Topic: 42 oppy build (for longevity)

[Rikk]

Hello all, I've been lurking and reading here for awhile now trying to gain some oppy specific mods. I'm not necessarily looking for racing type performance, but for anything that will help a work engine without decreasing life. I've built a couple B&S Raptor flatheads and one Animal for my son's race kart. Restrictor plate stuff, but I did everything that I was allowed to by the WKA tech manual. His best finish was 3rd in points overall without any engine failures. I'm not claiming to be an engine builder, but I have been a machinist for 20+ years and have turned wrenches since I was a kid.

What I am working on is a 42 opposed for a friend of mine. It's for a stump grinder. It seemed low on power and smoked /used oil a lot. I pulled it down and found huge carbon deposits in one of the heads and a fair amount in the other. I lucked out and there isn't any type of a ridge that I can feel in the cylinder, so I think we can just freshen it up with a hone, rings and valves, but I need to measure everything yet. I plan on making sure everything is perfect spec wise before assembly. The list of things I am doing is below and I wanted to know if there is anything else I should be doing that may help with performance/efficiency without making it unreliably. This thing sometimes runs at the max rpm the governor allows for 6-8 hours at a time with only a break for lunch and fuel. Will verify rpms with one of my Mychron tachs just to know where we are at.

-Opposed rod oil mods
-Deburr everything in the rotating assembly
-Ensure the rod/piston assemblies are as close as possible in weight
-Grind the valves to within .0005" or better face/stem runout and lap with progressively finer diamond compounds
-Install bronze valve guides if they are worn
-Exhaust valve springs on both valves
-Clean up and match the intake and exhaust ports (nothing major, just improve flow, take out obvious restrictions)
-Set ring gap at .002-.003
-Maybe new rods depending on how they mic.

I normally don't run flywheel keys in flatheads and lap the taper to the flywheel so the nut holds it and I can adjust timing using a degree wheel, but IF timing mods will help this engine, I'd like to use an offset key. Thoughts on that?

I know this isn't what y'all normally build, but I've been around racing all my life and a lot of things that help add life and performance to a race engine can be used for an everyday engine with success, I'm just out of my element with this opposed.

Thanks in advance, I really appreciate the help.

Cheers,

Rikk

[chavez]

I'm not overly familiar with the oppy, but as you are intending it for commercial use, I would recommend a larger ring gap .008" or so. You will be operating in an environment that doesn't promote cooling due to the buildup of foreign matter impeding air flow.  I might suggest using the flywheel key as well, just to keep that flywheel from creeping timing.  Racers are always checking and adjusting things, not something you want on the job site.   I believe there is something to flipping the top rod over as well, maybe someone will chime in on that one.
  Everyone has their opinion on lapping valves, but I tend to the theory if things are done properly, lapping only wears more material off the valve and seat. If your angles and concentricity are correct, they will seal, and seal better once worn in.

[Rikk]

Thanks Travis, I was wondering about the ring end gaps. And you are right, this engine will get run for long periods and in dirty conditions.

I was planning on a flywheel key, as you said, it won't get checked ever again, but I was more curious as to if there would be any advantage to running a few degrees of advance or if it wouldn't be worth the trouble seeing as how the engine will be used.

With the valves, I agree with your thoughts as well. My plan was to have the faces a set of new valves touched up to make them more concentric than they come stock. I've seen factory Briggs valves with significant runout. The valve seats will just get a light cut to make sure they are round and the angle is correct. Nothing major. With the restrictor plate engines I am used to working with, having the best seal possible is crucial.

I appreciate the advice! Happy new year.