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Autor Thema: R27 crank assembly in R26?  (Gelesen 599 mal)

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BSA Bob

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R27 crank assembly in R26?
« am: 08 März 2021, 02:41:27 »

I have what I'm told is an R27 crankshaft fitted with a steel rod.
Firstly, would this fit an R26?
Secondly, is there any point? Are these cranks an advantage over the R26?
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R27 crank assembly in R26?
« am: 08 März 2021, 02:41:27 »

OldsCool!

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Re: R27 crank assembly in R26?
« Antwort #1 am: 08 März 2021, 08:04:41 »

Firstly, no. The R27 crankshaft has a different front-end, only carrying the rotor. R26 also carries the centrifugal advancer (R27: on the camshaft).
Secondly, I don't think so. Just be sure you get an R26 crankshaft with the big webs that fit the steel rod. There are still many around with the small webs, carrying an aluminium rod before and having been reworked to steel rod. For sure the weight balance does not fit and those cranks are known as the "shaker cranks" ("Schüttelwelle").

Best regards,

Steffen
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Re: R27 crank assembly in R26?
« Antwort #2 am: 08 März 2021, 09:49:18 »

Hello Bob,

The R27 crankshaft has just one slot for the woodruff key for the sprocket of the camshaft chain drive. The R26 crankshaft has two slots. The slot at the cone is for the rotor of the alternator rotor to ensure a certain angular timining. The excenter shaft to actuate the ignition points is bolted on the alternator rotor. So if you use the R27 crank in the R26 the ignition timing may change, because the position of the rotor is not fixed....

The R27 conrod can be used in the R26, but then you have to use also the R27 pistion. R26 and R27 pistions have different compression height in order to compensate the different conrod length (R26 has 125mm, R27 has 120 mm)

Best regards
Stefan, the one clindrist
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Taz

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Re: R27 crank assembly in R26?
« Antwort #3 am: 08 März 2021, 10:24:57 »

Hello Bob.

For a first overview you might want to refer to this table...comparing a number of components that will provide a first rough idea about whether parts might be interchangeable between models or not...

https://www.bmw-einzylinder.de/Vergleichstabelle/default.html#EN

Kind regards, Taz
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rolf.soler

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Re: R27 crank assembly in R26?
« Antwort #4 am: 08 März 2021, 20:14:23 »

Hello Bob,
The R27 crankshaft has just one slot for the woodruff key for the sprocket of the camshaft chain drive. The R26 crankshaft has two slots. The slot at the cone is for the rotor of the alternator rotor to ensure a certain angular timining. T
No, the R27 has and needs 2 slots in the crankshaft too...one for the gear drive wheel (this toothed wheel that drives the chain), and one for the rotor (Anker). But if they are in the same angle position ? I don't know...
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Re: R27 crank assembly in R26?
« Antwort #5 am: 08 März 2021, 20:18:12 »

Rolf.

NO. That is not correct. The Rotor of the R27 does not need any angular orientation. The ignition is driven by the camshaft....

regards
Stefan, the one cylindrist
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Re: R27 crank assembly in R26?
« Antwort #6 am: 08 März 2021, 20:27:02 »

yes, but I think the rotor still is fixed by a woodruff-keil...even though the position does not matter...if I'm not seriously wrong... Just dismantled a R27 engine.....but I'll check again in the workshop this evening


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Re: R27 crank assembly in R26?
« Antwort #7 am: 08 März 2021, 21:23:57 »

This is the end of the crankshaft of my second R27, currently in assembly...see the slot ? In the slot there was a woodruff-key....and a corresponding slot in the rotor...
though I now wonder why, the position of the rotor does not really matter
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Re: R27 crank assembly in R26?
« Antwort #8 am: 08 März 2021, 21:36:17 »

studying various catalogues, I wonder if this slot is really original in a R27 crankshaft.....maybe this was in ancient times a R26 crankshaft, later reworked for R27 with the shorter conrod ? In that case I should not put back the woodruff-key ?
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BSA Bob

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Re: R27 crank assembly in R26?
« Antwort #9 am: 09 März 2021, 03:39:11 »

Thanks for all this, & thanks for the comparison table Taz.
I've examined my crank, & it has 3 keyways.
At the front of the taper
In front of & behind the bearing surfaces.
My 1972 parts book list the front keyway on the R26 only.
So perhaps I've got an R26 crank, which would suit me.
The steel rod measures about 140mm centre to centre.
The counterweights form a half circle about 58mm radius measured from the shaft centre.
It has some surface pitting but I hope is salvageable.
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Re: R27 crank assembly in R26?
« Antwort #10 am: 09 März 2021, 07:50:09 »

Hello Bob.

140mm conrod length is too much. R26 had an 125mm conrod, R27 had 120mm.

Please measure also conrod small end diameter for the piston pin. This must be 20mm. If it is 18mm you have probably a R25/x conrod (136mm) or a complete crank from R25/x. R25/x has also smaller main bearings. So you should also measure the main bearing jounals of the crankshaft.

best regards
Stefan, the one cylindrist
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Re: R27 crank assembly in R26?
« Antwort #11 am: 09 März 2021, 11:05:36 »

Hullo Stefan
Well, that's torn things. I'll measure the crank tomorrow. I've looked at the comparison table & I assume the bearing surface of an R26 crank is 35mm?
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Re: R27 crank assembly in R26?
« Antwort #12 am: 09 März 2021, 23:18:20 »

I've measured my crank assembly again.

Rod 135-40mm (can't measure exactly).
Rod little end 20mm
Journal diameter 30mm.

So would this be from an R25?
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Re: R27 crank assembly in R26?
« Antwort #13 am: 10 März 2021, 07:21:00 »

Bob, if you could supply a picture, we could tell you right away...
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Re: R27 crank assembly in R26?
« Antwort #14 am: 10 März 2021, 08:25:16 »

According to our comparison List 30mm main bearing jounal at flywheel side is R25/x....

best regards
Stefan, the one cylindrist
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Re: R27 crank assembly in R26?
« Antwort #15 am: 11 März 2021, 11:31:01 »

So the crankshaft is from an R25. Interesting about the little end size of the rod.
I'm trying to work out how to get a look at the crank currently in the motor to see what's in there. If I can get to that I'll try to send a picture.
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4Taktix

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Re: R27 crank assembly in R26?
« Antwort #16 am: 11 März 2021, 11:55:24 »

Hi Bob,
there's no other way than to remove the oilpan and the weave/grid (2 slotted screws ) below the crank. Simple as that.

Cheers,
Sascha
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Re: R27 crank assembly in R26?
« Antwort #17 am: 11 März 2021, 14:56:56 »

Hi Sascha
Yes, I'll have to remove the oilpan. I'm trying to work out a way to get the bike elevated so I can see underneath. I'd prefer not to remove the motor from the frame at this point.
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Re: R27 crank assembly in R26?
« Antwort #18 am: 11 März 2021, 15:40:08 »

Tip it over a bit onto something soft?

Kurt
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Re: R27 crank assembly in R26?
« Antwort #19 am: 11 März 2021, 23:27:16 »

Kurt
I may have to try something like tipping the bike on its side. What I need is a foxhole size drop pit but I haven't got one.
I definitely want to know what sort of crankshaft issues I've got before proceeding further.
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Re: R27 crank assembly in R26?
« Antwort #20 am: 11 März 2021, 23:36:08 »

Hello Bob.

Oilpan removed, one old smart phone with torch function pointing upwards and the second one taking a picture or two...should to the trick without any drop pits...the differences in the crankshaft design are pretty obvious...

Kind regards, Taz

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Re: R27 crank assembly in R26?
« Antwort #21 am: 11 März 2021, 23:52:16 »

Thanks for the reply, Taz.
I'll be away for a few days & then I'll start.
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