I was taught that before adjusting valves, you back off the headbolts about a quarter turn FIRST before re-torquing. (This in reference to pre-'69 BMWs specifically.)
A friend thinks I'm crazy because doing that will require the replacement of the head gasket. He further says that the BMW manual does not specify backing off the headbolt first.
Does anyone know of any BMW/Porsche/VW data that specifies anything like that- or am I nuts (at least in this regard, more than usual)?
Uncle Ernie,
I'm still a newbie to airheads, but have been fiddling with air cooled VWs for over 20 years (and I'm not as old as that may make me sound). Not that this is particularly relevant to your question...regardless of the application, if you don't break the stiction in the threads (ie: by backing off the 1/4 turn or so required to 'crack' the nut) before checking the torque (re-torqueing) you are wasting your time. This seems as plain as day to me, but if it matters, it is also specified in VW literature I have. Regardless, I do it on all my engines, including later model water-cooled stuff, prior to adjusting valve clearances. Doing this on one stud at a time and immediately re-torqueing (I do it in the specified torque pattern) will not affect the head gasket. Frankly, I think you could crack them all 1/4 turn and re-torque in sequence with no ill-affect but I only do this with VWs (which have no head gasket to coause problems of course...)
The most important thing with aircooled BMW and VW head torques is not to overdo it, unless you really enjoy fitting thread inserts.
Stout
No direct /2 experience, but correct procedure for /5 and newer. Probably a torque check is not needed every valve adjust.
This is all pretty much what I thought.
Mr Stout, might it be possible to tell me what your reference material is so I can shut this guy up?
I thought it was good practice to back the head bolt's off, then put a "G" clamp onto the rocker block's so as to minimise the end float of the rocker's. Then re-torque to lowest value. Hey-preso quieter engine.
Safe riding Karl
P.S. I've been doing this for 20yrs and not had to replace a head gasket.
As long as the head is not removed or broken loose from its seat then the gasket shouldn't suffer any damage. 1/4 turn isn't likely to allow that to happen.
Uncle Ernie,
If nothing else, my Haynes "BMW 2 valve twins" book says exactly what I said in my previous post (not that I think I've ever read that section before...). Under 'Minor service', step 10 says: Check the tightness of the cylinder head nuts and adjust the valve clearances. The method it describes for checking the head nuts is exactly as we have already discussed, including the stipulation of following the specified torque sequence (backwards if you please!), loosening each nut 1/4 turn and tightening to the specified torque. I like to use no more than 25ft.lb for my BMW (an R90S engine) and no more than 22ft.lb for VWs. I've never had a head come loose yet, but if I never have to install another thread insert it will still be too soon...unfortunately, I inherited my current BMW with a stud already pulled, so there is at least one more in my future.
I'm certain the VW factory manuals I own and a fine book by John Muir "How to keep your VW alive" also stipulate the same process for re-torque of head fasteners and checking valve clearances, but I can't verify this right now as those books are packed away in a box somewhere on the other side of the world.
Hope this helps.
Stout
Back in the middle 70's when leaded gas was dropped and the octane went down BMW had a fix, it was a base shim, if I remember correctly it was a .020 shim, I pulled the barrel and head as one on my 74 R90/6 and replaced it, my son has it now and at last count had over 178,000 mi on it( I know, hard to beleive) and the OEM head gasket, as long as the seal isn't broken there good, a 1/4 to 1/2 turn isn't going to hurt a darn thing.
Two points here:
mount the nuts with anti seize
When one stud comes out in an aluminium block >fix all of them! Long helicoils give more resistance then Aluminuim and you dont have to redo the job again a few thousand km later. (CB650 40.000km 1. time 80.000km 2. time> all studs > and never again until 148.000 and as I was told it still runs without mayor repairs allthough I sold it in 1989.
Sven
Ive worked on 911s for years and have never come across this
procedure in any Porsche teck book.Maybey its a good idea though.
Old 2.7l engines head studs would loosen up.
Cheers
o
