1972 Combat crankcase mod

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Aug 8, 2005
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Has anyone had this crankcase mod done to their Combat Commando?http://www.oldbritts.com/n_c_case.html I am thinking of getting this done but I am not to sure as to weather it is needed for street use. Is this crank mod for race engines only or is it a big improvement to the existing design. Your input. Thanks
Probably not necessary for street use but I think I'll have it done anyway. Good opportunity while the engine is apart. Don't think I'd bother tearing down just for that mod though.

'71 Commando with '72 motor, in pieces
The 1972-73 cases had there breather in a stupid place - right where the oil's going to pool in the case. In addition to the oiling problems Fred noted - with the breather full of oil, crankcase pressure forces leaks out of a few more seams than usual. Just what you need - a leakier C-do. :roll:

Yes, I've had Fred fix my 1973 cases. FWIW, Mick Hemmings was the origin of the practice. You can see the relocated breather and polished blanking plate in this photo:

1972 Combat crankcase mod

If you're going to push the bike hard for any real period of time, I'd say go for it. Cheap insurance if you've got everything apart already.
Thanks for the good Pics. I think I will get the breather placed where you have it also. What was the total package worth to get what you have done? You also show a pic on a modified chain tensioner on the timing side. I was wondering who does this....and is there any great things said about this mod. I am going to try to make my Combat as robust as I can and I want to improve as much as I can before I get to far into the reassembly of the engine.

No sweat, my pleasure.

Was it worth it? Well, it wasn't nearly as expensive as some of the other things I've had done to make my engine as "robust" as possible. So, I'd say yes.

As for the Automatic Cam Chain Tensioner - the benefit is threefold: the teflon slipper block is much quieter and offers less drag than the steel one, it eliminates the need for regular adjustment, but the big one is it provides more consistent cam timing. (Cam gears aren't totally round, so with a normal slipper/tensioner setup you adjust it to the correct tension at its tightest spot - so cam timing actually "pulses" a bit from tight to loose.)

RMA Engineering
447 Santa Mesa Drive
San Jose, CA 95123 rmaten4@aol.com 408-578-4032
Price $110 US

That said, I have a disclaimer to offer - I've just gotten the motor in the bike and am working on the primary belt drive, final drive and electrics, so I've no expeience with running my $7k hot-rod motor. However, everything I've done has a lot of personal research behind it as well as recommendations from people I really trust.

The bike sure looks better with an engine in it:
1972 Combat crankcase mod

(Yeah, I'll polish the instrument bezels and z-braces... It's just tedious work and I'm avoiding it.)
Its looking great so far Dr
Its comming together man and the disks are they the cast iron stock units with holes drilled to lighten things up? Hard Chromed? very cool.
Not trying to jack your thread NutNuck.... but, yes, those are stock disks, blanchard ground and lightened. They're not chromed though - just painted with JP1 Yamaha Silver and thrown in the oven for an hour on 250deg F.

The first set of pads will wear through the paint at the contact area, leaving the rest of the rotor painted at with at least a little rust resistance. Then I'll get a set of nice, sticky pads from Vintage Brake (if they don't sell out before I order them).

There are those who recommend you don't drill iron rotors, but I've seen them raced, season after season, without any problems. However, they have to be drilled by someone who knows what they're doing. Each rotor is about 1lb lighter than stock. With the alloy rim, the front wheel lost 3lbs.

I did a MkIII disk conversion for the rear brake - wouldn't do it again for the cost and hassle. Lots of different parts - and they're all expensve. Still waiting on a custom billet sprocket carrier (c'mon Fred, summer's almost over!) that will replace the cast iron unit and accepts a standard 2-piece motorcycle sprocket. Also waiting on a one-piece rear axle that uses a spacer in place of the flange on the stub-axle. In all , rear wheel will be 10+ lbs lighter than the stock drum was.

What's the rhyme, "yeah its fun, but I'd rather be done?" That's where I'm at right now.
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