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To Hone or Not To Hone : BMW Airhead Quandary

When the old BMW Airhead refused to hold a valve adjustment, the question became: to hone or not to hone? Rich Burgess has been working on a refresh of his 1982 BMW R80G/S for a while now, but with 80,000 km and 33 years on the bike’s clock, some work has to be expected. Recently he got a heads-up from fellow CB writer Nancy Irwin that the G/S’s rockers develop a flat spot as the miles pile up. That, and other conditions got Rich thinking.

A refresh of my 1982 BMW R80G/S has been ongoing for a number of years now; finally I have decided to tackle the engine. With over 80,000 km on the clock it was time to open it up and have a look. What I found may be more or less typical of old airheads (I am talking about the bike). 

In fact the BMW Airhead had been running reasonably strong when it got parked 18 years ago although with an occasional small puff of blue smoke. The more concerning issue was it didn’t seem to be holding its valve adjustment for very long.  CB’s own Nancy Irwin gave me a heads-up on that—the rockers develop a “flat spot” as the miles pile up. So upon teardown, inspection revealed flat spots on the rocker arms, valves that were still sealing but badly worn (too bad to be reground), valve springs that were just a little tired, cylinder bores that measured like new, piston rings that were worn to the point of being way out of spec. And of course there was some carbon buildup on the heads and pistons.

The rockers were easy: I found some super-low mileage ones on eBay. As an option I could have had the originals welded, reground and heat-treated. BMW started using “hardened” Niskasil cylinder bores in 1981—Nikasil is a trademarked coating for engine components, mainly piston engine cylinder liners.

The question is to hone or not to hone? Some say don’t hone, put new rings in and “ride it like you stole it,” reasoning the hardened surface is not suitable for roughing up. I know this works for some racers but my GS won’t be a racer. Some say a light hone is needed or you will have an oil burner. Well, there are times you have to ask the pros who will know for sure. 

In my case it was a trip to Motorrad Performance in Turner Valley, Alberta.  Mike Gambler has a great reputation and when it comes to engines he is about as good as it gets. Paul Shore also knows his way around a BMW, and what the two say carries much weight. Mike explained that the confusion could stem from people using the wrong kind of hone and ruining cylinders. I got him to hone mine and they look great, with no worries about the break in process.

Anyway I did look into honing Niskasil; it is a little different from doing, say, a Harley-style “iron liner.” With most cylinders there are three steps (some cylinders require torque plates so the machined bore is straight when the head is torqued down).

They get bored to about 1.5 thousands of an inch under the size you want with a boring bar.

Then a rough hone with a ridged hone to make sure they are perfectly straight, true and round. 

Next is a fine hone (about 200 grit in roughness) to give the desired surface roughness and a crosshatch pattern for the rings to seat against. 

Then it’s a must to wash the cylinders in hot soapy water, dry and oil right away. My wife actually let me use the kitchen sink (you might have to wait till yours is out shopping or something). Take your time and don’t stop washing until the drying rags come out spotless.

With a BMW or other “hard” cylinder bore that measures in spec you can ignore the first three steps and wash after going directly to step four:

Plateau-honing is done for “surface smoothing.” This extra-fine surface is what the old Beemer needed. Surface smoothing is kind of like a headstart on breaking in the rings to cylinders interface. There are companies such as Mongoose machine that can bore and recoat hard surface cylinders if they are too far-gone but it was not needed in my case.

In coming issues I’ll discuss pistons and rings in general and reviving the cylinder head for the old BMW Airhead.

by Rich Burgess Canadian Biker Issue #311


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