Little advice re:r100 cylinders

[ME 109]

I just had a little lookie around the Motoren Israel site.
Drool. All down my chin.

[lrz]

So, I had posted a reply yesterday which seems to have vanished?

Just to get back on track-

What I want to do is plug & play, no mods,no performance upgrade- just fixing some problems.

I don't think post '81 barrels fit the '77 case and that seems to be confirmed here.
If they did, I'd try to source a jug & piston combo.

I also note that the '77 wristpin is held w/ spring clips not circlips, I believe this changed in '78? I'd think, therefore, the piston differs as well?

So, in short, other than '77, is there a barrel which is direct fit?

& cutting to the chase, anyone have a pair?

[Garnet]

I just had a little lookie around the Motoren Israel site.
Drool. All down my chin.

Come on mate, it's only money. From what I read over on another thread, it's imortant that you Ausi's don't have too much money as you head into retirement. The guy's at MI are just trying to help you live a better life in your advancing years.

[George Ryals]

You are right Garnet, an older fellow in our archery shop heard another customer(a friend of his) lamenting the cost of a new sight for his target bow. The older fellow told him, "You can have a stack of $100 bills a foot high and it ain't worth a damn unless you spend it".

[the quinner]

So, I had posted a reply yesterday which seems to have vanished?

Just to get back on track-

What I want to do is plug & play, no mods,no performance upgrade- just fixing some problems.

I don't think post '81 barrels fit the '77 case and that seems to be confirmed here.
If they did, I'd try to source a jug & piston combo.

I also note that the '77 wristpin is held w/ spring clips not circlips, I believe this changed in '78? I'd think, therefore, the piston differs as well?

So, in short, other than '77, is there a barrel which is direct fit?

& cutting to the chase, anyone have a pair?

If you have pistons with the internal clips on your R100, chances are pretty good that they are the original (i.e., non-oversized) pistons. The different clips won't matter for your selection of barrels...

You should be fine with any 1977 to 1979 cylinder...many parts sources (including BMW) will give incorrect years as to when the large O-ring started to be used. I have worked on many early R100s and have yet to come across the large O-ring on a 1979...both of my late 78s don't have them. I'm not sure about 1980.

Check your email...I have some cylinders that should work.

[Major Softie]

How come they fit R75/6, R75/7, but not /5? What change causes the difference?

Bikes before 9/1975 have 97mm instead of 99mm cut outs in the block. But all is not lost, you order this kit.....http://www.motoren-israel.com/product_i ... -1975.html
That's what I needed. Very happy with the results.

Ah, I misread. I just glanced past this line "all from year 9/1975 on, engine block drilled out to 99 mm," and thought they were saying you needed to drill the block out to 99mm. If you had to drill out the block anyway, I couldn't see why they wouldn't fit a /5.

Now I get it.

[vanzen]

Although many BMW piston specifications are similar
the notable difference of concern will be the specific design and shape of the crown of the piston.
That particular shape of the piston crown is matched to it's particular head / combustion chamber shape.
Several sets of crown and combustion chamber shapes were used throughout the years of production.
Even as these different "series" of shapes varied only slightly,
and then only at the perimeter, or "squish band" (to promote turbulence and complete combustion) -
Performance will suffer if pistons and heads are mismatched "out of series" unless modified to work together.

John Falconer's excellent page explains the differences in detail.

[vanzen]

Sounds like a can of worms. Maybe this is a better route if you want Nikasil on an older block. http://www.motoren-israel.com/product_i ... -1980.html

These words from the MI page:
"An modification at the cylinder foot makes this use possible."
The ambiguity of the translation leads me to question
whether the MI Nik-kit ALSO requires a chamfered engine-case spigot ?
Or has MI machined the cylinder itself to fit the non-chamfered spigot ?
I'll bet on the latter, and suggest that this machining of the cylinders
would be a 2nd option in order to fit Nik-cyls to a pre-Nik case.
(i.e. engine still in MC, cylinders on the bench)

[the quinner]

Although many BMW piston specifications are similar
the notable difference of concern will be the specific design and shape of the crown of the piston.
That particular shape of the piston crown is matched to it's particular head / combustion chamber shape.
Several sets of crown and combustion chamber shapes were used throughout the years of production.
Even as these different "series" of shapes varied only slightly,
and then only at the perimeter, or "squish band" (to promote turbulence and complete combustion) -
Performance will suffer if pistons and heads are mismatched "out of series" unless modified to work together.

John Falconer's excellent page explains the differences in detail.

In theory this is correct...in practice, BMW violated these principles.

During the 1978 model year, the pistons were changed from the perimeter squish band type to the fully domed (roughly, but not exactly corresponding to the internal/external wrist pin clip change). It wasn't until sometime during the 1979 model year that the heads were changed.

Both of my 78s (one built in april 78, the other may 78...very late for the 1978 model year production) came from the factory with fully domed pistons and "perimeter squish band" heads...giving a "reverse" squish band. Interestingly, the few 1979s I've had over the years (with matching domed heads/pistons) were FAR more prone to detonation than the 78s I've owned...

My theory is that the squish band is wide enough on the mis-matched stuff that it doesn't matter...the fuel charge is theoretically being squeezed "out" towards the cylinder wall. I suggest that an airhead's combustion chamber is so bad to start with...that any turbulence, whether squeezing in or out, is beneficial. It seems that the profile of the cylinder head seems to have more impact on real-world performance than whether or not the head/piston profiles match. Too much focus is on the very moment when the piston is at TDC...I believe that the "sharper" edge of the earlier heads creates more turbulence while the piston is moving from BDC to TDC than the later heads.

[Garnet]

"Too much focus is on the very moment when the piston is at TDC...I believe that the "sharper" edge of the earlier heads creates more turbulence while the piston is moving from BDC to TDC than the later heads."




Your theory may be enough to get me off my lazy butt and try something.

I have a pair on small spigot cylinders that where bored out to take std R100 pistons. I also have a pair of fully domed R100 (78) 9:1 pistons for them. I have never used them as they where intended for my /5 and I thought that I should machine the heads to match the pistons or machine the pistons to match the heads to make an effieciant combustion chamber. I also considered getting some R100 heads to match the pistons.

By the time the dust would settle I would probably have enough $$ into the experiment to make a set of MI cylinders look cheap, and we know they do work.

But from what you say I may be overthinking the head part. Maybe I'll slap together what I have, tripple check the clearnaces and see how it works.