Helicoil - knackered thread on engine block.

[Barson]

What's the difference between an M8 1MM and M8 1.25MM helicoil thread?

Don't say size :oops: :lol: !

Is it the width of the coil piece, i.e. the difference between the external diameter and internal diameter?

for what reason would there be two different size, is it just dependant on application?


cheers me dears
:wink:
Dave.

[bozzzydmc]

Thread Pitch or Count
Thread pitch or thread count is used only on machine thread fasteners (those that take a nut or thread into a tapped hole). The thread pitch or count describes how fine the threads are.

Thread Count (TPI)
TPI stands for Threads Per Inch. This is simply a count of the number of threads per inch measured along the length of a fastener. TPI is used only with American fasteners. Metric Fasteners use a thread Pitch (see below). In general smaller fasteners have finer threads, so the thread count is higher.

Thread Pitch
Metric fasteners are specified with a thread pitch instead of a thread count. The thread pitch is the distance between threads expressed in millimeters (measured along the length of the fastener). For example a thread pitch of 1.5 means that the distance between one thread and the next is 1.5mm. In general smaller fasteners have finer thread so they have lower thread pitch.

Therefore you need to use the correct thread pitch set bolt / screw

[Bandit]

Its the 'pitch' of the thread mate. i.e 8x1 is 1mm between the point of each thread, if you get me & 8x1.25 is obviously 1 & a quarter mm between the pitch therefore the 1.25 is the coarser of the two.

[bozzzydmc]

And interestingly :-

Size for size a fine thread is stronger than a coarse thread . This is both in tension (because of the larger stress area) and shear (because of their larger minor diameter).

Fine threads have also less tendency to loosen since the thread incline is smaller and hence so is the off torque.

Fine threads can be more easily tapped into hard materials and thin walled tubes.

Fine threads require less torque to develop equivalent bolt preloads.

[Barson]

yep - cheers lads

makes more sense than what I was suggesting. I feel silly now :oops:



The pitch should be stamped on the head of the bolt, right?

/goes off to think about what I've done.


Dave

[stunned_monkey]

Which bolt is it? Water pump ? There aren't many M8's on the block, and it's a boggo standard 1.25 on those three.

[Daniel Shane]

Normally the Head of the bolt shows the tensile strength of a bolt, for example 8.8 10.9 etc

For the pitch you need a pitch gauge, they normally come included in a good tap and dye set.

However most are 1.25 on the car.

This is copied from Wikipedia :-

High-strength bolts usually have a hexagonal head with an ISO strength rating (called property class) stamped on the head. The property classes most often used are 8.8 and 10.9. The number before the point is the tensile ultimate strength in MPa divided by 100. The number after the point is 10 times the ratio of tensile yield strength to tensile ultimate strength. For example, a property class 5.8 bolt has a nominal (minimum) tensile ultimate strength of 500 MPa, and a tensile yield strength of 0.8 times tensile ultimate strength or 0.8(500) = 400 MPa.

Very technical ah.

[Barson]

stunned_monkey Wrote:Which bolt is it? Water pump ? There aren't many M8's on the block, and it's a boggo standard 1.25 on those three.

Well that answers that Thread closed!


cheers

Dan, thanks for the clarification - always wondered what those numbers meant...