Engine Mount Bolt Torques

[ChrisH]

I recently upgraded my lower engine mounts with inserts, and discovered that some of the torque settings in the instructions were incorrect (discovered the hard way - stretched bolts, but fortunately on the ones easiest to replace and that used nuts) . The big concern is that overtightened bolts can break under load. The torque settings are not listed in the service manual. So, I tried to examine the bolts to determine their size and class, and looked up the proper torque value for the bolt. Of course, this says nothing about what the female threads in the engine, transmission or the frame can handle, but the bolt specification is a start.

First, here are the two best references I could find on recommended torque for metric bolts. I liked the first one better, but the second does give some lubricated values. Unfortunately, there is a lot of variation in recommended torque settings out on the web, but these two sources seemed consistent with most, had more information, and in general appeared to be more reputable. You might want to save them to your computer in case the web sites vanish.

http://www.imperialsupplies.com/pdf/...rqueCharts.pdf
Metric Bolt Torque Table - CNCexpo.com
(By-the-way, Bolt Depot's recommendations seemed consistently higher than other references, so I didn't use them. Nor were they as complete in their specifications as compared to the others.)

I was upgrading the lower engine mounts. Here are the bolts I examined:
1) Rear lower mount:
1a) mount to frame, rearmost pair (with nuts)
Class: 8.8 (says so on the bolt head)
Diameter: M10 (10 mm)
Pitch: 1.5 mm (thread spacing, which is how metric bolts are specified)
Plated (looks like zinc)
Recommended torque for this specification: 38.25 foot-lbs

1b) mount to frame, forward pair (bolts into frame)
Class: 8.8 I think (not listed on the bolt)
Diameter: M8 (8 mm) I think. I forgot to measure it before installing, but it was definitly a size smaller than the other pair of bolts (1a).
Pitch: 1.25 I think (again, forgot to measure, but the threads didn't seem as fine as a 1.00 pitch)
Plated (looks like zinc)
Recommended torque for this specification: 18.75 foot-lbs
(of course, you can't fit a torque wrench in the space, so you just have to estimate your strength)

1c) big center bolt through mount
Class: 8.8 I think. (The only marking on the bolt head was a lower case b with a dot over it. I don't know what that means, but Lotus generally doesn't use the higher class bolts.)
Diameter: M12 (12 mm)
Pitch: 1.5 I think (measured it, but lost my note. This my best recollection.)
Plated (not sure with what)
Recommended torque for this specification: 69 foot-lbs


2) Forward lower mount
2a) mount to transmission housing
Class: 8.8 I think. (The only marking on the bolt head was 11. I don't know what that means, but Lotus generally doesn't use the higher class bolts.)
Diameter: M10 (10 mm)
Pitch: 1.25
Plated (looks like zinc)
Recommended torque for this specification: 40.5 foot-lbs
(of course, you can't fit a torque wrench in the space, so you just have to estimate your strength)

2b) big center bolt through mount
Class: 8.8 I think. (The only marking on the bolt head was 11. I don't know what that means, but Lotus generally doesn't use the higher class bolts.)
Diameter: M10 (10 mm)
Pitch: 1.25 I think (measured it, but lost my note. This my best recollection.)
Plated (not sure with what)
Recommended torque for this specification: 40.5 foot-lbs

It would be good if others could check the bolt measurements and classes, and add in the bolts I didn't list, such as the upper side engine mount bolts.

The bolts I damaged were listed in 1a above. I ended up replacing them with higher class bolts (10.9), and torqued them to the appropriate specification for that class in the Imperial Supplies chart above.

[EliseAtLeast]

Thanks for taking time to post this info.

[ChrisH]

One caution for anyone trying to determine a bolt torque: DON'T estimate the torque of an installed bolt by seeing how much torque is required to turn it some more.

It will generally take much more torque to start the bolt moving (breaking the static friction) than it took to tighten it to that point. This is because the static coefficient of friction is much higher than the kinetic (moving) coefficient of friction. (The bolt was moving when it was originally tightened.) And, if the bolt had seized a bit (fairly common due to corrosion or other slow chemical bonding), then that would lead to an even greater overestimation. If you used that method to estimate the proper bolt torque, you will overtighten the bolts and damage them, with potentially catastrophic results.

[tvacc]

I agree completely. I put mine on this past weekend and nearly stripped one of the rear bolts trying to tighten it up to the specs in the install instructions. I figured something was amiss and did not tighten mine up so much.

[ChrisH]

Originally Posted by tvacc

I agree completely. I put mine on this past weekend and nearly stripped one of the rear bolts trying to tighten it up to the specs in the install instructions. I figured something was amiss and did not tighten mine up so much.

You should replace the bolt(s) that you nearly stripped or that you overtightened. They are damaged and could break under hard acceleration, fast releasing of the clutch (if your rev matching isn't so good), etc. Your local autoparts store should have them. I upgraded my rear lower frame bolts (the ones with the nuts) to class 10.9 after stretching the original bolts.

[300hpMR2]

I just finished installing my inserts yesterday. I was able to torque the large bolts but couldn't get my torque wrench on the smaller ones. Those numbers you have are WAY lower than in the installation instructions. Yikes!

[PECivil]

yes mucho gracias to Chris for this info, I am installing mine tomorrow!!

[ChrisH]

If you have any concerns about the torque being so much lower (and I think the lower values are correct), then put some thread locker on for insurance. I didn't bother, but it may be worth the peace of mind.

[glacialmoraine]

Re threadlocker. The linked table lists dry and lubed torques, use the lubed torque if threadlocker is applied.

gm

[caboydman]

So other then this thread is there a place where I can get the exact torque specs for all the engine mount bolts.
I paid the $25 for the online lotus catalog but it didn't have any information I needed.
I also called a dealer and couldn't get an answer.

Thanks

[ChrisH]

Originally Posted by caboydman

So other then this thread is there a place where I can get the exact torque specs for all the engine mount bolts.
I paid the $25 for the online lotus catalog but it didn't have any information I needed.
I also called a dealer and couldn't get an answer.

Thanks

There is no place that anyone has found (or at least reported). Hence, the reason for this thread.

[ads_green]

I've just been out to have a quick go with the engine mount inserts and failed on the first mount (lower rear).

There's nothing special here with this bolt? I suspect it's just me only having a relatively small wrench (9") so probably need something with a little more length. Just want to check first before I have another go with a longer wrench.

Ta

[ChrisH]

Originally Posted by ads_green

I've just been out to have a quick go with the engine mount inserts and failed on the first mount (lower rear).

There's nothing special here with this bolt? I suspect it's just me only having a relatively small wrench (9") so probably need something with a little more length. Just want to check first before I have another go with a longer wrench.

Ta

Which bolt is causing you trouble? Getting them off or on? On the rear mount, the front bolts for the bracket to the frame are hard to access. If I recall correctly (it has been awhile), I just had to use a box end wrench and use muscle to get them off.

[ads_green]

It's the rear lower mount M12 bolt through the middle of the mount - the one recommended to torque to 69lb/ft - and the damn thing wont' budge. The only wrenches I have to hand are about 9" and I don't thik I'm getting enough leverage. I'm hoping it's just a case of a longer wrench

[ChrisH]

You need to invest in a breaker bar. I got mine at Harbor Freight.

Or, hit the wrench with a hammer. The shock will often break it loose. You could even try some penetrant, or something to effectively extend the handle on the wrench to get more leverage.

[choi0706]

Originally Posted by ads_green

It's the rear lower mount M12 bolt through the middle of the mount - the one recommended to torque to 69lb/ft - and the damn thing wont' budge. The only wrenches I have to hand are about 9" and I don't thik I'm getting enough leverage. I'm hoping it's just a case of a longer wrench

double up on the wrenches..

[ads_green]

success
Big wrenches sorted.

Thats the rear one done now onto the joys of the front one