Skrunkwerks/Siebenrock 1000cc Powerkits for R75’s, R80’s and R90′


Skrunkwerks have partnered with Siebenrock to create power upgrade options that are more efficient than the standard, purely bolt-on kits that Siebenrock traditionally supply.

‘There’s no replacement for displacement’ as the saying goes. Siebenrock have been delivering high quality 1000cc upgrade kits for many years and cover just about every airhead model there is. The secret behind their popularity is the fact that they are truely bolt-on. However, there is a compromise in making this so: While the Powerkit cylinders are true 1000cc items, the pistons are not. In order to fit the larger 94 bore diameter of the 1000cc cylinders to the smaller combustion chamber of your R75, R90 or R80 heads, the Siebenrock pistons have been designed with a step to prevent collision with the heads.

This system works, with many happy Siebenrock customers content to leave as is, but for those willing to go some extra steps, there are additional efficiencies to be had.

The stepped piston that comes with the standard kit upsets the all-important ‘squish’ band of the combustion chamber resulting in reduced combustion efficiency and power than a true R100 setup. The problem is illustrated in the following diagram.

Figure 1 shows the flat squish band design of R75’s, R80 and R90’s (and the first R100 models up to 1979) and figure 3. shows the later angled squish design that was used from 1979 in R100’s. In both cases, you can see how there is a clear path for mixture to be ‘squished’ to the center of the combustion chamber creating the turbulence necessary for a more complete combustion process. The later angled squish band does a better job at this than the earlier flat squish band.

Contrast this with figure 2. The step in the piston interrupts the path for fuel mixture to efficiently make its way to where it can more completely ignite and make more power. Importantly, the reported tendency for the stepped arrangement to promote detonation or ‘pinging’ (particularly when compression ratio is pushed) is effectively dialed out.

Skrunkwerks overcomes this limitation by machining your flat squish band R75, R80 and R90 heads to replicate the angled squish band and 94mm diameter combustion chamber of later heads per figure 3.
We have arranged with Siebenrock to supply kits with non-stepped pistons to suit your modified heads and 1000cc cylinders to suit earlier engine cases creating a true R100 configuration that is in fact more efficient than the stock R100 due to the use of 9.5 CR pistons. Throw in a higher lift Skrunkwerks dual pattern performance cam regrind and you won’t recognise your bike!

The package includes:

  • You send us your R75, R90 or R80 heads, we machine them to later R100 specifications with angled squishband accurately matched to Siebenrock 1000cc pistons. Includes cleaning, bead blasting and checking the heads.
  • Flycutting head mating surfaces to optimise squish band thickness and ensure a true compression ratio of 9.5-10:1 with Siebenrock pistons.
  • Special Siebenrock 1000cc power kit to suit the above including 2 x Nikasil 1000cc cylinders with stainless steel pushrod tubes, forged 150g lighter than stock high compression pistons, pins and rings.
  • The stock airhead ports are rather average, sometimes with pretty dodgy machining. A 1st stage porting job is part of the package.
Price : A$2,550 (Plus GST)

Additional costs apply if we dissassmble and re-assemble your top end.

With the compression ratio increase of this package, dual plugging is recommended. Go here for details

Also talk to us about having your cam reground to the  Skrunkwerks performance street profile while we’re at it..

Contact here

A pair of R90 heads. The left one is stock with flat squish band. Its sibling on the right has been machined out to 94mm diameter with sloping squish band to suit the high compression Siebenrock piston – a true 1000cc upgrade!

On the left is the Siebenrock stepped piston as supplied with the standard Siebenrock ‘bolt-on power upgrade kits. Machining R75, R90 and R80 heads allow the use of  the high compression piston on the right resulting in a true 1000cc upgrade.

Here’s a few pics of the kits:

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