Thoughts....

• If first oversize is 0.5mm, then that's about 0.020", or second oversize on most other engine brands. 0.020" will clean up everything if the guy doing the boring knows what's going on. At 0.5mm you shouldn't need to worry about any scars (ring lip and/or ovality) being left behind from the standard bore.

• You should give the machine shop the specs from the workshop manual AND the new pistons. Cylinders are bored to the piston size PLUS the OEM clearance.

• A lot depends on who is doing the boring... a true "machine shop" or a motorcycle shop. A true machine shop bores to size and may not hone. A engine shop will know to come just shy of the size because final honing will possibly remove as much as another thou, depending upon what type hone they use. You want your cylinders bored, but you also want your cylinders honed to final size.

• The grit used for honing is important for ring seating and engine life. Just like sandpaper, hones come in different grit sizes. If this work is being done by someone who is not an "Airhead guy" then you may need to specify a grit.

• DO NOT forget to set the ring end gap. Unless the shop specifically tells you that they gapped the rings, they didn't. 

• You can do a lot to help the bore job. Make sure the head gasket surface is absolutely clean and flat before delivering the parts. Make sure you remove any dowels, because they can easily get lost during the bore process.

Hope this helps

If you have the extra cash, I highly recommend that you have the cylinders Nikasil plated by Millenium Technologies. They will plate the cylinders back to factory specs and match your pistons at the proper clearance and then you run standard iron rings. This plating should be good for the life of the bike.

I had them do my 74 R90 a year back and couldn’t be happier.

Greg Z.

PO had installed a fairing on the bike. Then the bike got knocked over in the flood and tore up the fairing so he removed it but never restored the wiring to the original configuration. What I don't know...

1)How the wiring was changed to accomodate the fairing, obviously headlight and turns

2)What he had done to wiring after removing the fairing if anything

3)What he did to the harnesses... looks like the left handlebar harness to headlight bucket is "newer"

4)...is much about BMW wiring.

I am ok with wiring schematics but after spending a few hours with the Clymer diagrams I have a few questions about what I am working with in the headlight bucket. So we are on the same page, I will refer to the main harness as the one that leaves the  bucket and heads to the rear of the bike, the headlight harness as the short one from the left bar (headlight switch) to the bucket, and the ignition harness as the short one that runs from the right bar (starter switch) to the bucket, and the board as the circuit board in the recesses of the headlight bucket. I have several questions.

My turn relay should have brown-yellow/green-black/white-green/black wires. Instead of the green/black it has blue/black. Schematic says blue/black is for the right turn? Why is it green/black? Green/black is involved with the horn,speedo/volmeter (don't have),clock (don't have) and front and rear brake switches...confused.

Headlight harness looks "newer" and there are two wires green/black and grey coming from that harness into the headlight that connect to nothing. Could this be a replacement harness for a different bike and these wires green/black and grey have nothing to connect to?

I have a red wire and a brown (ground) wire coming from the board at the back of the light connected (severed) to nothing?

I have a black (with fitting) and a green/red (with fitting) not connected to anything that disappear into the main harness?

Nowhere in the bucket do I see the wires (yellow and white) for the headlamp. Ground could be the severed wire...or not. The headlight relay has all the correct wires attached...grey,brown,green/pink,red and yellow/white. Are the wires for the headlight supposed to come diectly from the board as a harness? And the headlight assembly I picked up from Dick P. has four wires attached...brown, white, yellow and a black/white?

The  turn relay is just sitting in the headlight bucket unattached...where is it supposed to be mounted?

This may all be easily explained by extra un-used wires that haven't been taped off, and a missing headlight harness off the board.

Any insight is appreciated guys!

So I just watched William Boxster2Valve repair the headlight carnage on an R90 and some of my questions were answered. Black wire with connector was left unused. I know the four wires for the headlight are yellow-white-brown and grey. But I am still a bit bewildered...thinking I am missing the headlight pigtail with those wires coming off of my circuit board. I am open to ideas guys. Anyone have acces to their headlight buckets?

Posted by: @lars-waldner

PO had installed a fairing on the bike.

The brand of fairing would really help. If it was a Vetter Windjammer then the only wring done was to extend the 3 headlamp wires about 18" to reach the new fairing (Vetter didn't use parking lamp, so no Gray.) And, you would remove the stock front turn signal pods and wire in the Vetter turn signal lamps.

• You can get the wire and head lamp plug to make your own headlamp bulb harness. Use 16AWG multi-stranded auto motive wire with 1/4" crimp spade terminals.

So I figured out some of my wiring problems. A PO had replaced the wiring harness (and maybe switches) for the left handlebar, so the harness coming into the bucket was different wires than I was looking for on my schematic. Didn't anticipate this. Looks like that harness is from a 1977 R100S. Different wire colors for the horn, dimmer and headlight switch. So I am used to power being red or yellow, but it appears now that power can be other colors? School me please...I am learning a new language.

Posted by: @lars-waldner

So I am used to power being red or yellow, but it appears now that power can be other colors? 

Red or Yellow... you must be talking about Japanese bikes. Each manufacturer uses different color codes for their harness.

Posted by: @lars-waldner

...but it appears now that power can be other colors? School me please... 

I know that Honda uses those colors, but they also use a main fuse to protect the entire harness. The Germans see things differently. 

Here's the basic layout... There is no "main fuse" on a stock BMW. The battery wiring passes up the length of the harness on Red all the way to the ignition switch. (Anywhere you see a Red wire [like on the Clock] it is "hot" full-time with no fuse.) At the ignition switch it splits into Green for ignition and Grey for lights. Those 2 colors (still unfused) come down to the multi-color circuit board in the headlamp. There, Gray goes through an 8A fuse and turns into Gray/Black; Green goes through an 8A fuse and turns into Green/Black. Those 2 colors feed everything on the bike.

Brown is used for DC Return (what most call "ground") all through the harness. 

Colors, other than Gray/Black and Green/Black, have passed through an electrical component and the color change is used to denote it is the output of that component.

Posted by: @lars-waldner

I am learning a new language.

Yes... and there's a test on Monday... so start cracking !

@wobbly 

Thats what I'm talking about. Thank you!

I see no black wire on my diagram...yet I have one with a female rectangular fitting...that disappers into the main harness and leaves the bucket. Same one in Brook's headlight video. What is that supposed to be for? Also I have a severed red and severed brown wires both connected to the board. I am guessing they were remnants of the PO wiring the fairing headlight? I have a pigtail for my new headlight. I am presuming I can pull the severed wires and free up some spades on the board? Also from that same left handle switch I have a green/black and white/brown from the horn. White/brown has a place on the board, green/black does not that I see. Where the wires actually connect to the horn, I have green/black and white/brown...so the green/black in the headlight needs to partner with similar on the board. I am open to suggestions. 

It will help you a great deal to get a cheap floor lamp from Goodwill and fit it with 5000K (bright white) LED flood lamps. Then you'll be able to see the true colors inside your headlamp bucket. The color sections on the multi-color circuit board are not all the same size, shape or number of terminals.

Typical multi-color circuit board

• And remember to add a very small amount of electrical de-oxidation paste to each male terminal before refitting the female connector. That will act in the short term as a lubricant and in the long term to promote the connection.

• Most definitely unplug any wires that have been cut. The reduces the possibility of short circuit and opens up space (and terminals) on the board.

• Solid Black ("SW") is used in and around the ignition coils on most models. On later models it is used for the electronic tachometer, running from the coils to the gauge cluster plug. (The tach uses the ignition pulses to gauge the engine RPM.) So this wire is in the harness, but does not pass through the HL shell.

• There are places for Green/Blk on the circuit board. On my 1978 RT there were more wires than terminals and so there was a Y-accessory plug used which had a single female that branched out to 2 inline males. 

• Now remember the difference between Green/Blk and Gray/Blk !! Your 1975 bike is wired for a time before most riders used headlamp ON during the day. In 2022 everyone rides with headlamp ON 24/7.... and this makes Green/Blk and Gray/Blk electrically the same. So if you need extra terminals for Green/Blk you can simply plug them into Gray/Blk for now.

► The lesson here is that you cannot look at everything with 2022 eyeballs. When you work on a 1975 bike, you MUST put on your "1975 glasses" and look at things from the historical point of view. That can be hard for youngsters to do. To give you some humorous insight... the most commonly fitted RT accessories in 1975 were cassette players and CB radios !! Some of the wiring you are dealing with could be for those long-gone accessories.

Hope this helps.

Thanks Richard! All make sense. BTW I am a 1976 R75/6.

If it were me, I would, as suggested above, remove all severed wires from the circuit board. Next turn on ignition switch. Make certain headlight comes on, high and low. Next try cranking in neutral-the light did come on didn't it? Then I would test each electrical function including lights and fix one at a time rather than wholesale trying to match wire colors. After everything is working correctly, make notes on your wiring diagram as to any differences in wire color and make a note of any wires not connected. Heat shrink to cover any unconnected ends will prevent exciting electrical occurrences.

Bob,

Those are all good ideas. Bike is disassembled at present and I am unable to check the various electrical systems. But...while we are talking about electrical systems, I may as well get the forum to weigh i on how to clean the internals of my starter. Bike was briefly flooded and the starter took on water. Solenoid worked but starter would not spin. Disassembled the starter (why not, rest of the bike is in pieces) and there is oxidation, but it looks recoverable. I have seen suggested an alcohol based electronics cleaner and tooth brushes and brass brushes to remove the oxidation. Before I replace the starter, I would like to try to restore it. What do you guys suggest? 

The only issue (and therefore experience) I've ever had with the starters is the grease inside the bearings. All the bearings should be re-packed (or in your case most proabably replaced). The OEM grease has oxidized by now, which makes the starter VERY hard to rotate. The stock OEM starter is actually fighting itself while trying to start the engine.

I've disassembled maybe 50 starter motors on all models of cars and motorcycles. It's a very dirty environment, so I wouldn't spend too much time getting things "spotless" because it will be a mess again in 1 week. Instead, vacuum it out and then work to stop obvious rust and corrosion which will cut the life of the starter short. Remember that the starter lives in a warm, dry place and will be naturally protected from most degradation in the future.

My top 3 things to replace would be: bearings, brushes, and starter-to-battery cable. Other than that, lube the moving parts of the mechanism and slap it back together.

So I tore the entire starter down and cleaned it out. The brushed were adequate so re-used as were the bearings. Casings were wire brushed and painted. Lubed and re-assembled. Bench tested and it is very happy. Working now on cleaning and re-assembly of the timing cover...and installation of electronics. I can test my electronics when the battery is re-installed.

Note...two extra screws were not supposed to be included in the rebuild.