R75 /6 Cylinder Recondition/Nikasil Coating

[sverige78]

1971 R75 /6 Cylinder Recondition/Nikasil Coating

Friends of MOA,

So first let me just say thanks for being such an amazing resource, I finally bucked up and bought my membership, as having 2 boxers in the garage (2106 R nineT, 1971 R50 /5) would like to start being a member of the community and giving back where I can. I am a Mechanical Engineer by education, been working on air-cooled engines since I was kid (VW's mostly, but the odd Honda Dream and Norton here and there) and am thoroughly enjoying the down to frame strip I am doing on my recent R50 acquisition. Introductions aside, on to my question....

Even thought the bike is a R50 /5, the previous owner at some point switched the engine/trans out to a R75 /6 (build date 01/74), and when I bought it had an unknown mileage and has been sitting on the side yard for 17 years. Anyway, after removing the heads and jugs/pistons, it is pretty evident there was a lot of blow-by and this engine has seen some miles and is in need of at the very least a top end refresh. Being as how it seems very difficult to find a new set, and doing a lot of reading, it seems the Nikasil cylinders are the way go which gave me three options; To either purchase one of the big bore kits, source a set 81-84 cylinders/pistons in good condition, or recondition and coat my existing cylinders. I opted for the latter for cost effectiveness, and reached out to Millennium Technologies and had a great talk with Jeff about the process, however was cautioned to not use chrome faced rings as it would damage the coating. That said, I need to source a set of rings for the 82mm pistons I have which will be compatible with the coating.

So, long story, short question, can I use the OEM replacement rings for this model R75 safely in my coated cylinders (details of the construction/material of rings seems hard to find), or is there another source for rings I should be looking at?

Thanks.

-Christian

 

[brook.reams]

Friends of MOA,

So first let me just say thanks for being such an amazing resource, I finally bucked up and bought my membership, as having 2 boxers in the garage (2106 R nineT, 1971 R50 /5) would like to start being a member of the community and giving back where I can. I am a Mechanical Engineer by education, been working on air-cooled engines since I was kid (VW's mostly, but the odd Honda Dream and Norton here and there) and am thoroughly enjoying the down to frame strip I am doing on my recent R50 acquisition. Introductions aside, on to my question....

Even thought the bike is a R50 /5, the previous owner at some point switched the engine/trans out to a R75 /6 (build date 01/74), and when I bought it had an unknown mileage and has been sitting on the side yard for 17 years. Anyway, after removing the heads and jugs/pistons, it is pretty evident there was a lot of blow-by and this engine has seen some miles and is in need of at the very least a top end refresh. Being as how it seems very difficult to find a new set, and doing a lot of reading, it seems the Nikasil cylinders are the way go which gave me three options; To either purchase one of the big bore kits, source a set 81-84 cylinders/pistons in good condition, or recondition and coat my existing cylinders. I opted for the latter for cost effectiveness, and reached out to Millennium Technologies and had a great talk with Jeff about the process, however was cautioned to not use chrome faced rings as it would damage the coating. That said, I need to source a set of rings for the 82mm pistons I have which will be compatible with the coating.

So, long story, short question, can I use the OEM replacement rings for this model R75 safely in my coated cylinders (details of the construction/material of rings seems hard to find), or is there another source for rings I should be looking at?

Thanks.

-Christian

Christian,

No, you should not use the original R75/6 rings in the Nikasil bores as they are too hard and will damage the bore. I do not know of a source for the correct size rings that will work in the R75/6 cylinder bore.

Be aware that in 1976, the engine case was bored out to 99 mm in preparation for the 1000 cc engines, so an early case such as yours will NOT accept a 1976 or later cylinder without machining the engine case which requires complete disassembly of the engine.

Hope this helps.

Best.
Brook Reams.

 

[20774]

Christian -

Welcome to the MOA and the forum! Snowbum echoes the warning against using chrome rings, rather iron rings should be used on Nikasil-like surfaces. See this page and the top section where he discusses Nikasil and Galnikal. Personally, I'd probably use BMW rings and discuss this with the supplier of your coated cylinders.

http://bmwmotorcycletech.info/cylinders.htm

Personally, I'd probably use BMW rings and discuss this with the supplier of your coated cylinders. That said, any supplier of quality iron ring products should work.

Update...my thoughts are in conflict with what Brooks just posted. Not sure what the issue is, unless if the R75/6 rings were chrome to start with.

 

[beemerphile]

I think the issue is that there was never a Nikasil 750cc motor and so the market would consist only of people who had Nikasil modded an older bike. Too small a market to service.

 

[sverige78]

Displacement Increase ?

Brook, Kurt, Lee,

Thanks for the feedback, and echoing the information I found from Snowbum... It seems that outside of trying to source custom rings (which I will continue to look for), opting for one of the 97mm "Power Kits" might be the easier way to go to ensure long life of the engine rebuild. I would prefer not to bore my case to 99mm to keep it original though, but will be stripping it out to check the condition of components and refresh as necessary.

If I do opt for the 1000cc power kit, will the current jets on my 32mm Bing carb be wide enough to accommodate the displacement increase and fuel needs, or will I have to re-jet? I ask as the jets are in need of replacement and I would prefer to buy once rather than twice

Thanks again for the info!

 

[rs36vh]

Rings

Take your measurements and look at the Hastings website , they have all kinds and sizes, I have a 1911 Indian and needed rings and they helped me out. P.S. they will take calls and offer help.

 

[20774]

I'm sure you would need to rejet. I have 32mm carbs on my R100/7 and put on the standard Siebenrock 1000cc kit, not the power kit, so I left carburation alone. But I'm sure your 750 cc carbs/jetting wouldn't be appropriate. Likely you would start with some common 1000cc CV carb settings and make adjustments from there.

 

[Jeff488]

Seems to me that whichever jet provides the correct amount of fuel for the amount of air flowing through the carburetor is the one to use. More air pulls more fuel through the jet.
I'm not convinced the carb cares much how big the cylinder is.

I'm certainly no carb expert and could be wrong.
Surely someone out there has the hot skinny on this.

 

[20774]

More air pulls more fuel through the jet.

True. But the hole in the jet is only so big and if the cylinder is bigger and draws more air, then the jet will restrict the amount of gas that can travel through the hole. Thus the hole needs to be bigger.

 

[Jeff488]

I still don't see it.

If the carb provides the correct fuel/air ratio with a 325cc cylinder, how does that ratio change with a 450cc cylinder?

After more consideration, I can see that if the 32mm throat will not flow enough air, then a lean condition will occur, or perhaps power will just drop off.

At 6k rpm a 450cc cylinder a needs about 47 cu/ft of air per minute and a 325cc cylinder needs about 32 cu/ft. I could not find air flow rate info on the Bing carb to see if it flows enough air.

I got the formula from the Holley carb web site.
(cubic inch times max rpm divided by 3,456 = flow rate desired.)

 

[20774]

I think we agree that the larger cylinder is going to pull/suck more air when it goes from top dead center to bottom dead center. More air is going to need more gas to maintain the same mixture ratio. Say you have two main jets (there are other jets involved, too) that could be used...one has a diameter of 0.001" and another has a diameter of 0.01". The piston is going to move regardless of what the jet is doing. But since gas does not compress, the smaller diameter will only let a small amount of gas through while the bigger diameter will let more through. If the smaller diameter is too small, then gas will not get through to match the larger volume of air and the bike will run lean. For a bigger diameter, more gas will get through and if the diameter is right, the proper amount will get through.

Have you heard about a way to size the main jet on a bike? I haven't done it but it's interesting to try. Select a main jet and let's assume that you select one that it's too small. When the bike is being ridden wide open throttle (WOT), the bike will reach a top speed that is dependent on the amount of fuel that can physically make it through the jet. Increase the size of the jet. The bike will generally run faster. Eventually, there will be no more gain in top speed as the jet size is increased. At that point, you've basically found the right jet size for the engine.

All the jets and the needle play a role in the amount of fuel that is metered to the engine and it needs to be sized to match the air being consumed by the pistons.

 

[Jeff488]

'More air is going to need more gas to maintain the same mixture ratio."

I agree.
As I understand it, air velocity past the venturi determines the strength of the low pressure condition which pulls fuel through the jet.
Therefore, increased air velocity pulls more fuel within the limitations of the system(jet and venturi size).

If the 32mm carb will flow enough air to fill the cylinder of the 450cc cylinder at normal engine speed, then enough fuel will be pulled through the jet because the fuel/air ratio remains the same.

If BMW fitted a larger throated carb to the 900cc engine, they certainly had vastly more knowledge on the subject than I, and fitting R900 carbs would be the thing to do.

I bet Snowbum has the most complete explanation!

 

[20774]

Therefore, increased air velocity pulls more fuel within the limitations of the system(jet and venturi size).

This is it exactly...within the limitations of the jet size...only so much gas can get through.

From the Bing manual for carb settings:

R75/6 - 64/32/13-14 carbs:
- main jet 145
- needle jet 2.66
- jet needle 241
- atomizer 591
- idle jet 50
- clip position 3

R90/6 - 6432/11-12 carbs:
- main jet 150 (BIGGER)
- needle jet 2.68 (BIGGER)
- jet needle 241
- atomizer 591
- idle jet 45 (SMALLER)
- clip position 3

Not sure I see the smaller idle jet, but there could be internal passageway changes. But for the two important settings where you spend probably 95% of your time, the needle jet and main jet are bigger. Bigger lets more gas through for the more air that is being used.

BMW didn't fit a larger throated carb to the R90/6...they just increased the jets.

 

[Jeff488]

Aha!

From the numbers it looks as though they are both 32mm carbs.

You are correct and I stand in humble deference to your knowledge.

I learn something, reluctantly, most days.

Thanks.

 

[cycleman2]

If you go the big bore route you will want to go to 40 mm carbs. With the larger bore you will need more air as well as fuel to get the engine to perform.

A first overbore of the stock cylinder and you could have kept everything else stock on the engine. That is likely the way I would have gone as there was nothing wrong with the older iron cylinders, they lasted a long time and caused few problems.

As you will find out when rebuilding your engine there was some differences in the early 750 block, when dealing with seals etc. Make sure you pay attention to the build date of your engine serial number when ordering parts or order both sizes ( they are cheap ) to make sure you get the right one. Only a few seals involved.

 

[20774]

Jeff -

You made me look!! I figured it would come down to numbers.

And I'll let you know what I find the truck stop!

 

[typ181r90]

This is it exactly...within the limitations of the jet size...only so much gas can get through.

From the Bing manual for carb settings:

R75/6 - 64/32/13-14 carbs:
- main jet 145
- needle jet 2.66
- jet needle 241
- atomizer 591
- idle jet 50
- clip position 3

R90/6 - 6432/11-12 carbs:
- main jet 150 (BIGGER)
- needle jet 2.86 (BIGGER)
- jet needle 241
- atomizer 591
- idle jet 45 (SMALLER)
- clip position 3

Not sure I see the smaller idle jet, but there could be internal passageway changes. But for the two important settings where you spend probably 95% of your time, the needle jet and main jet are bigger. Bigger lets more gas through for the more air that is being used.

BMW didn't fit a larger throated carb to the R90/6...they just increased the jets.

I think you mixed up the numbers for the R90/6, it should be a 2.68 needle jet, not 2.86.

R100s did come with 32mm carbs, MaxBMW sells I think 4 or so main jets when you look at the fiche for an R100/7 if you want to play with tuning and deviate a little from stock.

 

[20774]

I think you mixed up the numbers for the R90/6, it should be a 2.68 needle jet, not 2.86.

Yikes! That would equate to 200 mph!! Thanks for the catch...I've updated my original post.

 

[paddlefoot64]

My bike is a 1976 R75/6 with 1000cc displacement now. Carbs are 64/32, but I'm not sure the main jet size. Regardless, it runs pretty good. Craps out around 105 mph, but gets there fast and has enough torque.
I may experiment with jets after I see what size is in there.

 

[washpark]

Nikasil vs 1000cc Kit

I am making the same decision regarding 1000 cc kit or Nikasil coating the worn cylinders on my 1975 R90/6. The shop doing the plating indicated the BMW rings would be fine. The top ring is chrome. Everyone else recommended that I not try the chrome rings.

Bud Provin at the Nickwackett Garage, a respected airhead mechanic, suggested contacting Ed Law at Total Seal, Inc. in Phoenix, AZ. for plain rings. I sent Ed my relatively new BMW top ring and he provided two iron rings that matched. The rings he provided were "cut" to match. Total cost including setup, cutting, shipping, and handling was $46.12. I still have not pulled the pin on the chroming. $500 plus shipping vs 1000 cc kit from Motobins in the UK. The kit includes top end gaskets, pushrod tubes, and tube seals.

My local Airhead mechanic thinks I would be better off with the original displacement. He feels the 1000cc kits result in additional vibration.

I am leaning toward refurbishing the existing cylinders.

Mike Washington
1975 R90/6, 2011 F800R