R65 timing sprocket position

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Re: R65 timing sprocket position

Postby windmill john » Wed Oct 10, 2018 6:37 am

I too am unclear.

If there is no mark on the crank sprocket but the flywheel is at ‘OT’ is that definitely right? It can’t be out by one rotation!?

The reason I say is when you set the left hand (for example) tappets at OT, the right hand valves are open; to a point.
But then again, that’s always going to apply whichever side you set.
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Re: R65 timing sprocket position

Postby Mjolinor » Wed Oct 10, 2018 7:42 am

No it cannot be out by one rotation. Angles are measured on the crankshaft, not the camshaft. One turn of crank = 1/2 turn of cam so you can split the cam rotation in two, one half turn does one cylinder, the other half turn does the other cylinder.

It would work just as well timing with the cam mark at the bottom, functionally it is exactly the same.

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Re: R65 timing sprocket position

Postby Mjolinor » Wed Oct 10, 2018 7:43 am

That is of course assuming your flywheel is fitted correctly.

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Re: R65 timing sprocket position

Postby Rob Frankhamr » Wed Oct 10, 2018 10:01 am

It doesn't matter whether you time the engine with the camshaft at 12 o'clock or at 6 o'clock. It is only convention that says you time it at 12 o'clock. The camshaft moves at (exactly) half of the speed of the crankshaft (it must) so the camshaft can be either at 12 o'clock or at 6 o'clock when the crankshaft is at TDC. You could time it with the camshaft mark at 6 o'clock and it would be just as correct as it would be at 12 o'clock just with the other piston on the power TDC.

As Mjolinor says, the crankshaft can't be out by one rotation...Both pistons are at TDC at the same time and the crankshaft doesn't know or care which of them is at the top of the compression/power part of the cycle and which is at the exhaust/induction part of the cycle.

To think of it another way... If you connect the chain with the crank at OT and the camshaft mark at 12 o'clock then rotate the crankshaft by 360 degrees. The crank will be back at OT but the camshaft mark will be at 6 o'clock (it will only have moved 180 degrees) exactly as though you'd connected the chain in that relationship. Rotate the crankshaft by another 360 degrees and you'll be back with the camshaft mark at 12 o'clock.

This isn't a 'BMW' thing, it's a basic principal applicable to all 4 stroke Otto cycle engines...

Finally, I believe that, with the airhead engine, if the camshaft is at 12 o'clock (or 6 o'clock for that matter) the pistons won't touch the valves if the crank is turned. Both of the valves on one cylinder are closed at compression and those on the other are very nearly closed on the changeover between exhaust and inlet strokes. I would always recommend, however, if you are working in any way with the camchain disconnected. That you undo the tappets so that all of the valves remain constantly closed. It's only a matter of minutes to reset the valve clearances once you've got it all connected up.

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Re: R65 timing sprocket position

Postby windmill john » Wed Oct 10, 2018 10:37 am

Thanks again all.

Rob, re your last line, at least cranking by hand confirmed I had definitely got the timing wrong as something caught.
I knew about camshaft moving half crank rotation, I think I got my underwear in a twist because it went wrong the second time.

John
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Re: R65 timing sprocket position

Postby Mjolinor » Wed Oct 10, 2018 10:49 am

What do you mean by "something caught"? If the timing is out enough to allow piston --- valve contact and you turn by hand it will lock solid and you will be unable to turn it. Depending on how gentle you are it may or may not bend a valve doing that. "something caught" to me says perhaps you have a lump of crap in your alternator or a stiff point on your oil pump.

Being a pedantic old git I would add it is not peculiar to the Otto cycle, the Atkinson cycle and lots of other variations on the theme obey the same rules. :)

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Re: R65 timing sprocket position

Postby Mjolinor » Wed Oct 10, 2018 10:57 am

Hmm, a further thought:

As far as I can see no one has mentioned that you should only rotate the crank in one direction. If you go past TDC do not rotate back to TDC then check the timing. The run of chain on the offside of the bike must remain taut. <<<< Grandma eggs suck (not necessarily in that order?)

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Re: R65 timing sprocket position

Postby windmill john » Wed Oct 10, 2018 11:40 am

Something touched, I am assuming valves to piston.

I was able to rotate the engine the other way.

Did it by allen key, so no damage.
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Schorsch - 1978-80 R65 - Has gone. No, not talking about it.
Max - 2009 F800GS- where’s the desert at!?
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Re: R65 timing sprocket position

Postby Rob Frankhamr » Wed Oct 10, 2018 11:45 am


Being a pedantic old git I would add it is not peculiar to the Otto cycle, the Atkinson cycle and lots of other variations on the theme obey the same rules. :)

Absolutely right...

You are a pedantic old git... :grin:

Actually I thought someone was going to say that sleeve valve engines as well as most radials and rotaries don't have camshafts as such and so don't comply with all of the description... but the principal remains the same.

Rob
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Re: R65 timing sprocket position

Postby windmill john » Wed Oct 10, 2018 5:41 pm

Blooming heck! Just got sprocket out of freezer, heated up my newly arrived bearing, welding gloves on..... hang on!..... this bearing is nowhere near the right size!
What an idiot! This bearing came from James Sherlock, sprocket from Motobins a couple of years ago. I didn’t spot the different bearing on the Motobins site! I just assumed the bearing was right.

I’ve made a lot of stupid assumptions on this job!......

Looks like I’ve been pedantic... with errors ](*,)

1902D62F-E501-48A9-A481-90FEA73A8D97.jpeg
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http://www.kittos.co.uk
Best roads: 623 Burgos to Santander. A back road to Metz; can't remember which!
Schorsch - 1978-80 R65 - Has gone. No, not talking about it.
Max - 2009 F800GS- where’s the desert at!?
Too many bikes have come and gone, trying to be sensible now!

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Re: R65 timing sprocket position

Postby Galactic Greyhound » Wed Oct 10, 2018 6:03 pm

Oh well..... it happens!

You begin to accept it after 70!

When you fit the crankshaft sprocket mark at 6 o'clock, is the flywheel at the OT mark?
Ced.

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Re: R65 timing sprocket position

Postby windmill john » Wed Oct 10, 2018 6:36 pm

See, you’re doing it now Ced, there is no mark :grin:

I’m going back in on Saturday, subject to weather.
http://www.kittos.co.uk
Best roads: 623 Burgos to Santander. A back road to Metz; can't remember which!
Schorsch - 1978-80 R65 - Has gone. No, not talking about it.
Max - 2009 F800GS- where’s the desert at!?
Too many bikes have come and gone, trying to be sensible now!

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Re: R65 timing sprocket position

Postby Mjolinor » Wed Oct 10, 2018 8:03 pm

Can someone please tell me why some are duplex chains and some are simplex?

Mine is duplex, that pic is simplex

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Re: R65 timing sprocket position

Postby beemer8 » Wed Oct 10, 2018 8:09 pm

Didn't they fit the single chain from 81 0n when they altered a lot of stuff.

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Re: R65 timing sprocket position

Postby Galactic Greyhound » Wed Oct 10, 2018 8:53 pm

by Rob Frankhamr » Tue Oct 09, 2018 3:14 pm
……….Not all crankshaft sprockets have a mark and it really isn't necessary... if the engine is at TDC, then the crankshaft sprocket is in the correct position for fitting the chain.

John,
Your last sprocket photo seems to have a punch-mark but I had forgotten that bit in Rob's post! :smile:
Ced.

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Re: R65 timing sprocket position

Postby Rob Frankhamr » Wed Oct 10, 2018 10:02 pm

Can someone please tell me why some are duplex chains and some are simplex?

Mine is duplex, that pic is simplex

The simplex chain was introduced with the '79 model (i.e. Sept 78). There were also a number of other mods at the front of the engine including the deletion of the mechanical drive for the tacho, a change to the camchain tensioner and the use of a bean can type ignition module (this initially contained points... the electronic bean can wasn't introduced till Sept 80 for the 81 year). In fact, the timing cover and front cover castings are quite different,

It isn't possible to retro-fit most of the components but I have heard of people allegedly using the simplex chain on an earlier engine with duplex sprockets.

There is also anecdotal evidence that the simplex chain lasts longer than the earlier duplex one... can't think why.... go figure.

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Re: R65 timing sprocket position

Postby Mjolinor » Wed Oct 10, 2018 10:04 pm


There is also anecdotal evidence that the simplex chain lasts longer than the earlier duplex one... can't think why.... go figure.

Rob
Now that is a conundrum needing thinking about.

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Re: R65 timing sprocket position

Postby Mike D » Wed Oct 10, 2018 11:19 pm

There is also anecdotal evidence that the simplex chain lasts longer than the earlier duplex one... can't think why.... go figure.
Don't know where that comes from.

In my experience, duplex chains last at least twice as long as simplex chains, and I have bikes with both systems fitted.

Mike

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Re: R65 timing sprocket position

Postby windmill john » Thu Oct 11, 2018 7:02 am

by Rob Frankhamr » Tue Oct 09, 2018 3:14 pm
……….Not all crankshaft sprockets have a mark and it really isn't necessary... if the engine is at TDC, then the crankshaft sprocket is in the correct position for fitting the chain.

John,
Your last sprocket photo seems to have a punch-mark but I had forgotten that bit in Rob's post! :smile:

You’re right Ced, that sprocket has a lovely mark. I even popped a little one on the tooth in case the bearing covered it.
http://www.kittos.co.uk
Best roads: 623 Burgos to Santander. A back road to Metz; can't remember which!
Schorsch - 1978-80 R65 - Has gone. No, not talking about it.
Max - 2009 F800GS- where’s the desert at!?
Too many bikes have come and gone, trying to be sensible now!

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Re: R65 timing sprocket position

Postby Graeme » Thu Oct 11, 2018 9:22 am

It doesn't matter whether you time the engine with the camshaft at 12 o'clock or at 6 o'clock. It is only convention that says you time it at 12 o'clock. The camshaft moves at (exactly) half of the speed of the crankshaft (it must) so the camshaft can be either at 12 o'clock or at 6 o'clock when the crankshaft is at TDC. You could time it with the camshaft mark at 6 o'clock and it would be just as correct as it would be at 12 o'clock just with the other piston on the power TDC.

As Mjolinor says, the crankshaft can't be out by one rotation...Both pistons are at TDC at the same time and the crankshaft doesn't know or care which of them is at the top of the compression/power part of the cycle and which is at the exhaust/induction part of the cycle.

Rob, I understand and agree with the mechanical principal of this, but it got me thinking about spark timing. If you can time it both ways, does it mean that the plugs both fire together every 360° (before TDC)? I.e. one plug fires into compressed fuel air mixture (power stroke) and the other fires into the end of the exhaust stroke?
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Re: R65 timing sprocket position

Postby windmill john » Thu Oct 11, 2018 10:06 am

Yes, as far as I am aware all points Airheads use the lost spark method; not sure about the way post 81 works.

You have one set of points, the coils are in series, both charging at the same time. Each coil is 6 volts making the 12 needed in total.
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Best roads: 623 Burgos to Santander. A back road to Metz; can't remember which!
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Max - 2009 F800GS- where’s the desert at!?
Too many bikes have come and gone, trying to be sensible now!

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Re: R65 timing sprocket position

Postby Rob Frankhamr » Thu Oct 11, 2018 10:32 am

It doesn't matter whether you time the engine with the camshaft at 12 o'clock or at 6 o'clock. It is only convention that says you time it at 12 o'clock. The camshaft moves at (exactly) half of the speed of the crankshaft (it must) so the camshaft can be either at 12 o'clock or at 6 o'clock when the crankshaft is at TDC. You could time it with the camshaft mark at 6 o'clock and it would be just as correct as it would be at 12 o'clock just with the other piston on the power TDC.

As Mjolinor says, the crankshaft can't be out by one rotation...Both pistons are at TDC at the same time and the crankshaft doesn't know or care which of them is at the top of the compression/power part of the cycle and which is at the exhaust/induction part of the cycle.

Rob, I understand and agree with the mechanical principal of this, but it got me thinking about spark timing. If you can time it both ways, does it mean that the plugs both fire together every 360° (before TDC)? I.e. one plug fires into compressed fuel air mixture (power stroke) and the other fires into the end of the exhaust stroke?

Yes absolutely, As WJ says, it's usualy referred to as a 'Lost Spark' ignition system. It's by no means unique to BMW twins... many engines that have pistons moving reaching TDC at the same time will have a lost spark ignition and that includes 4, 6 and 8 cylinder engines. Effectively it reduces by half the number of ignition systems a vehicle needs to have.

The only slight anomally with the BMW setup is that they chose to drive the ignition from the camshaft. This means there has to be two ignition trigger points (i.e. cam lobes with points or triggering impulses from an electronic sensor) which need to be very accurately made so that both firing points are in exact synch. This is what gives rise to the 'twin image' that occurs when using a strobe to time an early airhead engine if the camshaft nose has been bent or damaged. In effect, the two cylinders are not receiving their spark at the same time in their cycle... which makes the engine virtually impossible to balance.

It would be perfectly possible to trigger such a system from the crankshaft using just one trigger point, in fact there are aftermarket ignition systems that do just that. My belief is that BMW triggered points from the camshaft to reduce wear on the points heel and, thus, increase points life. There is no reason why, with an electronic trigger, it shouldn't be done from the crank but I guess it would have meant a major engine redesign to overcome a non-existent problem.

Rob
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Re: R65 timing sprocket position

Postby Graeme » Thu Oct 11, 2018 10:56 am

Makes sense to use wasted spark I suppose - keeps things simple
Cheers,

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Re: R65 timing sprocket position

Postby barryh » Thu Oct 11, 2018 7:59 pm

There was a very lively debate on the Airheads list some years back where a guy claimed converting wasted spark systems to single spark showed power benefits on the dyno. No one remotely believed that could apply to a flat boxer twin like an airhead as the wasted spark cylinder is on the exhaust stroke but V twins could be affected if they have aggressive cam timing as the wasted spark gets close to occurring when the intake is open. You might wonder why use wasted spark on a V twin but Harley Davidson still do it.

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Re: R65 timing sprocket position

Postby Roy Gavin » Thu Oct 11, 2018 11:01 pm

Most digital electronic ignitions are programmed to fire at even intervals, which is one of the befits of using them-------.


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