a novice fitting glowplugs

[campergeek]

I'm planning on replacing my glowplugs this weekend. Have read the fact sheet and various posts (magnet on stick purchased ) and it sounds like something I can handle. Couple of questions though...

I assume I should put a bit of copperslip on the threads to stop them seizing?

The box for the plugs says tighten to 15 to 20nm. Do i need a torque wrench? I've no idea if this is tight, so will finger-tight plus a quarter turn will be ok?

Cheers.

[kawasaki kid]

Hi, A smear of copaslip on the threads is a good idea and as you said screw the plugs in till finger tight and then nip them up - don`t overtighten them as it is an alloy head. I had one snap off - a dodgy plug not over tightened - cost me a replacement head.

[Velocette]

I'm planning on replacing my glowplugs this weekend. Have read the fact sheet and various posts (magnet on stick purchased ) and it sounds like something I can handle. Couple of questions though...

I assume I should put a bit of copperslip on the threads to stop them seizing?

The box for the plugs says tighten to 15 to 20nm. Do i need a torque wrench? I've no idea if this is tight, so will finger-tight plus a quarter turn will be ok?

Cheers.

If the torque setting is for a dry thread you have to reduce it if you use a lubricant as part of the resistance is friction. Angle tightening as you suggest is not affected by this so IMO much better.

Remember if you are ever replacing a used plug the tightening angle is much less because the gasket is already compressed. I am assuming the glow plugs have metal gaskets and not a bevelled face as I am only used to sparking plugs.

[campergeek]

Thanks guys, all done now. Was very straightforward until I realised the old ones were 102mm and new ones 83mm. Bent the rail a bit and got it to fit. Now need some cold weather to see if the lumpiness is cured or not.

[dandywarhol]

I was under the impression that angle tightening was mainly for "stretch" bolts - not conventional threads where torque settings are used

[widdowson2008]

I was under the impression that angle tightening was mainly for "stretch" bolts - not conventional threads where torque settings are used

dandy has a VERY valid point here I believe.
Torque setting 15~19Nm. (Source - Mazda manual).
Don't forget it is an aluminium head. Overtightening could result in stripped threads (not recommended). Best stick to Mazda spec IMHO.

[mikeWalsall]

I've always been a bit bemused why 'torque to yield' (stretch) bolts in particular are 'angle tightened' ...

Surly there is no difference in torquing a bolt down to say (stage one) 100lbs foot ... then (say) 120 lbs foot .. then a final torque of (say) 140 lbs foot ...

Rather than the initial torque ... then the 'clumsy' use of an Angle gauge for the final 'torque' figure ... ??

[g8dhe]

I think you will find that when the bolt starts to stretch the Torque required may well REDUCE, by angle tightening you force the bolt to stretch by a given amount in length.

[mikeWalsall]

I generally use what ever method is recommended in the manual ...

I can remember some years back replacing the rear wheel bearings on a RWD Vauxhall Carlton you torqued first then angle tightened ... given the location of the retaining bolts it must be one of the most awkward jobs to do efficiently ..!!

I have a several angle gauges and torque wrenches ...for example ... the green and gold 1/4" drives in this pic; are pre-set for CVT transmission cases on mopeds / scooters ..

[campergeek]

blimey, i appear to have opened a can of worms Hope a light tightening was enough - pretty sure I haven't over tightened it anyway.

[widdowson2008]

I think you will find that when the bolt starts to stretch the Torque required may well REDUCE, by angle tightening you force the bolt to stretch by a given amount in length.

The reason it's done by the angle tightening is as Geoff said, that is when the bolts actually START to stretch. The bolts are made of a very strong material (high tensile) which, for a given stretch produces are finitely known grip force. The actual bolt stretch is very small yet the grip force is identical in each bolt. VERY PRECISE.
The ONLY place I disagree with Geoff is the torque.. It certainly DOESN'T reduce in the final tightening. It goes WAY beyond the initial torque setting. Take my word for it Geoff. Been there - Done that. There is no way you can do the final 'stretch' using an ordinary torque wrench.
We had to use a 5 foot pipe to complete the final 2x90 degree setting, and then it took some force.
We followed the Mazda manual to the letter, even applying oil to the threads as specified (which makes a significant difference).

The head we replaced has now done in excess of 2000 miles and all seems well. (Head bolts were re-checked after 600 miles as recommended)

As far as overtightening of the glowplugs is concerned, you are far safer sticking to the Mazda recommendations (15~19Nm). Why? - Aluminium Just my opinion though.

[g8dhe]

You are probably right their Steve, I was thinking of measuring tensile strength where you stretch material to its breaking point, once metal materials get beyond there elasticity limit then the force required to stretch it to breaking point actually reduces. I suspect however that for stretch bolts your not taking them beyond there elasticity limits normally ! So yes the torque will not reduce in that case ....... goes off to check........
Ah yes here we are, and I was right ; http://en.wikipedia.org/wiki/File:Stres ... _A36_2.svg I guess Stretch bolts are working in the region marked as "4" on the red graph line, but before you get to the "4" region you go through the Elastic limit point 2 at which point the stress does reduce, before rising again as the bolt stretches, once you go to far however you enter the "necking" region where the bolt would start to reduce in diameter - not an area you want to get into!!
The main article is here http://en.wikipedia.org/wiki/Tensile_strength and I note that it varies with material for instance Aluminium doesn't go through a reduced stress region like steel does.

Yes and some interesting write ups like http://www.honestjohn.co.uk/forum/post/ ... 79#m244836 with more detail etc.
Worth a google search http://www.google.co.uk/search?q=stretch+bolts

[widdowson2008]

In one Geoff - good find and exactly what I was trying to get at.
It's all about staying well within the elastic limits of the bolt material when producing the extreme forces needed to hold the head down.
That's why ordinary bolts are no good in this application. The material would leave the elastic state and enter the plastic very early on with those sorts of loadings.

Spent a year of my apprenticeship in a test lab for a large steel company doing these tests. Interesting stuff.

[mikeWalsall]

Google is my friend and I did search for info on ANGLE tightening against simply torquing ..

I understand the principle of stretch bolts, and appreciate the relevant ease a gauge can be utilised on head / crank bolts etc:

But ... I am still bemused to why angles are used for some 'out of the way' bolts ...when moving a torque wrench by a pre determined about should achieve the same effect ..

[widdowson2008]

But ... I am still bemused to why angles are used for some 'out of the way' bolts ...when moving a torque wrench by a pre determined about should achieve the same effect ..

Don't know how much of this you know, so I'll take it from 'entry' level.
The PITCH of a screw thread is the distance moved in ONE complete revolution of the bolt.
For example, if the pitch is ONE mm, then the bolt movement :
- in ONE complete revolution of the bolt is ONE mm.
- in HALF a revolution of the bolt is HALF a mm.
From material tests of the bolt material, the part of revolution applied, will give the resultant grip force.

In the case of Bongo cylinder head bolts, they are initially torqued to 29 Nm, at which point they will begin their 'stretch' cycle.
They are then turned through 90 degrees (which equates to 1/4 of a thread pitch), and then another 90 degrees, which will give a total of 1/2 a turn past the original 29Nm. In other words, the bolts will have stretched in length by ONE HALF the thread pitch length, at which point the head will be clamped to the block by a KNOWN force.

Having read through that I hope it makes sense, but if not, I will produce a graphic to explain.