I just snapped a bolt using the "proper" torque spec...

After quite a bit of wrenching, I'm finally finishing up a project that had me pulling a head to change some piston rings. It's time to torque down the suspension bolts, so as I've been doing the whole time, I check the workshop manual.







Ok, 185 Nm for the lower rear bushing bolt. Hmmmm, seems a little high, but I just torqued the lower damper bolt to 175 Nm, and it says the same here as well.








Must be right. Alright let's torque the first one. Ok, no problem. On to the second. SNAP! The bolt sheared. After quite a few expletives, I looked into what happened. It turns out the twice-recommended torque spec for that specific bolt is almost 50% more than what that Aston says that bolt should be torqued to.







Unfortunately I hadn't yet discovered this sheet when I was tightening the bolt, and this particular bolt has a torque spec of 126 Nm +/- 24 Nm. So this bolt can theoretically handle up to 150 Nm, but the typo was saying to go almost 25% more than even the maximum.

After a little more digging, I found this picture tucked all the way in the back of the manual...





It seems both lower bushing bolts are supposed to be torqued to 115 Nm. Fortunately, the only damage caused by this typos is to the bolt, so I'm only out about $50 to replace it, and also the over-torqued bolt that didn't break.

I called up a friend of mine who works on Astons and he confirmed there are tons of typos in the manual. He gave me some advice I will certainly be heeding from now on.

Always compare the manual's recommended torque spec to the recommended torque spec for the size and grade of bolt you are tightening (I've attached the full matrix). If the specs don't match up, always use the lower of the two. Even if the bolt can handle the torque, the nut or material it is screwing into may not be able to, so ALWAYS use the lower number.

 

Hi @Bob Duato ,

Very good catch and good warning for future wrenchers.

Another important point about torquing bolts, that probably most know, but worth repeating is that specs are meant for dry thread unless they specifically say lubricated. Lubricating the threads before toqruing can add another 20% and more to the clamp force, Here too, you could be at the correct dial reading yet still snap a bolt if the threads were wet.

Best,

 

Thank you. Yes, clean, oil-free threads are important as well unless otherwise stated. And that reminds me, there's generally a lot of dirt and grit around when working on those lower bolts. It's a good idea to spray the holes out with compressed air to make sure there is nothing in female threads to cause any problems.

 

Omg. Appreciate you posting this.

 

That is very interesting to know. Thanks for sharing. I also snapped the lower control arm to the suspension bolt that is rated for 137 ft-lbs. Scared the **** out of me as it was so loud when it sheared. However, I didn't think the torque specs were incorrect as someone point out to me that those bolts are stretch bolts and are one-time use. Sure enough, I went to Aston to replace the bolts and it handled the full torque spec.

 

What makes you think they are TTY? Perhaps it's different on the V8V, but according to the manual, all the suspension bolts have a basic torqueing spec with no additional angular movement, which is generally how you determine. But then again, I have known the manual to be wrong before. It only explicitly says TTY bolts are used in two spots on the car - main bearing bolts and connecting rod bolts.

I just did a quick search and these are all the bolts that require additional angular movement after the the torque spec:
-cylinder head bolts
-sump bolts
-crankshaft damper bolt
-main bearing bolts
-main cap side bolts
-connecting rod bolts
-windage tray
-all brake caliper bolts

 

Thanks for posting this. What you are saying about the error makes a great deal of sense and is a good warning - if it doesn't look right it's probably not! I can remember in the 60's my 64 MGB manual said "Torque to a good mechanics fit"... Not being a good mechanic this wasn't helpful!

FWIW, there are lots of places where non-TTY bolts are spec'd with an angular torque. IOW, just because they are tightened with an angular spec doesn't mean they are TTY... And on many engines, they will claim you should only use the bolt once even if they aren't TTY.

 

I'm definitely more careful now. When tightening the intake manifolds I used the "good mechanics fit". I don't trust 5 ft lbs to my torque wrench and the thought of stripping a bolt in the head just makes me cringe.

I agree about the TTY stuff. Another member had mentioned that some of the suspension bolts were TTY, so I was trying to figure out where that notion came from.

 

Good catch. I am planning to change my lower arms too. I can believe this was a typo. All the bolts around it are at 115 N.m or 85 lb.ft. Someone might have copied the 85 from a lb.ft table and wrote 185 N.m

Ins't the rear bolt (front lower arm), used for wheel alignment? Wouldn't it be re-torqued at an alignment shop?

 

I was thinking the same thing about the typo, but you’d have to confuse the ft lbs and Nm and also add the “1”. Who knows…

Yes, the rear bolt has a cam for alignment. I marked the alignment before removing

 

I don't know for sure if they are torque-to-yield bolts, but after talking with a suspension shop and the Aston dealer, they recommended that those bolts are replaced and one-time use because of the torque they take on. The time I tried to reuse them and torque to spec, they sheared. The new bolts took the torque with no problem. I had a picture of it compared to the new bolts and it was longer.

 

Mistakes don't surprise me, and not just this marque.

The way these "manuals" (in any media) are created, a "subject Matter Expert" data dumps information to what is best described as a "Technical Writer" and the operative word here is Writer. They're usually an English and/or Language Major, non-technical. I know as I was a subject matter expert for several technical data field products at times in my career.

The English major is tasked with assembling the data received by the SME in a "readable" fashion (usually written to the 4th to 6th grade reading level). Then, of course, the manual is supposed to be proof read (usually by the SME). It can go back/forth numerous times, but it is amazing what a boring blurr that massive amounts of technical information can be, especially if it originated from you. The proof read routinely catches about 99% of the errors....as evidenced here.

 

I believe it. Interestingly I replaced the bearing cap bolts which the manual explicitly says are single-use TTY and they were the exact same length as the new ones down to ,01"