Announcing the Merc 888

[Yellowjacket]

Most outboard rods are forged steel, copper plated (carburize mask), rough machined (removes copper plate from intended surfaces), assembled, carburized, then finish machined (ground/honed). Copper plated areas are soft, only the bearing/wearing surfaces are hardened. To prove this to yourself, clamp a spare rod in a vice, apply torsion with a pipe wrench, and watch it twist without shattering.

For the proto rod, copper will be removed from the cap, the extension blade tig welded in place with the rod assembled and fully torqued. Following, the journal bore will be honed round and to size, and the pivot bore will be machined.

After swing fitting, the proto rod and cap bearing surfaces will be Brinnell tested. If there is no significant difference, the 888 might be run with welded rods. If the difference is unacceptable, custom rod caps will be machined, and the bearing surface will be carburized, honed, etc.

Side note: It might surprise you to know that mod outboard racers have been honing .002" - .0025" from the big end bores for many years. This procedure followed Mercury's change from .880" to .882" crank pins. It allowed rods to survive race conditions without overheating (bluing).

While the outer surfaces of the rod are copper plated to mask the carburizing process (so that the rod isn't hard and brittle on the surfaces that are under tension) and only allow the bearing surfaces to be carburized, the rods aren't in the fully annealed condition. The rods are strong due to the forging process and subsequent heat treat, but not brittle, as they would be if they were carburized all over. Doing a Brinell test on a regular (unwelded) rod and comparing the hardness with the modified end cap is a good plan. If you knew the alloy you could heat treat the cap after welding and it would be as strong as it could be.

I'm well aware that these rods are commonly honed out to increase the bearing clearance. I had a set done when I built my 44 in motor to APBA stock specs. But when you hone these rods to take about .001 inches off of the surface (x 2 to open the diameter up by .002). That doesn't go through the carburized hard surface. I don't know how deep the carburization is taken in these rods. Since the purpose of carburizing is done to make the surface hard enough to be a bearing surface it most likely isn't deep. Deeper carburizing wouldn't serve any purpose so I doubt it's very deep. The only way to know is to grind the rod and then do a hardness test on it, or get a copy of the rod print where it is probably noted on the drawing. My point was that you don't want to grind off too much of the bearing surface or you will risk having a soft bearing surface and that will deteriorate quickly under the contact stress of a rolling element bearing. If you only have to take off .003 from a surface (3 x what you take off in the process of loosening up the rod clearance) you are probably fine.

[smittythewelder]

Lots of rods get re-sized not to change the clearance dim but just because they get stretched to an oval shape after lots of running; for instance, the rods in a lot of 20H and 55H engines are out-of-round, if the rebuilder happens to mike the big ends. A good auto machine shop can probably save them, if you bring a specification.

[Tim Kurcz]

Lots of rods get re-sized not to change the clearance dim but just because they get stretched to an oval shape after lots of running; for instance, the rods in a lot of 20H and 55H engines are out-of-round, if the rebuilder happens to mike the big ends. A good auto machine shop can probably save them, if you bring a specification.

Thanks all for your thoughts on rods. Welder extraordinaire Chris Razor suggests clamping the rod to a 6"-8" long cylindrical aluminum heat sink. He says it will absorb heat from the blade welding process, which will reduce the annealing affect on case hardened bearing surfaces. The plan is to:

Turn the heat sink, build a blade, and weld. Once complete, the cap and rod will be Rockwell tested. If C-scale hardness is mid-50's, welded rods may be used for the prototype. As pointed out, grinding of surfaces and precision honing the bore round and to size must be accomplished following. A local machine shop uses a piston pin hone to accomplish the task.

A couple more weeks will have data. Cross fingers!

Tim

[Fastjeff57]

Was reading about the Napier-Halford aircraft engines used in WW II. The first one--a 16 cylinder H type with four opposed banks of four cylinders and two crankshafts--used the master/ slave rod concept. With a square 3.5 inch bore and stroke, their rated rpm was 4,000--very high for a continuous duty aircraft engine. After suffering rod failures at the crank end, the following engines (24 cylinder H type) went to blade and fork design and were trouble free.

Jeff

[Tim Kurcz]

Was reading about the Napier-Halford aircraft engines used in WW II. The first one--a 16 cylinder H type with four opposed banks of four cylinders and two crankshafts--used the master/ slave rod concept. With a square 3.5 inch bore and stroke, their rated rpm was 4,000--very high for a continuous duty aircraft engine. After suffering rod failures at the crank end, the following engines (24 cylinder H type) went to blade and fork design and were trouble free.

Jeff

Great research! It's always great to learn from the experience of others. While the blade and fork rod would most certainly be a stronger and more elegant arrangement, there is no room for luxury on the petite Merc crankshaft.

[Fastjeff57]

I understand. I'm hoping blade and forks won't be necessary, at least for the trial motor.

(Rubbing hands together with glee) I can't wait to see her run!

Jerf

[Tim Kurcz]

Expert welder Chris Razor suggested a conforming aluminum heat sink might allow welding of the blade/cap joint with little or no damage to the hardened case. An aluminum bar was turned to .003" larger than the big end bore, and the rod was bolted/torqued into position. Our first attempt saw minor heat soak through the cap section, only at the ends - not bad! Rockwell testing will be conducted at the burn sites and between to determine hardness. During the next attempt, peripheral welds will be allowed to cool before the end welds are made. If RC hardness is in the low-mid 50's, the rods will be honed round and to size, and run. More soon! Tim

[Powerabout]

Nice one....
love to see the actual process of making an outboard rod

[Tim Kurcz]

Nice one....
love to see the actual process of making an outboard rod

It took over twenty trial fittings and adjustments to make this one (note beginning shape). It was necessary to cut intake and exhaust side rod reliefs deeper, and remove 3/8" of the intake side piston skirt to allow interference-free swing: The link rod was striking the intake side skirt! That's how the umteenth prototype link rod was made.....

[Tim Kurcz]

At long last, Rockwell C hardness test results are in. Interestingly, the area between oxidation marks showed the lowest surface hardness(?) Evidently, discoloration is not necessarily an indicator of a change in metallurgy. The un-welded upper rod section tested at 59C, the cap is 54-55 in the discoloration circles, and 49-50 between the circles, dead square in the center of the cap.
Plans are to rifle drill the heat sink and pump water through the bore to draw more heat during the welding operation. It is expected that hardness will be maintained in the mid-50's in the center position. Back to the lab!