R100 cylinders on an R80/7 - ?

[tsa]

Whilst taking off the barrels to deal with a PRT seal oil leak, I thought I'd try out a set of R100 cylinders on my '78 R80/7. I assume the secondhand R100 cylinders (the old iron lined version, without the large O-ring groove) that I found have 1:9.5 pistons fitted, since they have domed piston tops, and use internal wire wristpin clips (which apparently only were used for the '77-'78 R100's).

Apart from the very fiddly task to get the wristpin clip in place, fitting the "new" cylinders & pistons (with R100 head gaskets) was fairly straightforward. However, when I had torqued down the cylinder heads, it was very difficult to turn the engine over. After I Ioosened off the cylinder head torque, I could turn over the engine - but felt a distinct hesitation or nudge just before TDC, when the intake valve opens. Which I interpreted as gentle piston to valve contact - which presumably isn't likely to get any better in a running engine. So the R100 barrels came off, and the R80 cylinders back on again.

When looking at the R100 pistons afterwards, I noticed that although they seemed fairly clean, there still was some deposits in the valve pockets that hadn't been cleaned out. The R80 pistons, heads and valves also had a fair amount of carbon deposit. So I'm wondering if a proper cleaning of the R100 pistons and R80 heads/valves would be all that is needed for them to go together with sufficient running clearance? Or, is the combination of high CR standard R100 pistons and R80/7 heads that I've got unlikely to be able to work together?

[Kurt in S.A.]

The '78 R100/7 was essentially 9.2:1...don't think it was as high as 9.5:1. The '70s era R80/7 was also the same compression ratio...I think with the '81-on bikes, it dropped down to 8.2:1. If you dealing with parts from the same time period, it should be a direct drop in.

Kurt in S.A.

[Roy Gavin]

There was a post a long time ago on the old Motobins forum about this, and they stated that you want the low compression 1000 cc pistons with the 800 heads.

Remember that just the capacity change would give a decent sort of increase in CR, probably too much, if the dome was the same on both pistons, so you probably want the 1000 cc pistons to be a bit lower.

It might be possible to fit compression plates at the bottom of the barrels, some bikes come that way from the factory, but I have had never tried it, and I would want to know if the piston is hitting the valves or the head before I tried.

[tsa]

Thanks for your comments.

I think this description is typical for this conversion
http://www.realclassic.co.uk/bmw_r80_r1 ... rsion.html
in that
a) little information is available on which pistons actually have been used, and
b) it seems the conversion works most of the time - ?

Unfortunately, little information seems to be available about how to identify different R100 pistons. As I have understood, the post-81 basic R100 CR 8.2 pistons were the only flat-top pistons. Before that, the early RS/RT/S CR 9.5, and the R100/7 CR 9.1 pistons both have been described as having domed tops. As also the post-85 standard CR 8.5 pistons. However, I've not seen any consistent info wrt which pistons had squish bands. Any info on cast part numbers etc on these pistons would be useful to know about.

I'll try and clean up my R100 pistons & R80 heads to see if this may provide the necessary clearance. I will also check out the use of cylinder base CR lowering gaskets - 0.75 mm ones are listed by my local suppliers (however without any mention of the resulting CR ratios). If they should lower the CR from 9.5 to 8.9 - 9.1, no harm will be done, considering what fuel is most commonly available these days.

[Kurt in S.A.]

John Falconer, sometimes a poster here, used to have a web page showing piston shapes. But after a divorce and issues with his server farm, I'm not sure the links or pictures have made it back to the web. You might search this forum for his posts.

Another place to check is Snowbum's site...people cringe when reading his stuff, but there might be something in there to shed some light.

http://bmwmotorcycletech.info/pistons.htm

Kurt in S.A.

[StephenB]

Maybe this helps (comparing pics of 8.2 flat vs hi-comp dome pistons):

http://advrider.com/forums/showpost.php ... tcount=176

[tsa]

I've been back to the drawing board on this one. Searching the net for further info; a pity John Falconers site with R100 piston info disappeared. Thanks also for the link to pictures of R100 pistons on advrider - they are actually from Phil Hawksleys business site, he is probably more known for running the bmbikes forum http://www.bmbikes.org.uk/Forum/index.php and the associated info bank on BMWs http://www.bmbikes.co.uk/information.htm. The fact that he also listed a pair of R100 pistons with "not certain of CR" http://www.boxerman.co.uk/engine.htm seems to confirm that BMW has managed to keep lots of their customers in the dark wrt exact specifications!

I managed to get my hands on and compare a piston from an aftermarket piston & cylinder replacement kit, made to fit the pre-81 R80s, with one of my secondhand (and assumed '77 R100RS CR 9.5) pistons by sliding both onto the same wristpin. Picture here https://www.flickr.com/photos/55376586@N02/14413129732/ - it is clear that the aftermarket one has a machined "step" at the edge of the crown (2 mm wide, 1.5 mm deep), and that the BMW piston otherwise has roughly the same height above the wristpin at the periphery, as well as dome shape.

I also happened to see that I had another pair of different R80 cylinder heads https://www.flickr.com/photos/55376586@N02/14227821070/. The one of the left had a machined squish band width of just under 91 mm, whereas the one on the right measure nearly 95 mm. Look closely at the relative widths of the cylinder stud hole, the distance between that hole and the machined "piston recess", and that of the squish band of the two cyl. heads. Then the difference in diameter of the machined piston recesses should be obvuous. When searching the net for info, I've not come across info on these two different R80 cylinder head versions.

If I mention that the cylinder heads on my R80/7 are of the variety on the left, it would seem that the reason for my problems are piston to head contact. So a pair of matching cylinder heads may be what I need to get my conversion to work. Hopefully.

[Bamboo812]

TSA, if you want your pictures to show here, you have to click on them to "view image" at the hosting site so the code ends in ".jpg" and then copy and post it here using the "[img]".

[tsa]

TSA, if you want your pictures to show here, you have to click on them to "view image" at the hosting site so the code ends in ".jpg" and then copy and post it here using the "[img]".

Bamboo, I tried to, but flickr seems to have developed a life of its own since I signed up. I could not find any "view image" option when clicking on the pictures. I tried using the [img] tag, but it appears it didn't work since the link didn't end in .jpg. Suggestions of other imaging host sites are more than welcome!

[tsa]

EDIT, incl. deciphering flickr source-code, to get hold of links to my pictures. Sorry for being so slow to sort out this; I don't do html programming for a living, nor as a hobby!
Re-post with pictures follows:
_________________________________________________

I've been back to the drawing board on this one. Searching the net for further info; a pity John Falconers site with R100 piston info disappeared. Thanks also for the link to pictures of R100 pistons on advrider - they are actually from Phil Hawksleys business site, he is probably more known for running the bmbikes forum http://www.bmbikes.org.uk/Forum/index.php and the associated info bank on BMWs http://www.bmbikes.co.uk/information.htm. The fact that he also listed a pair of R100 pistons with "not certain of CR" http://www.boxerman.co.uk/engine.htm seems to confirm that BMW has managed to keep lots of their customers in the dark wrt exact specifications!

I managed to get my hands on and compare a piston from an aftermarket piston & cylinder replacement kit, made to fit the pre-81 R80s, with one of my secondhand (and assumed '77 R100RS CR 9.5) pistons by sliding both onto the same wristpin. Picture here


- it is clear that the aftermarket one has a machined "step" at the edge of the crown (2 mm wide, 1.5 mm deep), and that the BMW piston otherwise has roughly the same height above the wristpin at the periphery, as well as dome shape.

I also happened to see that I had another pair of different R80 cylinder heads
.

The one of the left had a machined squish band width of just under 91 mm, whereas that on the one on the right measures nearly 95 mm. If you look closely at the relative widths of the cylinder stud hole, the distance between that hole and the machined "piston recess", and that of the squish band of the two cyl. head, the difference in diameter of the machined piston recesses should be obvious. When searching the net for info, I've not come across info on these two different R80 cylinder head versions.

If I mention that the cylinder heads on my R80/7 are of the variety on the left, it would seem that the reason for my problems are piston to head contact. So a pair of matching cylinder heads may be what I need to get my conversion to work. Hopefully.