Reboring R75/6 Cylinders

[Andey]

I have a spare set of cylinders that I'm looking at an overbore to go up 1 size and to install larger pistons.

Any info or advice on this? I'm reading Brook Reams take on the cylinder/piston measuring and didnt realised the piston is oval... is that right?

Before I took measurements and just compared it to the Clymers manual to see if its within spec... but just seeing if theres any advice on what size I should actually bore the cylinder down to, for the next size up.

[schrader7032]

I don't think cylinders are oval...probably what Brook was discussing is measuring to see how much ovality there is as a way to determine how much the cylinder is worn. On my /7, I tried my own measuring of the cylinder but I could never get consistent results. I took it to an engine repair place and they did it for me.

Snowbum mentions that there's a special hold-down approach to ensuring that the cylinder remains stable during the process. He lists his sources for cylinder work in this link under the letter "C":

https://bmwmotorcycletech.info/references.htm

Typically you get your dimensions and decide if you need to bore to an oversize...they are in 0.5mm increments. Then you buy the piston and rings and give them to the shop doing the work. They bore to match the pistons to meet the required specs for piston-wall clearance and ring end gap. A place like Bore-Tech can apply a carbide coating after boring to help with future wear...this process is similar to the Nikasil coating used on post 1980 cylinders.

[Andey]

I don't think cylinders are oval...probably what Brook was discussing is measuring to see how much ovality there is as a way to determine how much the cylinder is worn. On my /7, I tried my own measuring of the cylinder but I could never get consistent results. I took it to an engine repair place and they did it for me.

Snowbum mentions that there's a special hold-down approach to ensuring that the cylinder remains stable during the process. He lists his sources for cylinder work in this link under the letter "C":

https://bmwmotorcycletech.info/references.htm

Typically you get your dimensions and decide if you need to bore to an oversize...they are in 0.5mm increments. Then you buy the piston and rings and give them to the shop doing the work. They bore to match the pistons to meet the required specs for piston-wall clearance and ring end gap. A place like Bore-Tech can apply a carbide coating after boring to help with future wear...this process is similar to the Nikasil coating used on post 1980 cylinders.

I was mentioning about the pistons being oval... I think that’s what Brooks was referring to... but I’m actually a little confused...

So if the cylinder is out by 0.010mm, I bore to the next size up... which is suppose to be 82.5mm?

[srankin]

I have never heard of pistons being oval. I have heard of cylinders wearing out in an oval shape.

As for the size of replacement pistons and the size to bore too, I will defer to Kurt and Snowbum. St.

[Andey]

I have never heard of pistons being oval. I have heard of cylinders wearing out in an oval shape.

As for the size of replacement pistons and the size to bore too, I will defer to Kurt and Snowbum. St.

Yeah I thought it was strange to hear that too... Cylinders being oval makes sense due to the nature of the design and how I imagine it would wear (gravity playing a big part I guess).

But not sure about pistons...

From Brooks:

Pistons are oval, not round. So the piston diameter is the maximum diameter which is found across the piston skirt perpendicular to the wrist pin holes. This is sometimes called the thrust side of the piston. The side of the piston perpendicular to the wrist pin is the side pushed, or thrust, into the cylinder wall by the connecting rod as it goes around the crank. This is the side of the piston that usually wears more against the cylinder wall due to increased frictional forces.

[schrader7032]

Brook certainly knows what he's talking about, but he specifically mentions the skirt. I don't know why the skirt would be oval. However, the rings are definitely not an oval shape and I don't see how the cylinder would ever be anything but circular. When I measured my /7 cylinders, the fact that they weren't circular and were beyond the required specs, I had to replace them.

[srankin]

https://bmwmotorcycletech.info/cylinders.htm
https://bmwmotorcycletech.info/pistons.htm

[jwonder]

Pistons are almost always oval, or should I say GOOD pistons are oval. Good explanation of why is here: http://blog.wiseco.com/pistons-arent-ro ... -explained.

[schrader7032]

Really? I just measured my old original /7 pistons at the top near the combustion chamber...they were as close to round as I could see. At the other end, around the skirt, it might have been 0.2mm less than round but could be how I was trying to measure it.

The cylinders are round, right? If the piston is not round, how does that affect the rings which must ride in the grooves in the piston? That would mean that the rings are not evenly supported all the way around because the piston ovality supports less of the ring in one region versus another.

Seems strange to me...oval pistons.

[srankin]

Personally, I think it is the skirt of the piston which may be oval, that makes sense to me because I can understand the forces on the piston imposed from combustion and transferred to the connecting rod.

I also measured a BMW cylinder top and it is round as far as I measure and I measured pretty darn well multiple points a couple of different ways. The skirt on the other hand is slightly oval.

I can also understand cylinders wearing in an oval shape but new, they are cylindrical. I can envision the forces of combustion coupled with mechanics restraints of the system causing oval wear.

Show me a BMW airhead piston which is obviously oval at the top and I will believe, until then, I think maybe some people are confused. I hate to say it but I believe my eyes in this one. St.