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Fatigue-resistant threadforms

fastenersWhen we design a fastener, or select one for use, there are a number of points to consider. Not least among these is whether the fastener will last for the life of the engine, or certainly between planned services.

Where a fastener is cyclically loaded, we need to consider the effects of fatigue, and in this regard we need to pay special attention to design features. Where a bespoke engine is concerned, or where we are looking to increase markedly the output of an existing engine, it is very likely that we will have to look beyond the realm of 'off-the-shelf' fasteners and towards something that is specifically engineered.

For popular conversions of production engines for motorsport, there are improved fasteners for critical applications such as main bearing caps and cylinder head studs, and it may be that these are sufficient for a great many engineers who are tuning production engines.

There will be many applications, however, when the fasteners available as standard from race specialists won't be up to the task. In this case, obtaining a component of sufficient durability might simply involve having the part made in a better grade of material.

In bespoke engines, where the engineering is better optimised, and space generally tighter, there are a lot of highly stressed special fasteners used throughout the engine, and almost certainly in critical areas such as the con rod bolts, main bearing cap studs and cylinder head bolts/studs. Common design features such as large blend radii and waisted shanks are used to improve fatigue life, in these cases by minimising the stress concentration factor and lowering the internal load coefficient respectively.

It is usually at the thread, however, where fastener problems are to be found, which may be due to reasons too numerous to list here. One action we can take though is to look to the geometry of the threadform itself for an improvement, and fortunately there are threadforms with a tightly controlled root radius in both imperial (unified) and metric dimensions.

These are known as 'J' threads, and you may see designations such as MJ or UNJF used to describe threads cut to this standard. They are generally compatible with standard threads of the same basic form, but have an increased radius at the thread root (minor diameter) compared with the standard type.

One point to be careful of is to check the specification of the nut or internal thread used with a 'J' specification thread, as it may not have a sufficiently large internal diameter to ensure clearance to the increased root radius. An increased root radius serves to decrease the stress concentration factor while slightly increasing the minor diameter, both of which serve to lower the critical stresses in the component.

Where a standard thread has a root radius which is a minimum of 0.125 times the thread pitch, a 'J' specification thread has a root radius controlled with the range of 0.1501 to 0.1804 times the thread pitch. Thus the minimum root radius of the MJ thread, for example, is 20% greater than the guaranteed minimum on a standard M profile thread.

Fig. 1 - Race engine fasteners such as these studs are often specified with 'J' form threads (Courtesy of T&K Precision)

Written by Wayne Ward

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