately, that idea was based on a few assumptions that turned out to be wrong–like, thinking the engine mounts were symmetrical…silly me. Even more unfortunate is that I went ahead and bought a 25′ stick of 3/8″ angle and 1/4″ plate in 6061 aluminum and cut and welded my creation together…proving once again that a fool and his money are soon parted. But on the principle of making lemonade when life hands you lemons, I’m re-purposing the fancy engine beds as spacers between the vibration isolators and engine stringers.

Back from the machine shop, with a longer slot

Back from the machine shop, with a longer slot

My uber-precise engine centering gauge

My uber-precise engine centering gauge

When I first attempted to install the engines, I used a piece of tape between the stringers with a mark at the mid-point. Every time the engine touched the tape, it ripped (surprise!). The tape ended up stretching, too, so it was pretty useless as a center indicator.  So now, I use a piece of aluminum angle with a mark indicating the halfway point between the outside edges of the stringers. I put a square on the angle, line it up with the mark, and then move the front of the engine until the square is centered in the middle of the damper pulley.

The mark at the mid-point between the stringers

The mark at the mid-point between the stringers

The middle of the damper pulley

The middle of the damper pulley

With the new slot cut, the vibration isolator lands on the stringer

With the new slot cut, the vibration isolator looks like it will land on the stringer

ShopSmith table saw quickly cuts up expensive engine beds into useful pieces

ShopSmith saw quickly cuts up expensive engine beds into useful pieces

Cutting expensive 3/8" angle to make plate

Cutting expensive 3/8″ angle to make plate spacers

On the upside, I'll have lots of aluminum to practice welding on...

On the upside, I’ll have lots of left over aluminum to practice welding on…maybe make a shelf or two

Starting to look like a spacer

Starting to look like a spacer

Putting an angle on the ends

Putting an angle on the ends

I need to keep the plate out of the radius

I need to keep the plate out of the radius, or a gap forms between the plate and the angle

That ought to do it

3/8" angle + 3/8" plate = 3/4" spacer

3/8″ angle + 3/8″ plate = 3/4″ spacer at the front

Reconfigured rear engine mount + 3/8" angle + 3/8" plate (x3) = 1-1/2" spacer

Reconfigured rear engine mount + 3/8″ angle + 3/8″ plate (x3) = 1-1/2″ spacer

From the beginning, I wrongly assumed that the mounts that came with these engines had the slots at the same elevation. It turns out that was completely wrong–the front mounts are lower. Having never done this sort of thing before, I’m pretty much guessing how many spacers I need. After putting in 3/4″ spacers at the front and 1-/1/2″ at the back, I still had at least an 1/8″ gap at the bottom of the couplers. So I removed the spacers at the front and started dropping the vibration isolator bolts lower and lower to close the gap.

Out with the spacers at the front

Out with the spacers at the front

In the pic above, you’ll notice that in addition to there being no spacers anymore, I’ve also bottomed out the vibration isolator. See the big nut in the lower left corner of the pic? That’s the locknut that’s supposed to go under the other nut and washer on the under-side of the engine mount. Taking it out allowed me to drop the front of the engine an additional 1/2″ and get the couplers to align. But I think it’s unwise to run without the locknut. Hmmm

Perfect fit...zero gap

Perfect fit…zero gap

And then, another problem that was right in front of me made itself apparent. If you look closely at the left side of the pic below, you’ll notice (as I belatedly did) that the angle of my uber-precise engine centering gauge is hovering above the engine stringer. rather than sitting firmly on it.

3/8" angle + 3/8" plate = 3/4" spacer

Hmmm

The angle on the stringer is not square

The original angle on the stringer is not square

It turns out that Chris Craft welded the 6061 angle to the stringer with the upright stringer plate impeding into the inside radius of the angle. This creates a gap between the angle and the plate and changes the angle between the upright stringer plate and the top of the stringer from 90° to ~95°. So, to avoid inducing stress into the vibration isolators from that 5° misalignment, I need to make a spacer that will level the stringer here. But if I do that, it will raise the front of engine, and there goes the perfect alignment of the couplers!

Dang it! Back to the drawing board!

Next up on our 1969 Chris Craft Roamer 46 Refit: Cummins Engine Install — Spacers II

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6 comments on “1969 Chris Craft Roamer 46 Refit: Cummins Engine Install — Spacers

  1. Doug says:

    Then grind down the stringers to flat

    • 1969roamer46 says:

      Well…that’s one approach. But the Cummins engines weigh 2x the Ford 427 OE power, with 30% more HP and 150% more torque. The 871 boats with 320hp used the same scantlings, so at full thickness I think they’ll be fine. The angle on top of the stringer is 1/4″ thick. Removing half of that thickness right under the spot these engines attach…I think it’s better to add than to remove. Modern epoxy and fillers are engineering wonders. Time will tell.

  2. Q – What about using a really tough dense rubber/delren/navcon anti-vibration material that would form fit to fill the gap and really dampen the vibration.

    • 1969roamer46 says:

      Hey Bill. I don’t have a way to accurately slice solid material to get a wedge that narrow. I’m leaning toward thickened epoxy. That stuff’s got very good compression properties.

  3. Doug says:

    Can’t you just make an aluminum shim wedge that leaves the high inside point of the stringer supporting the motor mount and fills in that angle with aluminum to make th emount supported all the way acros?

    • 1969roamer46 says:

      Hi Doug. I could use aluminum, but I’m not sure if my equipment can cut a wedge that thin accurately. I’m thinking thickened West will do the trick. But either way, the bigger problem is that it’ll raise the front of the engine, and there’s no more adjustment in the isolators. Stay tuned…

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