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Head bolt replacement issue
Ryon C Warren wrote:
>On Thu, 26 Sep, btoski@domain.elided (Bruce Toski) writes-
>
>>...Shop suggests replacement to all customers with old bolts still in
>>head. Pull valve cover and replace ONE AT A TIME...
>
>My question, what is the strength rating (i.e. the number stamped on
>the head) of the recommended replacement head bolts. I believe
>the head bolts on my '81 528i are stamped 10.8, which if memory
>serves correctly is 10,800 psi tensile strength. All this talk of
>head bolt failure has me slightly paranoid, and wonder if I should
>change to higher strength bolts.
Ryon and others, the new style TORX bolts are special design, they
are 'stretch' type that are supposed to increase in length as they
are tightened. Once you have tightened such a bolt, it cannot be
refitted and re-tightened -- beware that they are one-time-use only!
The bolt breaking problem has nothing to do with the bolt strength
grade (nor has it anything to do with the bolt head style, but the
bolt head shape is how you identify the different types).
The bolts break because of a design flaw; the radius of the 'joint'
between bolt stem and bolt head is too small. If you have even the
slightest mis-alignment between the bolt itself and the contact surface,
it is a high risk that microscopic cracks develop, and eventually
the bolt head snaps off.
The stretch-type TORX bolts have much bigger radius in the joint,
and in addition the 'stretchability' evens out mis-alignments.
BMW swithed to TORX type head bolts late 1989 production, and took
all 'old' style hex bolt off shelf in 1990. Alldata at
http://www.alldata.tsb.com has the Service Bulletins (october '90).
To my best knowledge, the problem only exists on the M20 'small six'
engine, the 'big six' and the four-cylinder engines don't seem to be
affected. The M20 was used in all 6-cylinder E21 and E30 3-series cars,
and in some E12/E28/E34 5-series (520 Euro, 528e, 525/E34 and possibly
a few others). If you're in doubt, the M20 is equipped with timing
belt, all other six'es have timing chain.
Conclusion: If you have an M20 engine with hex head bolts - be worried,
and replace with new TORX stretch bolts ASAP.
If you don't have an M20 engine - relax.
Thanks for the bandwidth,
Gunnar Elmgren
Stockholm, Sweden BMW Club Schweden no.1436
'71 1600-2 Vollcabrio
'75 2002 tii
'90 M5
'87 320i (Father-in-laws, now with new cylinder head and TORX bolts)
Gunnar Elmgren ______'
AS/400 Software Support, IBM Sweden o/______\o
/// (oo=00=oo)
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