R27 piston (or not!)

[ChrisR27]

Hello everyone,
We had a discussion about my R27 in July where I said the bike did 80kmh maximum and the compression was 80psi, this post is just an update really. I said at the time the piston had four rings and the opinion was this was an R26 piston. The bike was running 'OK' and I wanted to use it but now winter is here I have stripped the top end. The piston sits about 5mm below the top of the cylinder, this is the same as the height difference between an R26 and R27 piston. Everything else is R27, I rebuilt the crankshaft myself with a new R27 rod kit, the cylinder has two oil holes and the head has the studs for the frame mounting. So only the piston is wrong 

I now need to measure the cylinder and decide whether a rebore is required or whether just to hone the cylinder and fit the correct piston. I plan to use a genuine BMW piston, there are pattern parts but they are almost the same price.

By the way, my bike has no BMW tank badges and NOBODY knows what the bike is when I stop. Single cylinder BMWs are very rare in the UK!

An early Merry Christmas wish to everyone.
Chris.

[ChrisR27]

[Einzylindär]

Hello Chris,

I am pretty sure R26....

Please find attachted a picture of my R27 with new KS piston. This is the newer piston design with incresed compression height of approx 38,6mm (from 1966 on). The earlier R27 design has 1 or 2mm less compression height (I guess it ist 37mm). But your picture shows much more height.

Best regards
Stefan, the one cylindrist

[Einzylindär]

Just a hint. I saw the dimension on your piston...
At German EBAY you find an old 1st oversize KS piston. It is in Spain...I bought from this seller a set of original BMW tappet guides some month ago. They were fine....

https://www.ebay.de/itm/KOLBEN-KOLBENSCHMIDT-BMW-R27-68-50-MM-/234305015114?mkcid=16&mkevt=1&_trksid=p2349624.m46890.l49286&mkrid=707-127634-2357-0

Best regards
Stefan, the one cylindrist

[strichzwojan]

High, our member "fraggi" offers different sizes of NOS Nüral Pistons ( without bolt, rings and clips). But its a nice price! R 27 only in standard bore 68,00mm. I ordered two pistons for 45 Euro.

https://bmw-einzylinder.de/forum/index.php?topic=20007.msg296831#msg296831


Jan

[ChrisR27]

Hello and thank you for the replies.

Very noticeable difference in piston height. In the UK we have a saying 'be careful what you wish for!'. I might have to put the R26 piston back in when my old legs can't kick start the bike 

Motorworks in the UK have listed all piston sizes, KS and pattern parts. I need to order gaskets too so will place an order with them.

https://www.motorworks.co.uk/vlive/Shop/Parts.php?T=6&Q=R27+piston&search=SEARCH

Regards,
Chris.

[Jollyjester]

The price of circa £158.00 seem about right, I purchased an R50S piston for my R25 back in the summer.

You could do worse than contact / speak with John Lawes  at Derby,

bemw.co.uk

He was the BMW ClubUK&I vintage register Secretary for 25 years.

He re-bored the barrel, fitted and reamed a new small end bush and supplied the correct piston.

Turn around a week..

One of the 'old school'

Nothing wrong with Motorworks, but with John you get face to face with the man who does the job.

you can find me if you wish on

dvla@bmwclubuk.net

Robert

[ChrisR27]

Hello everyone,
I had a local company measure and check the bore for me, South Cerney Engineering. Second oversize piston was required and ordered by me from Motorworks, this was out of stock but arrived early January. I dropped the piston off with the engineers and they completed the work a few days later, £57.60.
I've fitted the piston and compression is now good and I'm looking forward to getting out for a ride. I don't mind the cold too much, I just have shorter rides, but I can't bear getting the salty, gritty, muddy, yucky crap off our roads all over my bike. That's what winter hacks and cars are for!
Belated Happy New Year.
Chris.

[rolf.soler]

The amazing thing ist that the R27 runs at all with a R26 piston....when I bought mine, it was the same situation...also made around 80 km/h
And the BMW-novice thinks that's normal for a 60 year old motorbike, as I did for a week or two. Look and see how it runs with the correct piston and enjoy

[ChrisR27]

Hi Rolf,
I'm all excited now and want to go for a ride!
Regards,
Chris.