1978 R100/7 Topend Overhaul

[20774]

Here is a composite shot of the interior of one of the pistons. The stamp "1275" probably indicates that it was cast in December 1975. The other marks are the Kolben-Schmidt logo, what looks like a 19 with a 7 underneath it turned sideways, the number "094022" which is probably the mold number, and a "2" and a "7+" underneath the mold number. Not sure what these last two numbers might mean. Maybe the "7+" has something to do with the weight or weight group.

[20774]

Heads are back! They sure look purty! Pictures later.

I also got my cylinders professionally measured and the readings aren't good. The measurements at the base (unworn part of the bore) are basically 94mm and the shapes of each cylinder are bell-mouthed from there. The left cylinder ends up being 94.06mm at TDC in one direction and 94.09mm in the other. For the right cylinder, the dimensions at the top are 94.09 in both directions. The machinist indicated that it was pretty much an even taper from bottom to top.

So, I have to decide what to do. I doubt I'll bore to next oversize (I need to discuss with Ted Porter) due to stability issues that I've read about. So, that likely means going to new cylinders and pistons. Which got me to thinking...do new pistons come with new wrist pins? And what about the small end rod bushing? Doesn't that have to be fit to the new wrist pin? Or do I have to have the old bushings pushed out and new ones inserted? The machinist mentioned that bigger engines usually have to be drilled out a pretty good size, but if the bushing only needs say a small amount (maybe 0.003"), then that is honed by hand. Getting it squared to the pin would be critical.

These are aspects of this that I hadn't heard mentioned before, and hadn't considered. I quickly getting in over my head!

[BC1100S]

So, I have to decide what to do. I doubt I'll bore to next oversize (I need to discuss with Ted Porter) due to stability issues that I've read about. So, that likely means going to new cylinders and pistons. Which got me to thinking...do new pistons come with new wrist pins? And what about the small end rod bushing? Doesn't that have to be fit to the new wrist pin? Or do I have to have the old bushings pushed out and new ones inserted? The machinist mentioned that bigger engines usually have to be drilled out a pretty good size, but if the bushing only needs say a small amount (maybe 0.003"), then that is honed by hand. Getting it squared to the pin would be critical.

This oughta help. Not hard to do either.

Under "engine parts", click on "power upgrade kits". Everything you need is included.

http://www.motobins.co.uk/bmw-parts....20valve%20Twin

[20774]

Thanks...I know I can get parts...I'm leaning towards just going down to my dealer. I wouldn't want to get high compression than I have (probably 9.0:1)...hadn't really considered trying lightening, either. But these details about new wrist pins, honing small end bushings, etc. I'll have to ask some more questions about what all is done and how to get it done. I wished I lived closer to one of the "gurus"!!

[Omega Man]

Heads are back! They sure look purty! Pictures later.

I also got my cylinders professionally measured and the readings aren't good. The measurements at the base (unworn part of the bore) are basically 94mm and the shapes of each cylinder are bell-mouthed from there. The left cylinder ends up being 94.06mm at TDC in one direction and 94.09mm in the other. For the right cylinder, the dimensions at the top are 94.09 in both directions. The machinist indicated that it was pretty much an even taper from bottom to top.

For what you are working on Kurt, the results don't sound horrible (wear wise) except in the expense dept. It's funny on that kind of wear, on small engines, where the sheet metal shielding has been removed, the cylinder out of round is frequently visible.
I'm interested in seeing the pictures. Gary

[20774]

For what you are working on Kurt, the results don't sound horrible (wear wise) except in the expense dept.

Gary -

The standard size for "B" cylinders is 94.015 to 94.025. I have a dimension that is 94.09! That's even bigger than "C" cylinders. So, I can't just put pistons in there and new rings...things would be pretty sloppy. At this point, it would have to go up by 0.5mm, the first over. Or buy new.

[Omega Man]

Yeah Kurt, I understand the wear but it was still running although not at tip-top. I always find it amazing what the older engines will put up with. I still have some older diesel engines that don't need any power to run..and run....and run. and I know the wear is substantial.

[140008]

I just priced those parts at Moto Bins,ted Porter has them too,and maybe the same price or a little less after ship. Go for the Up Grade,dual plug the heads,then you can handle the low octane fuels.Ted does this cheap(dual plugging,just has spare heads done)and run the 9.5 pistons in the new cyls. In for a dollar in for a dime.

This oughta help. Not hard to do either.

Under "engine parts", click on "power upgrade kits". Everything you need is included.

http://www.motobins.co.uk/bmw-parts....20valve%20Twin

[20774]

In for a dollar in for a dime.

At this rate, the dollar is turning into three dollars! It looks like I'm going to be spending as much on this rebuild as the bike is worth.

Sure, I guess dual plugging is an option, but it certainly adds more complexity which I'm not in favor of. The other thing I see on the MotoBins (and Ted's) kit is the internal snap rings for the pistons. I hate those with a passion. I struggled quite a bit when I did this on my R69S...I was very pleased to see external snap rings on my /7.

[DennisDarrow]

Kurt, folks sure do spend your money for you don't they. I agree with Gary about the "slop" that is acceptable in the older style motors. I just got my 79 Troybilt tiller with a B&S 8HP original engine replaced with a "modern" engine. The old engine had umpteen million hours on it and was drinking oil every couple of hours. No compression felt with one's thumb over the plug hole or when pulling it through.......BUT.....It ran like that for years putting in garden after garden until, finally every time I used it I had to clean the plug because of fouling............Anyway....what I am trying to say is that these old engines will go and go and yes replace what ONE HAS TO replace to avoid something catastrophic like a popped valve...........A total rebuild????..........Who knows..........Yes, one could replace wristpins if a bit of slop is there and golly go with the light ones, or those special rod bearings will be great, and how about lighter rods???????

On and on.........For me, folks spend my money really easily; but I do have to listen to GOOD common sense which is usually a combination of advice and knowledge from folks who REALLY know.....HERE????.........Have you talked with as many of the old wrenches as possible on the phone?????........Not just guys pushing parts but wrenches??????

Anyway.........Glad you got your heads back and are on the way to getting ready for just around the corner..........Golly, in SA it's already that time.......God bless........Good luck.......Dennis

[20774]

I've had an exchange with Ted Porter about the Siebenrock kits. They are ready to install - cylinders honed, pushrod tubes installed - and have lighter wrist pins and pistons. I'm not a fan of the 9.5:1 compression but he indicates they can be "detuned" with a base gasket to get them back to 8.8:1. I also don't like the internal snap rings, but for the price savings on these versus stock parts, it's a very strong contender for me.

I'm normally a person who looks first to BMW for critical parts and I would consider the top end parts in that category. I'm sure I could save some if I were to go the used route, but there's that uncertainly about how "used" they are. So I want new with the confidence going forward. From what I've been hearing from others, these aftermarket kits really sound like a good way to go.

I'll probably swing by BMW tomorrow to do some pricing so I really know what I'm working with. I've heard that BMW replacement parts that I need will be close to $1700. The Siebenrock kits are nearer to $1000-1100.

[20774]

Picture of left head...very nice!

[20774]

Here are the old valves. Can you pick out the exhaust valves? It doesn't look there much of a change to the face of the exhaust valve...it still appears to be a flat surface, not dished. The exhaust seats were removed by welding a ring around the inside...I suspect the seat just fell out. The intake seat was reused, just recut to match the new valves.

[Omega Man]

Geeeze Kurt, none of those valves are even bent I did my first valve job when I was 14...the cause...real mean down-shifts on the XL100 Honda. I did it with an issue of mini-bike magazine (article on a big-bore kit on a mini-trail) and some guidance from my step-father. I was sorry I killed a Honda..but I brought it back to life. You'll be fine and the bike will be a winner
Gary

[140008]

Why not just put some new rings in and break them in.I think you mentioned that it was running fine.This would be a low cost alternitive and you could run a compression check.All could be well and if not happy then bust for one of the cylinders or ...