Con Rod Bolt Torque

[AlfaLofa]

I've bought a set of Burton's "Heavy Duty Con Rod Bolts" (part # FL245).

They say these are suitable for standard rods only (not steel) - my rods are 125E which I assume is what they mean as "standard".

Burton recommend torqueing these to 31-35 ft/lb - but in Miles Wilkins' book he suggests 44-46ft/lb.

Which torque setting should I use????

The new bolts are dry - are these torque settings for dry or oiled bolts???

Also

I assume that these bolts should be fitted without washers (my S1 workshop manual shows lock washers being used in the exploded diagram).

[512BB]

Couple of sugestions Steve.

The Burtons recommended torque seems a bit light to me. Phone them to check. Speak to someone in the technical dept if they have one.

What does the Lotus workshop manual say? I suspect a little more grunt on the wrench, 40 - 45?

I would torque them up dry with some thread lock as well.

Here are some I made earlier!

Leslie

[AlfaLofa]

Thanks Leslie.

I'll take your advice.

The S1 manual says torque to 20-25 ft/lb - which must be for the weak original con rods.

Later manuals state 44-46 ft/lb as per the Miles Wilkins book - so this is what I shall do - but dry with thread lock.

By the way your pistons/rods look excellent!!

I was only saying to my other half the other day how pleasing a freshly painted set of diff output shafts resplendent with their new bearings looked. They could be used as household ornaments!

[rgh0]

The Lotus manual quoted torques are with clean and lightly oiled threads. There is no need to use thread locker on the con rod bolts when torqued correctly and they must be torqued correctly to avoid cyclic fatigue failure. I don't know if a thread locker is potentially a problem in this application on con rod bolts but it is certainly not normal. If the bolt toque comes off such that the tread locker is required to hold the bolt in place then the bolt will fail anyhow due to fatigue.

cheers
Rohan

[512BB]

I prefer a belt, or bolt, and braces approach. And a dab of thread lock will act as a slight lubricant on the bolt.

And no washers Steve.

Leslie

[Galwaylotus]

Have to say I agree with Rohan. Lightly oiled and torqued. The "heavy duty bolt" supplier should have given a recommended torque with the bolts.

[AlfaLofa]

Hi Galway - if you look at the first post you can see that Burton do recommend a torque setting, but it's considerably lower than the torque recommended by either the workshop manual or Miles Wilkins.

EDIT:
Earlier I sent a query to Burton - they have just replied saying:

"These are a heavy duty bolt and the torque setting for these is 31 - 35lbft they may well have a different torque setting to the original bolts. We would advise a torque of 35 lbft".

So why the difference in torque between a "heavy duty" bolt and a "non-heavy duty" bolt ??

And what is a "heavy duty bolt" ??

[Galwaylotus]

Probably the material spec. That is what determines the correct stretch required for the application. Torque is what translates into bolt stretch and is affected by thread lubrication. The optimum way to determine the correct installation loading is to measure the stretch; however, that is far easier said than done, hence the torque spec which will get you to the approximate correct load.

[rgh0]

The clamping force of the bolt at the same torque is nominally the same for a bolt of the same diameter thread and same thread form no matter what material it is made from. I say nominally because the actual torque to clamping force can vary significantly depending on actual thread surface finish and lubrication. A stronger bolt of a stronger steel could be torqued to a higher value and achieve a higher clamping force before failure but at the same torque it achieves the same clamping force and at a lower torque it would achieve a lower clamping force

For conrods it is critical that the bolts are torqued sufficiently so that the clamping force is high enough such that there is no cyclic loading on the bolt each stroke. i.e. as the rod goes over TDC the acceleration loads that tend to stretch the rod bolts do not change the load on the bolt itself as they are balanced by a reduction in the clamping load the rod bolt is already seeing in the other direction as the join is pull apart a microscopic amount relaxing the compression loads on the join.

Use of a stretch gauge to measure the bolt stretch and thus directly measure the clamping force rather than infer it from torque is recommended and normal practice in race engine assembly and it is what I use but in its absence make sure you torque to the Lotus recommended values for the 125E rods and bolts. It is wise to ensure the threads are clean with no grit and burrs and to lightly oil them. Then tighten and release them 2 or 3 times before the final torque up to ensure the threads are running smoothly and have bedded in with each other, this then gives the most consistent bolt loading.

I do not understand why Burtons would recommend a substantially lower torque as there appears no engineering basis for that change and i suspect it is an error and confusion with the smaller bolts used in the early 116E rods used in the Ford 1500 engines and in the early twin cams. A "heavy duty" bolt for the smaller rod bolts could be torqued to a higher value than the original smaller bolts and this may be the source of the torque recommendation.

cheers
Rohan

[AlfaLofa]

Rohan thank you for your comprehensive reply. I feel much happier now and shall use the 44-46 lb. ft. recommended in the manual.

The info from Burtons created doubts in my mind - hence this posting.

Originally (i.e. yesterday) I torqued them to 45 (as recommended by Leslie).

Then I got the reply from Burtons - so I loosened them off and re-torqued them to 35.

It now appears that I have inadvertently begun to follow your advice concerning a tighten/loosen cycle.

So I will now put them back to 45.

[Quart Meg Miles]

I've not had a failure at 45 ft-lb in 200,000 miles though I may have changed bolts once. Always lubricate threads, except for exhaust components, even if only with copper-slip.

[AHM]

From the little I know or can remember about conrod bolts.

It is all about cyclic loads and fatigue. If you preload the bolt to a level that is higher than the cyclic load then it doesn't see any cycles and doesn't fatigue. (or something like that!)

Tightening torque is a very rough guide to the preload, elongation is related to the Young's modulus so is more accurate.

I can't work out why you would want reduce the tightening torque. But without knowing the material properties neither can we say what the effect is. "eavy duty" isn't a good enough answer.

Edit - A day late and repeating what others have said! I put that down to having to unbolt the body lift it up enough so that I could get the bolt out of the handbrake tree stem, put the handbrake tree on and put the bolt in the diff side (with the diff in the way).