S Series Bolts - What grades?
Discussion
Got a few "left overs" you're welcome to.......................
Easier to buy a "set" from somewhere like RT?
For general purpose use 8.8 is fine, the higher grades 10.9 & 12.9 tend to be for stuff like propshaft and drive shaft bolts, caliper mounting.... things that take a lot of stick basically!
mk1fan said:
And if you go allen socket head then go up a grade again.
Slightly puzzled by that? If you go 'stainless' then you need to specify A4-80 grade (which should be marked) which is almost exactly equivalent to the old imperial 8.8 'high tensile' strength rating which is used for alloy steel bolts. If the bolt isnt marked, dont use it!
(A2-70 is better than the standard 'mild steel' bolts but considerably below 'high tensile')
Stainless of this grade is fine for almost all the applications, except that I would be wary of using it for bolts such as suspension mountings. 'Stainless' has a different corrosion reaction to the chlorine in salt than alloy steel, which can result in cracking.
Do NOT use anything other than 12.9 for drive shaft bolts, loctite them in and mark them! Many of us have had them come loose. I would go for Unbrako hex socket heads. Quote often multipoint 'star' ones are offered (the drive shaft joints now usually come with multipoints) , these are fine for assembly machines but are too easily 'rounded' when assembled by hand tool.
greymrj said:
mk1fan said:
If you have 10.9 (hex head) bolt then use a 12.9 allen socket head bolt.
I am still puzzled by that, enlighten me please.In general it makes no difference to the tensile load the bolt will take whether it is hex or SHCS provided they are of equivalent materials and equivalent standards of manufacture. Failure of the head is more likely to be due to the profile where head joins shank and whether poor manufacture has left a profile which concentrates stress.. You will have noted that SHCS are now the usual choice for even cylinder head bolts. You do not need a bigger SCHC if you replace a hex bolt provided you use a similar grade.
There are a couple of differences.
The hex is considerably cheaper to make as it requires less processes, so hex is used for cost reasons.
SCHC were introduced first for positions where the close location of adjacent surfaces made access for the normal spanner a problem. Then, as machine assembly became more common, SCHS became more widely used as they are easier to machine load and it is easier to keep the machine engaged. (for similar reasons, when did you last see a cross head rather than a posidrive screw in a new machine?)
The corners of a hex head can dig in and produce an inaccurate torque reading, a problem which largely goes with SCHS. Quite often washers were placed under hex screws (again this happened with some cylinder head bolts) to obviate this problem.
If a thin material is to be fixed to a thick material hex heads can do damage but the greater surface area can help keep the thin material flat, in such cases an SCHS with a washer may be needed.
If a washer is used with a SCHS (or hex to that matter) then it is necessary for the washer to be of a similar specification or the washer may compress under load and reduce the torque. Washers used under hex cylinder head bolts/rocker shaft bolts etc are noticeably harder than most washers.
Using SCHS widely usually means more compact tools and a smaller tool kit is needed.
Good quality hex screws are less likely to round off than hex heads, provided the tool used is of equal of better quality than the SCHS. But they do need more care by the mechanic, their principle assembly advantage is in machine rather than hand use.
Have i missed anything?
There are a couple of differences.
The hex is considerably cheaper to make as it requires less processes, so hex is used for cost reasons.
SCHC were introduced first for positions where the close location of adjacent surfaces made access for the normal spanner a problem. Then, as machine assembly became more common, SCHS became more widely used as they are easier to machine load and it is easier to keep the machine engaged. (for similar reasons, when did you last see a cross head rather than a posidrive screw in a new machine?)
The corners of a hex head can dig in and produce an inaccurate torque reading, a problem which largely goes with SCHS. Quite often washers were placed under hex screws (again this happened with some cylinder head bolts) to obviate this problem.
If a thin material is to be fixed to a thick material hex heads can do damage but the greater surface area can help keep the thin material flat, in such cases an SCHS with a washer may be needed.
If a washer is used with a SCHS (or hex to that matter) then it is necessary for the washer to be of a similar specification or the washer may compress under load and reduce the torque. Washers used under hex cylinder head bolts/rocker shaft bolts etc are noticeably harder than most washers.
Using SCHS widely usually means more compact tools and a smaller tool kit is needed.
Good quality hex screws are less likely to round off than hex heads, provided the tool used is of equal of better quality than the SCHS. But they do need more care by the mechanic, their principle assembly advantage is in machine rather than hand use.
Have i missed anything?
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