R27 Piston

[ChrisR27]

Hello everyone,
Well, my R27 is now on the road and running. My longest ride so far is only about 50 miles (80km) but already it's clear to me the bike is a 'keeper'. When I rebuilt the engine the piston / bore clearance was 'just' Ok so I decided to leave it knowing that a later re-bore would be an easy task for this engine given how easy it is to work on.
However, the bike only manages 50mph (80kph) so clearly something is wrong and a compression test only gave 50psi, I believe I should be seeing 120 - 140psi. Pouring oil in through the plug hole increased the value to 80psi, I also get a lot of pressure out of the oil filler / dipstick hole indicating piston blow by.
My question is which piston is correct for my bike? The current piston has 4 rings, 3 compression and 1 oil control, I believe this piston was only used for one year. My engine number is 374923 so an early model. Looking at the specification table the piston design looks to have changed at engine number 385083 so which piston do I need to order, I really have no idea?
The usual spares dealers have only one piston type for the R27 in various over sizes.
I might be over complicating this 
Regards,
Chris.

[ChrisR27]

[Taz]

Hello Chris.

As far as I am aware the piston design has been the only parameter being changed at that time so imho the "one design fits all" approach of the dealers should do. You mentioned rebuilding the engine so I trust that all the "deeper" work like replacing/cleaning of the slinger plate were done at that occasion and the only job left to do now is re-boring the cylinder and replacing the piston?

I hope that some of the more R27 centered members will also contribute to this thread...

kind regards, Ute

[Rütz]

The current piston has 4 rings, 3 compression and 1 oil control,

It really seems, that your bike got a R26 Piston installed, which ist much(!) too low for R27.
(R27 has a shorter conrod while same dimensions of barrel and head)
You can compare dimensions in our Table:
https://www.bmw-einzylinder.de/Vergleichstabelle/default.html#kolben_EN
Or post a picture.

The latter pistons were an improvement (down compatible), it makes little sense to look for an early originally shaped one.

[ChrisR27]

Hi Rutz,
So my engine has the wrong piston fitted. This would make sense, the bore measurement seemed within specification (just) but the compression is much too low. However I think the piston was flush with the top of the cylinder when installed, I need to remove the head to be sure though. The engine is now assembled so no picture, sorry. I do remember my piston was marked 68.44, I guess this is the diameter.
Regards,
Chris.

[Taz]

Hello Chris.

You have guessed right, 68,44 would be the piston's diameter in mm, so the engine got re-bored once in the past already (starting diameter of a virgin engine would be just under 68.00 mm).

Kind regards, Ute

[rolf.soler]

It really seems, that your bike got a R26 Piston installed, which ist much(!) too low for R27.
(R27 has a shorter conrod while same dimensions of barrel and head)

I had exactly the same scrap situation in my R27 when I bought it "fresh from MOT"
A 26 piston in a 27, it ran like a bag of potatoes... 80 km/h downhill , compression was about 5 bar
The piston was about 1 cm below the cylinder top...
as Rütz explained, due to the different conrod, a 26 piston will not do...
Just buy a standard R27 piston from one of the trustworthy dealers and be surprised how it runs !
Good luck

[Taz]

Hello Rolf.

I don't know about the Swiss "MOT" or the UK one (yet, regarding the BMW never will ), but in Germany the MOT counterpart would not care about lack of power...as long as the vehicle doesn't leak on the tarmac and the brakes are ok they engine may provide as little power as it "wants"...

Kind regards, Ute

[rolf.soler]

Hello Rolf.

I don't know about the Swiss "MOT" or the UK one (yet, regarding the BMW never will ), but in Germany the MOT counterpart would not care about lack of power...as long as the vehicle doesn't leak on the tarmac and the brakes are ok they engine may provide as little power as it "wants"...

Kind regards, Ute

They check a few more things unfortunately (or fortunately) especially if "Veteran" oder "Oldtimer" status is asked for...
advantages are MOT only every 6-7 years and an unlimited number of bikes on the same number plate (and insurance fee..)
However, lack of power would not disqualify for "normal" MOT; but for "veteran status" it could... if the tester has some idea about how it should run...
anyway, this was not the only problem... gears 3/4 did not work, Tacho was from a R50 showing completely wrong speed, tank treated with 1 kg of filler and spray painted silver....it had passed as a veteran, but in the canton Wallis/Valais, where everybody knows / is related to everybody else. It should have failed even normal MOT in any reasonable test center.
But it was relatively cheap (2500 €) and I fixed it all...and I learned a lot of things. Not a bad deal, after all, today it looks and runs as in 1960 straight from the dealer.