When measuring the diameter of a bolt

Murv

Murv

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where most of the shaft is smooth and just the bottom is threaded, do you measure the smooth bit thickness or the threaded bit?
 
Assuming the bolt isn't stepped from the threaded section to the plain part then the plain shank of the bolt, but not the cross flats dimension that the spanner grips across.The length is measured from below the head to the free end of the thread.
 
Superb, thank you.
Just to check mine isn't stepped then, I'm reading 7mm across the plain shank, but 7.6 across the threaded part, so it's an M7 bolt?
 
I would probably think it is imperial at 7.6mm, possibly 3/8. To be safe before damaging any threads pop down to the chandlery or wickes and try a nut on it.
 
Murv said:
Superb, thank you.
Just to check mine isn't stepped then, I'm reading 7mm across the plain shank, but 7.6 across the threaded part, so it's an M7 bolt?
Click to expand...
Sounds more like an M8, with a narrow shank. Normally an M8 bolt would measure 8mm at the plain shank, and marginally less at the thread. 7.6mm is 0.3" which is odd for an imperial size (not checked the old BSW/BSF dimensions).

M7 is definitely an odd size, and the thread OD wouldn't be more than 7mm.

How are you measuring the diameter e.g. caliper or micrometer ?

Is this for the volvo penta 2003 with the ripped out thread on the water pump housing ?

Some VP pumps were Johnson which do use obscure non metric threads. If unsure use a normal M8 nut to see if the thread matches. A standard M8 will have 1.25mm pitch, with fine at 1mm pitch, and very fine at 0.75mm pitch.
 
superheat6k said:
Sounds more like an M8, with a narrow shank. Normally an M8 bolt would measure 8mm at the plain shank, and marginally less at the thread. 7.6mm is 0.3" which is odd for an imperial size (not checked the old BSW/BSF dimensions).

M7 is definitely an odd size, and the thread OD wouldn't be more than 7mm.

How are you measuring the diameter e.g. caliper or micrometer ?

Is this for the volvo penta 2003 with the ripped out thread on the water pump housing ?

Some VP pumps were Johnson which do use obscure non metric threads. If unsure use a normal M8 nut to see if the thread matches. A standard M8 will have 1.25mm pitch, with fine at 1mm pitch, and very fine at 0.75mm pitch.
Click to expand...

Cheap M8 bolt with rolled thread thats under size perhaps ???? Thread larger than shank suggests a rolled thread I think.

Got Zeus here but not found anything to match 7.6 mm

Check pitch. M8 is 1.25mm
 
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I'm measuring with a micrometer, it's the bolt that holds the charge air cooler onto the bracket on the inlet manifold. I've stripped the thread on it, so wondering whether to have a dabble with helicoil kits.
I guess it wouldn't matter too much though really, the thread in the bracket that holds it sits at the bottom of a tube (poor description, sorry) but the bolt is 70mm long, but only threaded for the last 25mm.
It held a very thick captive washer, I've just cut that off so hopefully that will give me enough extra length to bite into fresh thread.
There are two bolts that secure it, a large 18mm and this 13mm one which supports the side.
If it doesn't though, I'll have to find a way of inserting a new thread.
I guess I could wind a helicoil into the smooth part of the fitting and fit (for example) a 25mm bolt instead?
 
Murv bolts are always referred to by the size of the threaded bit and not the spanner size, makes things very confusing, especially nowadays where for example an m10 bolt would normally have a 17mm head now it can have a 15 or a 16mm head!
So in your case the 13mm may be an M8 and the 18mm an M12.
Theres a bolt place on the medway estate 30 yards away from the truck cafe, if not Aghabridge opposite mcdonalds will have/get what you need.
Pirats cave chandlers on the estate has a decent selection of bolts
To use a helicoil you will need space to get a drill in there to open the hole up, then be able and again have space to use a tap, the tap cuts a new thread then the helcoil screws into the new thread and your bolt then screws into the helicoil
 
Oh yes bolts have a plain bit as yours does and if its fully threaded its called a set screw, it can get real confusing!
 
lynall said:
To use a helicoil you will need space to get a drill in there to open the hole up, then be able and again have space to use a tap, the tap cuts a new thread then the helcoil screws into the new thread and your bolt then screws into the helicoil
Click to expand...

Helicoils (or the various equivalents) do a good job. Problem, as stated, can be access. Depending on the material, it may not be necessary to drill the hole; the Helicoil tap has the same pitch (obviously!).

A rechargeable drill will sometimes be a space-economical method for turning the tap and will also be good for alignment. Use it on a low torque setting to avoid breakage. If access is really tight, you might get by with an open-ended spanner quarter turn at a time.
 
superheat6k said:
A standard M8 will have 1.25mm pitch, with fine at 1mm pitch, and very fine at 0.75mm pitch.
Click to expand...

I am going to have to check out the same thing for the stripped thread in my VP 2003, see earlier thread - any recommendations for sourcing M8 bolts/studding of a pitch other than standard ?
 
Murv said:
where most of the shaft is smooth and just the bottom is threaded, do you measure the smooth bit thickness or the threaded bit?
Click to expand...

Depends how it's made, if the thread is machined measure the plain diameter, but the thread can be rolled so the thread diameter is larger than the plain section, in that case measure the thread.

Brian
 
Thanks all, a fair bit to digest then!
Thanks Lynall, I didn't know about those suppliers (except pirates cave,) that's extremely useful to know.
I need to get down to the boat early to fit the spare prop as mine hasn't come back yet from Steel Developments. Once I've fitted that, I have a couple of hours spare until my first training session at high tide (assuming I don't get swept away in the morning darkness fitting the prop) to have a really good look and see what the best option is.
 
Before helicoiling how about tapping the thread out to 10mm and fitting an M10 bolt. You may need to drill the existing thread to get to a tapping clearance. Buy a good quality bolt and use Locktite on it.
Got to be cheaper than helicoil.
 
This is a useful source of reference for Helicoils:-

http://www.helicoil.in/pdf/HeliCoil Catalogue.pdf

I don't think that tapping out to M10 or fitting an M8 helicoil is a job for the inexperienced. On the basis of "don't make a bigger problem" I'd suggest practice first on something similar.

To buy all of the kit required to fit a helicoil will be fairly expensive.

Good luck!
 
Thanks for that, I'm starting to get that impression!
I'm fairly sure that the thread is in cast iron, from what I've read, that's not the easiest material to tap either.
I think I'm probably better off investing in a torque wrench...
 
7.54mm is 19/64" - Not common, but not unheard off either
and 7.94mm is 5/32" - more common - indeed quite a common size
 
Graham_Wright said:
Helicoils (or the various equivalents) do a good job. Problem, as stated, can be access. Depending on the material, it may not be necessary to drill the hole; the Helicoil tap has the same pitch (obviously!).

A rechargeable drill will sometimes be a space-economical method for turning the tap and will also be good for alignment. Use it on a low torque setting to avoid breakage. If access is really tight, you might get by with an open-ended spanner quarter turn at a time.
Click to expand...

a 16 sided socket on an extension with a t bar works well.
 
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