can you substitute class 12.9 for 8.8 bolts?
 

Hi all,


Is it safe to use bolts that are stronger than specified by Rotax, or will there be an unintended consequence (e.g., damage to the internal threads, etc.)?

I over torqued the metric bolts on my water pump housing and will be pulling the engine to remove & replace the affected bolts. Unfortunately, LEAF is quoting 6 weeks to get replacement M6x90 bolts. I have located a domestic source of M6x90 bolts, but they appear to be considerably stronger as they are Class 12.9, vs Class 8.8 as found on one of the ruined bolts in my possession.

A quick internet search shows that Class 8.8 has a yield strength of 660 MPa while Class 12.9's yield strength is 1100 MPa. Is it possible to do damage using bolts that are too strong? Will the torque specifications be the same? If not, is there a known conversion? If not, then I will not use them and will just wait for Christmas.

thanks in advance,
-dbh

Stick with 8.8
 

The long answer to this requires a thesis on bolts. The short answer is to replace the 8.8 bolts with the same grade as specified by the engine manufacturer. This is not a moment for backyard intuitive engineering on your engine which is, after all, your life support system.

12.9 bolts have a high ultimate tensile strength but a low ductility (they're more brittle). If they're plated they're also much more prone to hydrogen embrittlement. That could be crucial in this instance. They also need to be torqued up to a much higher proof load than 8.8s.

In engineering, more is sometimes less. Play it safe and stick with 8.8.

Bob,

I agree with your observation, but look at the application. I wouldn't lose any sleep over more brittle bolts in the application of water pump pressure boundary retention. Landing gear would be a different story.

Rich

Have you tried contacting Lockwood in FL? They may have what you need in stock.

Rich, put it this way. If the OP uses the specified fasteners he will be 100% safe. If he does not then there could be ramifications that the layman cannot understand. For instance the depth of female thread available may be specifically engineered for an 8.8 bolt and may be insufficient for the higher torque required for a 12.9 bolt. Consequently he may strip the female thread or damage it such that it fails down the track in flight. Neither scenario would good. Alternatively there are other dangers in not torquing a 12.9 fastener to its specified torque.

The fact that the OP is asking this question on this forum suggest to me that he is not an engineer and therefore my recommendation to him is to use the Rotax specified fasteners at the Rotax specified torque.

My other concern would be whether when the OP overtorqued the original bolts he damaged the female threads at that time.

Try McMaster Carr
 

Dave,
Did you try McMaster Carr. They usually have most in stock

Gary

Don't Change the Torque
 

There's no change to the assembly torque if you go with a higher-strength bolt, provided -

The thread form hasn't changed,
The material remains steel,
the head type is the same,
the thread engagement is the same,
the same washers or nuts are used,
and the same lubrication or lack of it is used upon assembly.

If all these are true, then the torque to preload relationship is unchanged, and you should use the original assembly torque to obtain the bolt preload, the clamping force, which the designers wanted. A higher-strength bolt DOES NOT need higher torque in this type of a replacement situation. It might well be damaging.

You'd use a higher torque in a wholly new design that fully uses the higher strength of the new bolt, but not for a replacement bolt.

Dave

May I request some clarification please? Here's my understanding, and I would appreciate the education if I'm misinformed.

The clamping force applied by a bolt is fundamentally a function of its elastic modulus, and all steels have roughly the same modulus of elasticity, about 3.0^7 psi.

The mechanical difference between the 12.9 and 8.8 is yield strength (roughly 90% of 1200 MPa vs 80% of 800 MPa). That means the 12.9 can be stretched further (i.e. torqued to some higher value) before it yields, which of course increases the clamp force. If an 8.8 and a 12.9 have the same modulus, then stretching them equally (same torque, assuming same thread pitch, form, and friction) should result in the same clamp force.

There's no reason why a 12.9 mustbe torqued to a higher level. The required clamp force is usually dictated by the anticipated cyclical stress; we want the clamp force to be higher than the cyclical stress which tries to pull the clamped joint apart. If an 8.8 was adequate in this regard, a 12.9 should be fine.

Lower ductility for a 12.9 has no bearing on this discussion. It merely indicates a short plastic region on the stress-strain plot. The stress applied here, using the torque specified for the 8.8, would not put a 12.9 anywhere near that plastic region.

So what am I missing?

POSTSCRIPT....just posted to find Dave has already covered the question.

My guess is that the original 8.8 bolt will be a mil spec product with cad plating. Cad plating is very lubricious and has a lower coefficient of friction compared to unplated steel. 12.9 fasteners are normally black steel due to the quality control problems associated with plating such a high strength alloy. Of course using a black bolt has it own shortcomings in terms of corrosion. But all other things being equal, the black 12.9 will have to be torqued to a higher level than the cad plated 8.8 to achieve the same clamping force. Now one could go to Milspec-HDBK-60 to get a detailed formula for calculating the torque based on various friction co-efficients. But I don't think the OP is going to do that.

So under the circumstances I still think that the conservative approach for the OP to take would be to obtain the engine manufacturer's specified bolts with the same plating and use the engine manufacturer's specified torque.

Yep, my thoughts exactly. And- I have never seen an engine bolt installed with dry torque. Preload, pre stretched, and torqued to length, but not dry.