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Siebenrock 1000cc Kits and Final Drive Ratio?

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Mathieu Dubois
Posts: 3
New Member
Topic starter

I'm considering installing this kit on my '71 R75/5 but I have a question about the effect it will have on the final drive gear ratio. It's been brought to my attention that going to 1000cc would impact the smoothness of the bike. Apparently, the "sweet spot" for these bikes with the normal final drive ratio lies in the domain of the 600-800cc range, and that a 1000cc configuration would lead to noticeable vibration and drastic changes in fuel economy when used with the final drive ratio of a 750 when crusing at 60-70mph. I'm not an engineer, so I'd like your thoughts on this. I'm running a 32/10 final drive mated to a 4-speed transmission. Also, I assume that running 1000cc, though it's claimed is a bolt-on set, would necessitate different jetting?

Can someone experienced with this "upgrade" provide some insight? Has the Seibenrock kit proven to be as long-lasting as the stock versions? Have you noticed a change in RPM, operating temperature, or any other symptoms which have lead you to regret the change?

The reason I'm contemplating this is because the bike I'm restoring suffers from pitting in the cylinder walls which require boring to 2nd over-size or more. Factor in the cost of machining and new pistons and it comes to about the same as the 1000cc kit. Plus, I'm having trouble finding a machine shop in my area that I trust to do the job - I live in muscle-car/lifted pick-up country, where close enough is considered good enough.

Thanks in advance,

Winnipeg, MB

Posted : 09/07/2016 14:41
Richard W
Posts: 2549

Also consider that...
β–Ί The kit only includes pistons and cylinders. The valves and carbs will remain at the smaller size (smaller than an R100) which will inherently restrict the breathing. This is OK for regular touring, but may be too restrictive if you have in mind some type of faster sport touring.

β–Ί R75's run 2 quart sumps, while most R100's run 3 quarts with oil cooler. Obviously the 1000cc engine likes more oil. The least expensive Mac Accessories large sump is $130 delivered. So the kit is not the only upgrade cost you need to consider.

Just my 2 cents. πŸ™‚

PS. I can bore the cylinders for you at the shop and they'll be right. Back in the Butler & Smith days we were a BMW shop. We're in the Atlanta area.

Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.

Posted : 09/07/2016 23:24
Marcus Priddy
Posts: 23
Eminent Member

Doesn't the Siebenrock kit expect a 99mm engine block bore intended for last year /6's though 1980?
Don't /5 bikes have a 96mm engine block bore (where the jug fits in)?
Perhaps the jugs from the kit could be ground down to fit 96mm?
Sounds like hiring Wobbly is the simplest solution! B)
good luck,

Posted : 09/09/2016 14:41
Jeff Benedict
Posts: 3
New Member

Siebenrock does have big bore kits for the earlier models with the smaller holes in the case. Motored-Israel lists them as does Siebenrock.

Posted : 09/20/2016 00:22
David Harding
Posts: 2
New Member

Im running a 1984 stock R80RT, considering the Siebenrock upgrade kit currently, and also have a followup request. Any updates or news, how did it work out?

For the R80RT. β€” are people changing valve sizes with this kit, upgrading to dual plugs, or what about going to the 32/11 ratio ? I have a chance to buy a final drive off ebay at that ratio and dont know if Im missing something or not here.
Plan was to go stock carbs, add SR kit, up exhaust valve to 40 mm , nothing else β€”-thoughts?

Posted : 06/01/2020 14:05
Richard W
Posts: 2549

Any updates or news, how did it work out?

I've contributed all I know, so I'm staying out and listening.
Just ducked in to say Welcome Aboard !

Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.

Posted : 06/01/2020 16:05
Gregory Zeronian
Posts: 5
Active Member

Send your cylinders and pistons to Millenium. They will Nikasil plate then grind to match the pistons. Better than new.

i just recently had them rebuild my R90/6 cylinders. Work was excellent. Cost is $600-700 ymmv.

They'll never wear out again.

You'll need moly rings, not chrome


Posted : 09/13/2020 07:29


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