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SteveH
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Im having a look at the heads of my R100R 1992 and was wondering if there was a measurement that i can check to determine if they have or have not been machined to increase compression ?
Does any body know a dimension that i can check ?

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Steve H

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Hello Steve, do you want to

Hello Steve,

do you want to check the compression ratio?

regards

dieter

SteveH
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R100 Heads

I am fitting a replacement Siebenbrock piston and barrel kit , it has 9.5:1 compression ratio with standard heads which is what i want.
When i stripped the engine down it had been fitted with Venolia pistons so i was wondering if the heads had been modified in any way .
I wouldn't want a higher compression ratio then 9.5:1

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Steve H

schrader7032
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San Antonio, TX
Joined: 10/27/2006
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I went the Siebenrock kit for

I went the Siebenrock kit for my '78 R100/7. I felt the 9.5:1 was too high, as the bike was 9.0:1 to start with and I was comfortable with that. I opted to insert a shim (I believe it 0.67mm thick) which dropped the ratio back to 8.8:1. Essentially the same as stock and I don't have to consider dual plugging in order to control the detonation. It might have been OK, but I chose not to take the chance.

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

SteveH
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R100 Heads

My heads are dual plugged so i think ill be ok with the 9.5:1 compression ratio.
Did you have any problems fitting the Siebenrock kit ?
Only thing i am nervous about is fitting the rings onto the pistons ?

Steve

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Steve H

schrader7032
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Steve - No real problems

Steve -

No real problems fitting the kit. The only thing that was hard was installing the clips that hold the wrist pins. On the /7, the clips are external and are easily removed/installed with a circlip pliers. The clips on the Siebenrock kit are internal and must be compressed into the wrist pin hole, then slid down the hole until it locks into the internal groove. I've done that twice now on my R69S and the /7 and I find it to not be an easy task. Lots of thumb pressure to get the clip to fit inside the groove...I think I still have the impressions in my thumbs to this day! Big Grin Other than that, it went fine.

The rings weren't too bad. I ended up using a very large radiator hose clamp to keep the rings in their grooves while I worked the cylinder into the engine case. I used a manual written by Oak Okleshen to guide me through a top end replacement. I don't do this routinely and was glad I had his writings to keep me from making a mistake.

All of the parts were lighter than the stock parts so that is good. And since the cylinders are all aluminum with the Nikasil-like coating, they dissipate heat better.

The key thing with a new top end will be break-in.

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Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

SteveH
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Clips

I thought those small clips may be a problem , when i took the old ones out .
The ring assembly tool i had in mind was as attachment.
Ill have a look and try to get the manual by Oak Okleshen .
On removal of the heads and valves everything looked ok except the exhaust valve was a bit gummed up and leaked when i put a drop of petrol in the combustion chamber before removal of the valves. So i have just cleaned the valves up and given them a light "lap" into the seats

Thanks for your previous post

Steve

  • piston_ring_assembly_pliers.jpg
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Steve H

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hello steve, 9,5 :1 is a very

hello steve,

9,5 :1 is a very high comression ratio with an aircooled cylinder head, i would go the same way to kurts recommendation to 8,8:1 .. 9:1 which will give you a lower piston dome temperature. engine pinging reduces the life of the pistons dramatically. you should use then a 2nd spark head.

bmw has made a lot of attempts during the series production to get a perfect compression ratio to the final version of the R100/7 and R80 build from 81 to 84.

an interresting page here in germany (unfortanetly no english version available), showing some tuning troubles you`ll find here

http://www.zeebulon.de/Mot/bmw_allg.htm#Wiseco

if the piston is made for cast iron cylinders, you can not always use them with nikasil coated cylinder surfaces and back.

regards

dieter

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hello kurt yes i`ve had

hello kurt

yes i`ve had trouble with fitting the 850ccm kit in my r65.

the pushrods had visible drag marks after the first test run.

reducing the outer diamenter from 10mm down to 9,7 - 9,6 mm was the only possiblity to avoid friction at the inner diameter of the pushrod pipes.
now the engine runs fine. compression ratio is now 8,5:1 (R65 was 9,5:1 and my oil temperature was always about 100°C), the increase of the torque is more important to me, because i have build in a 37/11 rear drive from an R80/7 and i`m using the odometer of the R80st.

another ugly thing was the assebling of the C-rings of the piston pin.
to avoid that this ......-ring does not always bounces up and away, i`ve wrapped an oily lug around the piston rods to close the engine cylinder bore and i manufactured a special plastic tool to protect the aluminum -5mm pin with a 1,6mm recess for fixing one side of the c-ring - to shift the other side of the c-ring into the piston pin bore.

regards

dieter

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