MKIII piston orientation

Anvill

Anvill

Joined
Feb 26, 2022
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Hi guys,

My MKIII engine reassembly continues. I had the presence of mind to mark my newly ringed pistons "L" and "R" in sharpie, but I did all of this long enough ago that I am now second guessing what the reference point was--proper L and R as in when mounted on the bike vs L and R with the engine facing me on the bench. After realizing I had marked my cylinder barrel lifters using the less intuitive bench orientation, I am now trying to figure out how to orient these pistons. They are flat topped original pistons, and don't appear to be stamped with an orientation. But they do have markings in the casting on the underside that might help orient them. I thought I'd post a couple pictures here in hopes someone might have ideas.


MKIII piston orientation
MKIII piston orientation


These photos are of the same piston, showing each side. One side has text on both sides, the other side has a circle stamp and a logo.

Thanks all,
Anthony
 
Anvill said:
Hi guys,

My MKIII engine reassembly continues. I had the presence of mind to mark my newly ringed pistons "L" and "R" in sharpie, but I did all of this long enough ago that I am now second guessing what the reference point was--proper L and R as in when mounted on the bike vs L and R with the engine facing me on the bench. After realizing I had marked my cylinder barrel lifters using the less intuitive bench orientation, I am now trying to figure out how to orient these pistons. They are flat topped original pistons, and don't appear to be stamped with an orientation. But they do have markings in the casting on the underside that might help orient them. I thought I'd post a couple pictures here in hopes someone might have ideas.


MKIII piston orientation
MKIII piston orientation


These photos are of the same piston, showing each side. One side has text on both sides, the other side has a circle stamp and a logo.

Thanks all,
Anthony
Click to expand...
Assuming new rings and at least a light hone, quit thinking about it! Otherwise, get a new set. When new, there is no orientation on 850 pistons.
 
Yes new rings and new hone, so the only thing making contact is the original wrist pins to original conrods. Hopefully swapping sides won't create a funky wear pattern there, but other than that its good to know there's nothing catastrophic to expect if I guess wrong.
 
The tops of the pistons if original will be marked A or B. On the underside of the barrel the bore type A or B is marked, hence if you have different piston/barrel fits you could match whichever piston goes where, however if both same fit that will not work.MKIII piston orientationMKIII piston orientationMKIII piston orientationMKIII piston orientation
 
Mine are both size B but thank you for this suggestion. Looking at your last photo I can probably sort out the orientation for the piston based on the internal stampings and the bit of cylinder barrel showing. But it sounds like pistons may not have been strategically oriented in the factory, bringing it back to a coin toss.
Lovel said:
The tops of the pistons if original will be marked A or B. On the underside of the barrel the bore type A or B is marked, hence if you have different piston/barrel fits you could match whichever piston goes where, however if both same fit that will not work.MKIII piston orientationMKIII piston orientationMKIII piston orientationMKIII piston orientation
Click to expand...
Mine are both size B but thank you for this suggestion. Looking at your last photo I can probably sort out the orientation for the piston based on the internal stampings and the bit of cylinder barrel showing. But it sounds like pistons may not have been strategically oriented in the factory, bringing it back to a coin toss.
 
Madnorton said:
If mixed up then a check of the piston clearances are would be where I would start to ensure one is not way out and the other too tight.
Click to expand...
If found good, you need to check identical compression height above wrist pins, as well as equal weight.

- Knut
 
Grew up with Starboard and port, Timing/PTO was confusing at first.
Have made the transition, now either one are instaneous.
 
When I started sailing years back I was taught “ port is left because there is no port left “

Slurp ……. Yummy !!! Drunk plenty of that stuff.

Is there any similar and easy saying for timing / mag / pto ??
 
Ain't using port and starboard!. Mind says port it towards the port and starboard means the view not blocked by the port! Never been a sailor and the old mind is not interested in changing. I have no clue where those terms came from.

British bikes, drive side and timing side work just fine for me. I understand when people say primary side, but I don't use it.

For wiring I use "White is bright" for the high-beam Blue/White wires and "White is right" for the timing side turn signals Green/White wires. I never considered them racially intensive until a white person pointed it out. I never even thought that way - they rhyme which is why I remember and use them.
 
Dan1950 said:
Use Starboard (right) and Port (left) in the direction of travel to avoid any future confusion.
Click to expand...
If that's all they mean then just use left and right and assume everyone is thinking only of the direction of travel.

On the other hand, I know a couple of grown people that cannot reliably raise the hand you tell him so left and right are not very good either. Righty tighty and lefty loosy also escape them without a good bit of thought.
 

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