Assembling the old /5 motor...

Discussion in 'Airheads' started by helion42, Dec 3, 2012.

  1. helion42

    helion42 Been here awhile

    Jan 16, 2011
    Salt Lake City, Zion Curtain, UT
    I'm about to put my lump back together and since my build thread gets buried/ignored/whatever I decided some fresh thread is in order. For reference, it is a all-original 1972 swb R75/5, but the motor was re-assembled for the PO by damn monkeys about 22 years ago, and I don't know the extent of badly they screwed up - there was red RTV on all surfaces - base, HG, VCG. Some even oozed into the pushrod tubes. Eww. No wonder he said it ran rough.

    I know most the basics from rebuilding a BMW M20 engine - very very light sheen of oil on it all, rings right side up and offset, head gasket and VC gasket dry. I plan to use a very thin bit of Hylomar on all 4 surfaces of the base unless otherwise convinced - that's just what I've been able to find on the interweb. Any other basic tips are appreciated, since I'm brand new to the boxers, and am pretty much just going off the Clymer manual, Snowbum and whatever else I dig up. just don't know what to look for, ya know?

    Question time.
    What I'm not familiar with these engines is if there's an o-ring at the base. There's a groove around the bottom of the jugs where it goes into the block, but I thought the o-ring was on later engines? I have no idea what that's about. Can post a pic if needed, there are also pics in my build thread for those not too lazy to check it out. Also how the fk does oil get to the rocker arms? up a block stud?

    The block surface (where the jugs mate to the case) is dented, very lightly on the edge of the hole where the piston skirts committed suicide on them. lesson harshly learned so shh. Can I just file & sand it smooth again? Or would it need machining? It's just the edge of the hole, not the whole gasket surface.

    Any other info about starting up a two-decades-dormant bike? Engine bearings all seem decent, tolerances are ok, everything has less than 9000 miles on it. Brand new gudgeon pins. 1st overbore pistons and rings; Jugs were professionally bored and honed for the pistons, although I have to figure out if I can check ring end gap without removing the rings if anyone can help on that.

    Any info helps!!!!! :deal
  2. Bill Harris

    Bill Harris Confirmed Curmudgeon

    Sep 20, 2008
    backwoods Alabama
    Completely different critter.
    That annular groove is the cylinder oiler. Note there is a ~1.5mm hole at 12:00 and 6:00 in the cylinder wall. There is an oil supply hole in the 12:00 position of the block bore.

    Hylomar used to be the sealant of choice. Nowadays Yamabond 4 non-hardening sealant is suggested. Same as the BMW Dreibond. Make for certain that you don't block any oil passage with sealant.

    The two upper cylinder studs. There is a recess around the stud and a 3mm drilling leading upward to an oil passage between the two studs (same on that supplies oil to the cylinder oiler). Make sure that the oil passage is not blocked with sealant, and if the stud threads have been repaired (helicoiled) make sure that the oil drillway is not blocked bythe thread insert. Oil flows around the studs up to the rocker arms.

    That happens, normally from the rods flailing around. Use a ziptie between the two lower studs when things are apart to prevent that. You can lightly and carefully touch up the dings.