Later R100 32mm carbs cylinder head vs R80

[sidkn33]

Can anyone confirm if the later R100 (exclusively monolever?) cylinder heads with smaller 32mm carb fittings are exactly the same as the R80 heads or are the valves/blocks modified for the larger pistons?
I am fitting R100 cylinders on my R80 and want to know if these smaller carb R100 heads are a sensible addition.
Noting that the factory R100 cylinders I have are in a block stamped "80" which I am told (by someone who built and raced custom Airheads) is because BMW rebored the R80 cylinders to 1000cc for some later R100s.

 

[mikeyboy]

The valves on the 100 are 42 mm inlet ,40 mm exhaust.
The valves on the 80g/s are 42 mm inlet ,38 exhaust.
Only other difference between the 80 heads and the 100 will be the squish clearance machining.
So, if you were to use the 80 heads with 100 cylinders you would need to machine the heads to accept the bigger piston crown.
We do it regularly.

 

[sidkn33]

Thanks - much appreciated.
I have read that R80 heads can be used safely without modification with the R100 cylinders, with a loss of top speed (power) but, possibly with some gain in torque - is that just hogwash?!
Would I actually cause harm by using the R80 heads with the R100 pistons?

I've now found this post here on UKGSER for example:

Thread 'R80 heads on R100 jugs & pistons'

Mar 11, 2015

I have acquired a nice set of R100 cylinders and pistons, vapour blasted with new rings and pushrod tubes installed.

I am going to fit them on to my R80RT. My bike is a 1986 and the new jugs are off an '88 so shouldnt have any issues with o-ring and casing differences. I have read that ideally the R80 heads should be machined to accommodate differences in the R100 pistons to optimise squish bands and compression, but I have also heard of people running them stock without any problems. I understand that there may be small areas in the chamber that suffer increased carbon buildup and a...

• gog
• Replies: 6
• Forum: Airheads

I recall that the guy who supplied my current R80 heads with new valves for unleaded fuel, also said they could be used with R100 cylinders without actually breaking anything.

 

[mikeyboy]

I doubt you’d have any power loss at all as the limiting factor is going to be with the carbs and inlet valves,
You’d definitely see an increase in torque though.
I’ve never considered or tried fitting unmodified 80 heads onto 100 barrels so can’t really comment on that ,
But why would you?
it’s not much of a job to machine the heads once they are set up on the lathe.
If you are going to the trouble of doing the job you might as well do it properly

 

[sidkn33]

The job isn't troublesome and if there's a torque vs top end equation, then I'd prefer more torque - so in that sense "properly" becomes a preference, rather than a choice.
That is, if we mean that the (reduced?) squish from the R80 heads, creates more torque?
I actually have a set of 32mm carb R100 heads arriving tomorrow as an option - so I can go with the larger valves without going near a lathe.
I'm just interested to know if damage could occur in not using the larger (proper) heads.

 

[mikeyboy]

You’ll be good to go then,

 

[sidkn33]

I doubt you’d have any power loss at all as the limiting factor is going to be with the carbs and inlet valves,
You’d definitely see an increase in torque though.
I’ve never considered or tried fitting unmodified 80 heads onto 100 barrels so can’t really comment on that ,
But why would you?
it’s not much of a job to machine the heads once they are set up on the lathe.
If you are going to the trouble of doing the job you might as well do it properly

It's likely that I am misunderstanding, is the torque improvement you mentioned due to the larger exhaust valve on the 100 heads?

 

[mikeyboy]

The torque increase will come from the larger engine capacity,
I doubt the bigger exhaust valves on the 100 heads will make much difference with a stock g/s exhaust

 

[sidkn33]

I did a bit more research and found this on Boxerworks

https://forum.boxerworks.com/viewtopic.php?t=125

The conclusion appears to be that unmodified R80 heads on R100 cylinders could/should/would work, but could/should/would risk damage, sooner, or later.
The OP on Boxerworks did not proceed.

As I mentioned, I have a pair of R100 heads so will use those.
Thanks for the advice.