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Thread: R90/6 Engine Rebuild: How Far Should I Go? Seibenrock Kit?

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    R90/6 Engine Rebuild: How Far Should I Go? Seibenrock Kit?

    This winter I am going to rebuild the engine on my venerable R90/6. I have a reputable automotive machine shop that is going to rebuild the heads with new hardened valve seats on the exhaust and new valve guides. I have settled on the idea of buying the Seinbenrock big bore kit.

    The engine in its present condition blows smoke on startup, is slightly down on power and has 150psi in the left cyl. and about 120 in the right. It doesn't make any funky noises and is generally still reliable. I would like to make sure that the engine stays together from here on out. I am going to inspect the cam chain and sprockets but unless they show obvious sign of age and wear I'm not going to change them. Question; who thinks I should go ahead and change them?

    There is no reason to suspect that the crankshaft or connecting rods have any problem. Question; should I go into this motor that far, i.e., inspect the bearing surfaces, plasti-gage, etc?

    I don't know the true mileage of this bike, I would guess at somewhere above 70k miles. Thoughts?

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    Big bore kit? What size is that? What year R90/6? The engine case opening changed for the '76 models to 99mm.

    I just did my R100/7 with 100K miles. Since the bore was out of spec and I didn't want to try 1st over, I went with the Siebenrock kit, just the 1000cc replacement. After considering the issues with going to the crank, I decided to leave well enough alone and just bolted on the top end without pulling the rods. Like you, I had no reason to suspect any problem with the bottom end...hopefully I won't come to regret that. But so far, so good.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

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    Which year R90 do you have?

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    It is a '76, engine # 496xxxx (or something like that). If I'm not mistaken Seibenrock has a kit that not only covers the 99mm cases but has one that will work on earlier 97mm cases also.

    I am also thinking about a new clutch, throwout bearing (or whatever they call it) and new seals all around. I want to run synthetic oil in it and I do not trust the seals not to leak. And yes, I fully intend to break it in with regular mineral oil first. Since the engine will be making more power I would think that slipping clutch problems would be right around the corner.

    What about the rocker arms and shafts? I hear tell that that is a point that is often ignored during top end rebuilds and that comes back to bite the builder in the ass.

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    I wouldn't hesitate to put a stock clutch in...my /7 had more power than the /6 and I've never once had a clutch issue.

    Again, I reused the rocker arms and pushrods on my rebuild. Since you have the '76 model, you have the same rockers as my /7 which were used to 1984 according to Anton:

    http://www.largiader.com/tech/rockers/

    They've worked fine for me. Rocker arms are something that can easily be replaced later. Same with the clutch. If you're doing an engine rebuild, then you wouldn't necessarily remove the clutch...unless you're going that far.
    Kurt -- Forum Administrator ---> Resources and Links Thread <---
    '78 R100/7 & '69 R69S & '52 R25/2
    mine-ineye-deatheah-pielayah-jooa-kalayus. oolah-minane-hay-meeriah-kal-oyus-algay-a-thaykin', buddy!

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    All years of R90/6's can be built up to accept more power and torque, the '76 just has a few more built in advatages. They got a better tranny, a little more frame bracing, the bigger diam. cases you mentioned. Which only means it doesn't have to be set up and bored out to accept the 1000cc jugs. Just on less step to deal with. If I were going to the trouble of going to 1000 cc's; I'd be looking to go with the big valve heads and running bigger 40mm Bings. I'd stay with the smaller diam. exhaust head pipes. Good street midrange. You be in the range of a '77 R100S which I believe was 65hp, 5hp up on a stock R90. Airhead bottom ends are known to be quite robust and have long life, if oil changes were even remotely on schedule. Your machine shop will probably inspect your bottom end and top end and make recommendations on what they find. I think you'd have a nice little package and a pretty peppy motor, with good service life.

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    Although I am looking for improved performance it is the type of performance that will allow me to go 90mph all day coast to coast. People have suggested a hotter cam and other race mods but I am looking to make the first and second priorities reliability and reliability. Does anyone know of a good oil cooler kit? I don't want to stick an old Lockhart under the horn. I'd like something with a thermostat.

    Since you brought up the subject of tranny's, does any one have a good fix for that 2nd gear "clunk?" It shifts almost louder than my ironhead Sportster.

    I intend to farm out all of the machine shop work but will be doing all of the assembly myself.

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    There is a tranny upgrade for that model. It's the camplate selector and it comes with more pronounced detents and a little beefier shifter spring. That spring is a known weak point. I've done that to my '74 R90/6 gearbox. I allways thought my '74 ran pretty good back in the day. Pretty much stock, I did run a Breil oil cooler, pancake style insert between the engine block and oilpan. Gave the engine 1 more Qt. of oil and the 3 holetubes were a rudmentary oil cooler. I just liked the look of it. I know my bike could run 80mph all day and not hurt it. The speedo would show 90, but that was way optomistic, a known deal on those speedo's. My engine now sits in an engine stand awaiting install. I did the top end work myself. New stock valves and freshened up seats. I dual plugged the heads mainly for a 70's style upgrade. I bored the cylinders out and installed 1st over Kolbensmitt pistons. I have stock compression and am running 32mm Bing carbs. It'll run strong and have long service life. The bike was 60hp and will still be 60hp, which is fine for this build. The package I suggested for you is I believe a good setup for a next step upgrade. I believe in matching components that give a pronounced upgrade, won't kill the pocketbook and are with end the parameters of the overall bike chassis.

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