Announcing the Merc 888

[Yellowjacket]

Basically, that's what is planned for the 888. A prototype will be built welding a "blade" to the existing Mercury rod cap. The matching "fork" will be machined into a custom link rod. These will be constructed to complete swing testing and clearancing of the opposing block.

Careful with welding on the rod cap. If you weld on it make sure that it is round when you are done. Welding will distort the cap and also anneal the rod cap (that is a forged part) so it will be softer than it was before the welding. Also the inner surface of the rod cap is hardened to work as a bearing surface. Grinding too much on that surface will also soften it and it won't last long under the contact stress... These rod caps are generally opened up for clearance but doing that only takes about a half thousandths off of the bore (or about a quarter of a thousandth off of each side) so there isn't much of a problem with taking too much of. But if you take off three thousandth's from a surface you are likely to be getting through the hardened surface and the surface will spalling off in short order.

If you change the bearing clearance by a half a thousandth then that's a lot, so you will probably have to clean it up when you are done welding.

[Tim Kurcz]

Careful with welding on the rod cap. If you weld on it make sure that it is round when you are done. Welding will distort the cap and also anneal the rod cap (that is a forged part) so it will be softer than it was before the welding. Also the inner surface of the rod cap is hardened to work as a bearing surface. Grinding too much on that surface will also soften it and it won't last long under the contact stress... These rod caps are generally opened up for clearance but doing that only takes about a half thousandths off of the bore (or about a quarter of a thousandth off of each side) so there isn't much of a problem with taking too much of. But if you take off three thousandth's from a surface you are likely to be getting through the hardened surface and the surface will spalling off in short order.

If you change the bearing clearance by a half a thousandth then that's a lot, so you will probably have to clean it up when you are done welding.

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).

[Powerabout]

why do they copper plate?
Do you need to freeze them to break the cracked cap types?
does the big end need to be harder to do this?

[smittythewelder]

After welding and BEFORE re-machining, you might consider shot-peening and even cryo-treating the rod . . .

[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.