Torque wrench calibration, torque specs and bolt strength questions

[sevensandeights]

As with most projects I get myself into, I've gone down a rabbit hole. I'm sure some of you can relate!

Backstory:
I rebuilt/upgraded a Dolmar/Makita 64 cc chainsaw to a 79 cc saw with an OEM piston and cylinder. Lots of others have done the same - nothing unique about my build. Used the saw for 2 months and it worked great until it developed a massive air leak. Found that the cylinder head bolts were loose. I didn't use a torque wrench the first time because the general consensus was just snug them tight with a t-wrench (I just used a screwdriver the first time). Asked around about torque specs (14 N-m) and bought "new" Blackhawk 3/8 torque wrench. When I tightened the bolts with the TW one of the bolts snapped before the specified torque spec was reached. Luckily it was easy to get out with my Norseman lefthand drill bits (bought on Epstein's Day a few years ago).

Ordered a new base gasket and 10 new M5-20 t27 head bolts from Ebay, not OEM (got extra just in case!). Started tightening the bolts incrementally in an x-pattern and got that uneasy feeling again at 9 N-m. Now I'm worried the TW is out of spec. Took a bolt and put it in a vise. Set the TW to 10 N-m and sure enough the bolt snapped. I also tested on an OEM bolt. TW would click at 8 and 9 N-m but snap the bolts at 10.

So, is there an easy way to check the TW? What do you think think shear strength of these bolts should be?

 

[designer485]

What is the torque range on your new torque wrench? Most are only certified for the upper 80% of the range. If you are in the first 20% of the range the value could be all over the place.

 

[APEowner]

14 N-m is really tight for an M5 fastener. The standard spec for a grade 8.8 is 7.0 N-m and even a grade 12.9 is only 11.7 N-m.

 

[garfunkle24]

As stated that is definitely on the low end for a 3/8 torque wrench and also a lot of torque for those fasteners. Also is that torque spec for dry or lubricated bolts?

A quick and dirty way to check a torque wrench or even torque a fastener is with a fish scale and wrench of a known length. eg, for 10 ft/lbs, 20 pounds pull on a 6" wrench.

 

[sevensandeights]

Great questions!

Torque specs are dry as far as I know. I'm getting them directly from the parts diagram and there is no mention of lube/threadlocker. A dealer also quoted the specs on a chainsaw forum I frequent. Not sure what the OEM bolts had since I bought this saw with an aftermarket P&C that I changed to an OEM set. The bolts I took out had no sign of lube/threadlocker.

In this case, the wet/dry debate is somewhat irrelevant since I locked the bolt in the vice

As for the TW, I'll have to check the exact numbers but 14 N-m is in the middle of its range.

 

[sevensandeights]

I think the range is 20 - 250 inch pounds. 14 N-m is 123 inch pounds.

 

[visionguru]

...

So, is there an easy way to check the TW? What do you think think shear strength of these bolts should be?

Is your torque number correct? I recently torqued some M6 bolts on a Honda engine, the spec is 12 Nm. Even with my Snap On Techangle, I stopped a couple of times to double check, for fear of snapping bolts.

14 Nm on M5 sounds too tight. M5 snaps easily even with a T-handle. I'm not surprised that it snaps at 10 ft-lb.

 

[garfunkle24]

Do you know the grade of the bolts? Seeing a max of 10Nm even for 10.9s..
Torque specs

 

[sevensandeights]

 

[sevensandeights]

 

[PoorUB]

Start with new screws, use blue lock time on clean, dry threads and snug them up with a t handle Allen wrench.
I ran a small engine shop for a few years, that is what I would have done. I have rebuilt hundreds of chainsaws and never used a torque wrench other than flywheel nuts.

 

[sevensandeights]

The new screws I bought did have some blue threadlocker on them. Torqued to 9 N-m so hopefully that's the end of it. Love this saw when it's running but I have messed with it so much that it is becoming very aggravating!!!!

Torque spec definitely seems too high but considering Dolmar sold to Makita and Maktia just announced it is going to stop producing gas powered tools I don't think I'll even bother trying to contact them about it.

 

[Joab]

This is an old thread; but I tripped across this same problem this week.

I was going over the my Makita 7300 (same as Dolmar 7310 and just a smaller cylinder than 7910) and checking everything for tightness -- took the 14Nm in the manual as gospel even though it seemed high -- twisted off one of the cylinder screws.

After spending considerable time in angry reflection, it occured to me -- there are two torque specs on that diagram. 8Nm for the decomp and 14Nm for the M5 cylinder screws -- I compared that to the torque specs on some Husqvarna models like the 562 and the 372 that use similar fasteners and the same decomp.

14Nm is about right for a decomp valve while 8Nm would be unusually low. 14Nm is unusually high for an M5 screw, while 8Nm is about right -- maybe a bit on the low side. (Screenshots of 562 manual included for reference)

My guess is that there's a typo in the manual and the two torque values for 1 and 2 on the diagram got reversed -- it should read 8Nm on cyclinder screws and 14Nm on the decomp. It doesn't seem they ever issued a corrected service manual, but I think that would explain it.