Siebenrock R80 1000cc Power Kit with R100 heads

[sidkn33]

Another question about head/cylinder combination.
So, the issue is that I have an R80GSPD on which I fitted a Siebenrock R80 1000cc Power Kit last year.
All good - it runs lovely and there was a definite, if not massive, power boost.

However, I have now realised (doh) that the cylinders I removed from the R80GSPD, despite being stamped "80" outside are in fact 100 inside - with 93.97 stamped on the pistons.
I am informed that BMW used R80 heads, rebored to 1000cc on some later models, hence the mismatch (and 32mm carbs with 100 cylinders).
No matter: The question I have is that assuming whoever installed the 100s on my 80, also changed over to 100 heads, I have inadvertently mixed 100 heads with a Siebenrock R80 1000cc kit.
The Power Kit, is designed to work with the original 80 heads - so am I actually downgrading performance (e.g. reduced compression) because of the mismatch?
Or causing other issues I am unaware of?

I emailed Siebenrock on Monday but no response as yet.

As I said, the bike runs well but I'd like to have an insight from those more informed....

Thanks in advance.

 

[mikeyboy]

Have you got a photo of the seibenrock piston crown?
If it’s a 800 to 1000 kit ,It should have a step machined to allow clearance in the combustion chamber.
If it’s a 1000 to 1000 upgrade there will be no machined step on the crown,
In which case the 80 heads should have had the combustion chamber machined out from 88 mm to 94mm
Hope this makes sense

 

[sidkn33]

Thanks @mikeyboy for the ongoing assistance.
Yes, that makes sense.
The kit is 100% an R80 to 1000cc, I ordered and fit it myself.

Power Kit 1000cc[br]Plug & Play

The ultimate 1000cc conversion kit for all BMW R80 models from 1981 onwards with TÜV approval and installation instructions. Increase in torque and…

www.siebenrock.com

This is the stock photo from Siebenrock


So the pistons are designed to work with the 80 heads, which is why I'm concerned about possibly having 100 heads on it.
I'd like to get a view on this before I resort to taking the heads off to check

 

[mikeyboy]

So, my view on this,
I prefer to fit a 1000 to 1000 kit ,then machine the squish clearance on the heads.
This gives a better profile to the combustion chamber and eliminates the chances of “ cold spots” and carbon build up.
Sure the 800 to 1000 kit works ok ,but I feel that it’s a bit of a compromise .

 

[sidkn33]

Understood, sort of!
Why am I confused at the idea that you prefer the 100 heads be enlarged with the Siebenrock 1000 pistons, when the larger heads are already, larger - for BMW 1000cc pistons.
Is it that I am thinking enlarged, when you mean something else by "machine the squish clearance on the heads"?

 

[sidkn33]

Had a reply from Siebenrock

"Hello Sid,
if you have original BMW cylinder heads 1000cc on your bike, then please use our Replacement Kit, art.no. 1100100 or 1100103. These hits gor perfect with the cylinder heads of R 100.
If you will use your Power kit 1000cc upgrade kit for R 80 modes, you have to have cylinder heads of R 80 models."

And the kit in question for R100 is this one - the pistons are quite clearly different to those in the R80 Power Kit = no recess on the top rim.
So Siebenrock's position is that I need to have R80 heads with their R80 1000cc Power Kit.

 

[mikeyboy]

I fit a 1000 to 1000 kit to 800cc engines,
Then machine the 800 heads to suit.
That way,the pistons have no machined “ step”

 

[sidkn33]

I fit a 1000 to 1000 kit to 800cc engines,
Then machine the 800 heads to suit.
That way,the pistons have no machined “ step”

Got it. Thanks for clearing that up.

 

[sidkn33]

In my case, I already have the R80 1000 kit, so should combine with 80 heads.

 

[sidkn33]

From Siebenrock re combining 100 heads with R80 1000cc Power Kit

"No damage!
You reduce your performance, less compression, less power, less torque, no fun."

I have confirmed that the heads on my R80GSPD are 100s with larger exhaust valves - to match the uprated R100 BMW cylinders as outlined in my OP.

 

[Mad Hatter]

On the plus side.....I doubt you'll get any pinking with the big heads and the siebenrock kit designed for small heads

 

[sidkn33]

Well

With the R80 heads, the R80 1000cc kit definitely feels different. More torque for sure, maybe a little less free top end, but this is completely unscientific as I have zero data.

Now think I have to look at possibility of running rich, given the amount of carbon on the Siebenrock piston tops after about 3k miles.