T H E O L D G R E E N B E E M E R
RIDING A 1974 BMW R90/6 FLAT TWIN
NEW GUIDES ARE IN AND ...VALVES STEMS DON'T FIT
Don't be alarmed this is perfectly normal
New guides can be pressed or hammered into the heated (190 + - celcius ) head
I went for the hammer and a snug fitting mandrel. Inserting the guides is a
straightforward operation, I had some worries first but this was
really an easy thing to do and nothing to worry about
FORGOT TO TELL YOU
Taking the old guides out is straightforward as well. First remove ( drill )
the upper part of the guide up till the circlip, heat up the head first
and knock out the other half towards the combustion side.
But ... as the guides in the head are in an angle you are probably going
to perform this task freehand, now you have to be carefull ...
To make sure I wasn't going to turn this job into a disaster I prepared the drill first
by grinding the tip into this shape. Now I was confident the drill would stay inside
the guide. Also important DON'T use a very sharp drill as it will bite away in
the guide material, you don't want that to happen! Slightly dull the cutting
edge and you will have good control over the removing process.
NOTICE: GUIDE TO VALVE STEM CLEARANCE
Important: Do not install original BMW guides as they are too loose a fit
for the valve stems ( almost at wear out specs ) If you do use original
BMW guides your bike will consume more oil than neccesary.
Wear out will be higher as well as the loose fit will allow the
valves to open and close in a sort of chewing action.
Have you ever seen a Camel chew? Have a look.
This is the mandrel I have used for driving the guides into the head. A piece
of outboard engine shaft and an 8mm bolt screwed into it. Note the small
recess into the end of the shaft, this is to clear the oil scraper rim
that's on the upper part of the valve guide. Don't flatten that !
During cooling down the head shrinks, guides inclusive, tightening them up
A tool is needed to bring it back to size, but instead of using an expensive
reamer tool it's far simpler and way safer to go for the ball burnishing
method. An 8mm hard steel ball is going to be forced down the
bronze valve guide, stretching the tightened area back up.
Normally this method is used on sleeved valve guides but as you
can see here it works for solid guides as well.
With the help of a 6mm steel rod with a dimple at the end
And again " Hammer Horror '' !
A single or a few passes will do the trick. This method gives total control over what
you will wind up with. A reamer is a cutting tool taking away material, if such a
tool is out of specs or run through the guide in an akward manner you're
in trouble. The steel balI is self aligning and if needed can be run
through a couple of times to get the right amount of play
See what I mean, perfect fit ... no side play to the touch
I am not the inventor of this method, the man below gave me the idea!
Thanks very much indeed Shopdogsam!
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