Re: Material for valve guide <cross posted>

[O2002@xxxxxxx]

In a message dated 2/9/99, Horst Apfelthaler writes:
>  I'm currently rebuilding an M20/B25 head ('88 325i) and noticed that some
>  valve guides are worn.
>  The problem is that I can't get new guides in the required diameter because
>  somebody 
>  already did a head rebuild.
>  So the only thing is to machine the guide myself.
>  Problem: what material should I use. 
>  Some sources say CuZnAl is used, other rebuilders use bronce.
>  Does anyone have specs for this material?
>  Regards
>  Horst from Austria

Hallo Horst,
Manganese bronze is an excellent material for replacement valve guides.  And
you can get any size you need, for cheap, through the US aftermarket.  Have
your shop contact Precision Engine Parts (no retail end-user, or DIY sales).
http://www.mspmall.com/pep/parts.htm
The manganese bronze material used is sourced from Germany, and they are the
largest US manufacturer of guides.  Buy a lot of extra pieces, as you will
find some with too large an I.D. to be suitable.  Since many sizes are now NLA
from BMW & other sources anyway, whatcha gonna do?  If anyone has a better
supplier, email me directly.  
I'm curious to know if you have determined the size required before stripping
the parts off?
I would suggest this is best decided after having removed the existing guides.
In the process of knocking them out, the 'fit' of the bore can be judged, and
you will know if an oversize is required. Does this cylinder head use the 7 mm
valve stems?  If so, I think the original guide OD is .514" and is
approximately 1.7~" in length.  Oversize guides are available for this
application in every .001" above standard up to .519".
The guides can be removed with an air-operated valve guide driver (like a
short stroke air hammer, but less BPM).  Many people use the same tool to
install, but it's better to warm the head and chill the guide in a nitrogen
bath.  That method prevents galling the bore and/or smashing the guide.  After
setting the guides in the head, the ID needs to be resized for the desired
stem clearance.  Using a reamer does not give much sizing flexibility.  If you
want a tighter clearance stem-to-guide, a worn reamer will cut undersize.  But
the bronze is tough even with a new reamer.  My personal favorite tool for
sizing the guide ID accurately is the Sunnen adjustable diamond hone.  Using
this along with a dial bore gage is much better than the common 'ream &
wiggle' method.
Of course, new guides will not be concentric with the old seat cuts, so the
work is just begun...  
Good luck,
 ///  J o h n   A h o
motor city chapter

------------------------------