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Old 01-20-2011, 07:10 AM   #1
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Airhead shorter piston longer rod experiences?

Just curious if anyone has any experience with running longer rods and shorter pistons in an airhead. What are the possible advantages of running these more modern style pistons? I'm not planning on doing it, just curious.

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Old 01-20-2011, 07:35 AM   #2
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In 1987 Jim Roche ran a R100 with a long rod engine in Mexico to over 150mph. I write about him and the motorcycle he called White Dog on my blog here

Mr. Roche has some of his writing on BMW's here:
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Old 01-20-2011, 08:16 AM   #3
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The kit on the picture is a 1070-kit and on this kit you don't have the thin sleeve on the cylinder that goes into the engine-case. This means that the usable area of the cylinder gets shorter and you have to move the piston out from the engine.

With a modern piston you can keep the total length of the (longer) rod and (shorter) piston standard.
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Old 01-20-2011, 09:35 AM   #4
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What years are these kits available for?

My dream is to build a fast airhead.

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Old 01-20-2011, 09:42 AM   #5
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Originally Posted by crazydrummerdude View Post
What years are these kits available for?

My dream is to build a fast airhead.

Thats Siebenrocks' kit. With the rods and camshaft and the 1070's its about $2400.

Check out their website... its for all years past 1980 I think.

Love it.
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Old 01-20-2011, 10:12 AM   #6
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It seems to me that Siebenrock went to longer rods for the same reasons the Harley guys did: Because they had to for running out of room hogging 'em out! A rare few of the HD guys figured out that they were getting performance gains beyond what the added displacement should have got them and narrowed it down to the longer rod length ratio. It seems that Seibenrock doesn't touch on that aspect of their big bore kits much.

The problem with those big bore kits is that the cylinder studs are in the same place as ever. The cylinder wall along the length of the cylinder's stud hole is paper thin as is the gasket area between the stud and pushrod tube holes and the bore. Plus the entire cylinder wall is way too thin to boot. I have seen big bore cylinders distort like a corrugated pipe 100% in unison with the cylinder's cooling fins! Talk about rings that can't seat!! I have also seen them leak oil like a mofo for having almost no gasket sealing surface in places.

Smokey Yunick is the guy to really read up on RLR. That is who Dr. curve got the idea from I think. I have helped put longer RLR rods in airheads and oilhead race engines that already had all the early and big performance gains done to them. The engines were at the point where small gains were hard to come by. The substantially longer RLR's net instant and substantial gains on the dyno! They give airheads more torque from low mid to high RPM and that translates into HP anyway you twist the throttle on!

I would love to run a long rod engine if I had the chance BUT I am leaving my bore at 94mm and my displacement at 980cc. There are GOOD reasons why BMW stopped hogging out their 750 at 1000cc's!!!!

supershaft screwed with this post 01-20-2011 at 10:18 AM
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