Snowbum's BMW Motorcycle Repair & Information Website

When you are ready to reinstall the cylinder, and assuming the piston was not left inside it, it is best to do it using the proper ring compressor; or, being careful, using your fingers.   You can use your fingers if strong and careful ...or make up a thin metal compressor from a tin can or shim stock ....or, get the correct type of ring compressor.  There are many types of ring compressors, be sure to get the type that allows usage at the area between engine casing and cylinder base ...these are types that work when the piston rod is still attached.

The cylinder base and the engine case must be very clean, and not at all oily either, before you apply sealant.  You want to avoid getting any sealant into the oil passageways at the two top studs.  You want the sealant to be applied very thinly &not leaving even a single brush bristle.  I usually do it with my fingertip and not a brush.  Less sealant and careful is better than too much sealant wrongly placed.  I want to emphasize, again, that the amount of sealant used is as thin as you can apply (but evenly).   For the cylinder studs areas, apply the sealant around the outer case portion at the top two stud areas, keeping sealant thinly applied (and out of the stud oil passageway at those top two studs).  If you have compression lowering gaskets, those need sealant on both sides.

DETAILS:
Sometime during the 1978 production year BMW did some minor machining changes to the cylinders;  they added a GROOVE.  The groove was for a large diameter O-ring, used at the base of the cylinder.  Always use a new O-ring. Have the pushrod seals in place, and everything ready to assemble, before oiling that large O-ring.  Just before re-installing the cylinder to the engine, smear the O-ring with engine oil.  That O-ring may swell, and you have only maybe 15 minutes or somewhat longer, to assemble the cylinder, or it may well not seal correctly from swelling ...and can actually cause problems if swollen too much. The original O-ring was 2.0 mm in thickness. Oak once told me that the 1978 models that use the O-ring (some 1978 do not), up until 1980...all without the Nikasil cylinders ...can use the optional 2.2 mm O-ring for better sealing.   There is some controversy over the color and size of these O-rings.  AFAIK, the early 2.0 mm were white, the later 2.2 were black, and if a 2.0 is ordered, you may get the 2.2.  It may be that sometime in the early 1990's that BMW changed the engine casting opening, again, for the O-ring, and thus started up the controversy?   I've never had a problem with any large O-ring, they all have fit OK, and not leaked; because I use sealant properly at the engine case.  I am fairly sure that only the larger 2.2 size is now offered. See my cylinders article:    https://bmwmotorcycletech.info/cylinders.htm

Last versions of the cylinders had a step machined into them, which must be machined off, or the engine case machined to match that cylinder, if you are swapping very late cylinders into an early engine.    See prior paragraph link to the cylinders article.

Sealants discussion:   https://bmwmotorcycletech.info/chemicalsetc.htm

I prefer to use engine oil on the outside of pushrod tube seals where they slide into the engine block, so they slide in smoothly as the cylinder is mated and pressured to the engine block. I have preferred a faint fingertip smear of silicone grease (common dielectric grease from auto parts stores) on the inside bore of these seals ...so the pushrod metal tubes can move faintly and smoothly during engine heat-cool cycling.  I believe that some pushrod tube rubber wear comes from metal heat expansion, not just repeated heating cycles, with dry tubes; ...silicone grease may help keep the seal from grabbing on the pushrod tube, especially in the critical new stage of operation. I have not seen incompatibility with pushrod seals, so far.  Install pushrod tube seals with the mold line directly downward.

In some instances of small oil leaks, and you have dressed the pushrod seal hole, etc., a small amount of RTV will effect an oil seal; this is for use with new pushrod seals. Never reuse pushrod seals.  The pushrod tube seals are one of the few places I may add a faint amount of sealant, where BMW never mentioned doing this.  While RTV is OK, Permatex #2 (non-permanent) Form-A-Gasket works nicely and does not tend to leak, even over a lot of time/miles. It does not peel or disconnect, like some RTV can.    Use the Form-A-Gasket in a quite thin amount ...use your fingertip. Use on well-prepared block and pushrod tube surfaces.  Nicks, brush bristles, old sealant and such are absolutely not allowed. I'm pretty careful about making sure the surfaces are good ...I never wanted to have a customer come back and say my pushrod seal job was leaking.   NOTE!   The rubber compound used on the pushrod tubes seals may have varied over the years.   If you want additional protection, then I suggest thoroughly cleaning the outside, after they, and cylinder, is fully installed and torqued-down. Use a strong solvent, such as acetone, MEK, etc.   THEN coat the rubber with your favorite rubber or plastics protection product.  I use a spray type.   You CAN use a product that is sold to protect the dash of cars, etc.   There are quite a number of these products available.   You can wipe off any excess that gets on the case metal.    These products will protect fairly well against the UV of the Sun, etc.     If you plan to use such a product, try to avoid silicon grease being on the seal outsides.

With the new pushrod tube seals in place and the mould line straight up/down; the rod re-attached to the piston if it was removed, piston & pin in place and cylinder contains the piston with its rings....and your sealant applied at the cylinder base (do this exactly as required, in the right place, no brush hairs, etc.), ....you are ready to reassemble the cylinder and then the head to the engine.  If you have a large O-ring model, now oil it sparingly, just before you are ready to draw up the cylinder to the engine. I am careful not to drip oil on the base sealant.    Install the cylinder onto the piston.  Install the head gasket (be sure the gasket is put on correctly, if wrongly, a small amount of the pushrod hole in the head will be covered by the gasket and the pushrod will rub the area).  Reassemble the rocker gear onto the cylinder head, and the 6 nuts, and you are ready to begin tightening.

I suggest giving a small amount of bias (slightly higher torque during initial tightening ...you are not likely even using a torque wrench yet for this) on the two lower rocker nuts ...to avoid any slight canting of the cylinder due to the pushrod seals acting like springs.   Be sure to 'stage' the torqueing of the 6 nuts, in a criss-cross-pattern.   The exact value of the staging is not critical, some use 11 ftlbs and then 18 ftlbs, and topping out at 25 ftlbs or a maximum of 26 ftlbs, all with a known good torque wrench.  I use 25 ftlbs as the final value on all Airheads.  Use of beam wrenches of the type without round dials on them is dangerous to your engine block threads. Round dial beam wrenches are extremely costly, and used industrially.   I suggest you use a known good tubular clicker torque wrench.    Although the non-Nikasil cylinders were originally specified to be torqued higher, don't do it.  Torqueing above 26 ftlbs is a BAD idea.   Some old literature will specify 29-31 ftlbs.  DO NOT!!

Avoid replacing pushrod tubes, as the process is not pleasant, and a special mandrel tool needed or you can substitute the front axle (sometimes), and you may ruin at least one new tube in the learning process.  Aftermarket stainless steel ones are the way to go if you do replace them.  You need a well-fitting mandrel. Seibenrock makes one.  More information later in this article, because some of you will someday replace tubes.  Perhaps yours are rusting or leaking.  Further down this article are details on how to replace the pushrod tubes.

Except for the earliest models, the top two cylinder studs have O-rings located at the engine case.  Always replace them if the cylinders are pulled even slightly off the engine base.   If you do not replace them, you will likely have oil leaks.   Be very careful about using sealant.  See my article,  https://bmwmotorcycletech.info/chemicalsetc.htm.  Be especially careful with sealant around the upper two studs ...that is where the oil, under pressure, comes out for the rocker's oiling.   The engine oil, under pressure, is applied to tiny holes located IN the engine case area where the TWO TOP studs screw into the engine casting.  All except earliest models use small O-rings at that point, and the design is such that oil under pressure follows the outside of the studs up to the cylinder head.  Any sealant around the studs must NOT be squeezed into the oil channel when the nuts are all torqued down, and that sealant is specified to be used on the OUTSIDE flat surface surrounding the studs.   Use sealant very sparingly!  Do not let brush bristles get into the coating ...if they do, remove them.  I do this job with a fingertip!  It is a good idea to clean the studs before re-assembly ...you do not need crud/dirt in the rocker bearings!   I clean the cylinder passageways too; something hardly ever mentioned anyplace.

I always use a sealant on all cylinder bases, no matter model/version.  Use sealants very smoothly, very little, using your fingertip to be sure it is thinly and smoothly applied.  Mostly avoid the area towards the case cylinder (97 or 99 mm) hole (from the studs), but do coat on the outer area of the studs, that case FLAT area.  That is, seal the outer portion of the cylinder base and case.  If you have an Airhead with compression-lowering plates, you must coat all surfaces. If you use a brush, leave NO bristles!  DO NOT slather on sealant!

It is critical that the base surfaces be nick free (absolutely no metal proud of a FLAT surface), and, flat.  Surfaces MUST be well cleaned with an evaporating solvent, before applying the minimal, THIN coating of sealant at the cylinder base area. Do NOT get sealant near/into the oil passageways at the top studs!    THIN layer only!   When I used Hylomar exclusively years ago, I diluted the Hylomar with a few drops of acetone and applied it with my finger ...especially carefully around the top studs. Hylomar is not necessarily the best sealant these days, although it is a safer sealant.   I have seen Hylomar failures, but not very often.   Some other sealants require setting-up time.   See my chemicals article for discussion of sealants: https://bmwmotorcycletech.info/chemicalsetc.htm.

The way most engine cases get oil weeps, besides improper sealant installation, is from the piston rod falling when you are removing the cylinder with the piston intact in it, creating nicks on the edge of the engine case cylinder opening.  Use a rag under the rod to prevent nicks.  If you have nicks, you MUST dress them out.  The faintest nick, that has proud metal on the case, will allow leaks.  Someone previous to you may have left nicks.

Keep in mind that the pressure exerted at the cylinder base (when you torque the 4 rocker mount nuts) will squeeze out the vast majority of sealant if you apply too much sealant ...and it can squeeze that sealant into the oil passageways  ...and thus, the engine innards. DO NOT over-apply the sealant!  I sometimes even thin the sealant.   Use your fingertip to apply the sealant. Do NOT lay a bead of sealant down!  DO NOT leave brush bristles on the cylinder base area ....they WILL cause leaks.

Retorque the heads & check rocker arm end play after a day of sitting. Then set valves, checking the full rotation of the rockers first (the valve clearance must be enough, that is, adjusters very well-loosened, in order to check full rotation of the rockers). Repeating: Check rockers end play too. Follow the procedures elsewhere's on this website.  The earlier the valve gear version, the more care in aligning the rockers (such as the very movable /5 types).  1985 and later models, or any earlier models having some or all of the 1985 parts, may have no rocker blocks alignment adjustment possible and 1985+ are shimmed models.

Since you have installed a new head gasket, check the torque and valve clearances at 500 or 600 miles, and again at 5000 miles at valve adjustment time.   If you have a fresh valve job, then I'd check them more often after the first adjustment ...noting when they become quite stable (less than .001" change per 5000 miles is considered stable).   I like keeping records of the valve clearances as found, before any retorquing & resetting.  Instructions on how to set the valves are elsewhere's in this website; primarily here:  https://bmwmotorcycletech.info/setvalves.htm

I seldom check head torque after 5K or so.  To do so you must back off the 6 nuts (ONE at a time) a small fraction of a turn, then re-torque each one separately to 25 footpounds.  If a nut squeaks, back it off, OIL the threads, then re-torque. (Do not specifically oil the two HEAD nuts, although any existing faintly oily film is OK).

Pushrod tubes: