ARP Bolts for Fly Wheel and Clutch Assembly

[EricB]

i have a tale of woe, which I am sure others have had too. Had the engine at my builder over the winter assuming that he would do some work for me but unfortunately, personal family things have kept him occupied. So I did bring the engine back to do the work which involved conversion to a Burton Water Pump among other things and hence why I have been quizzing the forum about for a little while, as well as gathering together all the great previous experiences.

One thing that can happen over time is that some of the hardware disappears. In this case, the flywheel and clutch assembly were removed when the engine was mounted from the rear on his engine stand. Could not find the bolts involved.

What I want to ask the forum is what size ARP bolts do I need for the fly wheel? My engine is really for a 1966 MK I Lotus Cortina so it is a 4 bolt flywheel. 3/8 UNF x ? length. Also ARP bolts (6) for the clutch assembly? Decided to upgrade to ARP.

I know I will have to use their special oil around the head, no washers and for the fly wheel, torque in 3 steps to 55 ft-lbs. Red Loctite seems to be recommended. Wonder if anybody has tried the orange stuff?

Re the clutch, last time I installed it, recommended 12 ft-lbs seemed too light and did tighten it to a higher "feel-good" value. What is the forum experience?

[2cams70]

All the usual suspects supply ARP flywheel bolts. You don’t need ARP bolts for the clutch cover. Absolutely do not be tempted to tighten the bolts beyond what is specified. The correct range is 12-15 ft/lb from memory. Just use the upper range figure. Just use bolts for the clutch cover equivalent to the original. What people forget is that the fastener torque is linked to the fastener grade. If you use a higher grade fastener than original but tighten it to a figure the same as the original lower grade one there is a risk that it won’t be stretched enough and will eventually come loose. Also you risk distorting components if you tighten them to their correct specification but the components being fastened weren’t designed to cope with the higher clamp load. In summary if original components are proven to work well don’t be tempted to tinker with them.

[jmcwpb]

ARP flywheel bolt part number was 206-2802 the last time I replaced them

[rgh0]

Youngs modulus for the various steels used in the various grades of bolts is essentially constant so the amount of stretch in a high grade bolt is the same as with a lower grade bolt of the same dimensions and the same clampimng load or torque. The higher grade bolt can stretch more when it is torqued higher as it yields at a higher stretch

cheers
Rohan

[2cams70]

Youngs modulus for the various steels used in the various grades of bolts is essentially constant so the amount of stretch in a high grade bolt is the same as with a lower grade bolt of the same dimensions and the same clampimng load or torque. The higher grade bolt can stretch more when it is torqued higher as it yields at a higher stretch

??!! Why is it the case then that higher grade bolts of the same size generally have a higher torque specification than one of lower grade? Doesn't it make sense then that if you have torqued a bolt insufficiently there's more of a risk of it coming loose?

[rgh0]

The general torque specs provided for bolts of differing grades reflect the torque the bolt can withstand and this varies with grade. Higher grade bolts can take more torque and provide more clamping load as they can be stretched further.

If a specific torque that differs from the bolt grade torque is recommended by the manufacturer of a component you need to understand the reason and whther you can deviate from that safely. Recognise that the clamping load may vary by as much as +/- 50% if tightening based on torque alone as the friction can cause signficant variation in coverting torque to bolt stretch to clamping load.

Torque to yield bolts are a different story but the principles are the same but the torque or angle turn takes the bolt above its yield stress point as compared to conventional bolts that the torque take to less than the yield stress point

[2cams70]

If you use a grade of bolt that has a higher torque specification than the actual torque you are tightening it to it has a greater likelihood of coming loose because it isn’t being stretched enough. I really don’t understand why you seem to find it so difficult to just agree or disagree with that statement Rohan. I don’t need any more corporate type gobbledegook!

If the rest of you think it’s always a good idea to throw away your original uk made GKN bolts and replace them with shiny chrome plated Chinese made grade 10.9 labelled Allen screw go right ahead. Please send me a PM so I can collect all your old bolts though!

[Craven]

You guys are omitting the important plastic phase of the bolt, this is the range where the stretch in the bolt is taken beyond the elastic stage where the bolt will return to its original length to where the stretch becomes permanent.

[snowyelan]

The plastic phase doesn't really apply to the majority of bolts that are properly torqued. The bolt remains in the elastic phase which allows them to typically be re-used.

A torque to yield bolt (or an over torqued bolt) will enter the plastic phase and permanently stretch, which is why they are single use fasteners.

Rohan explained that regardless of the bolt spec the amount of stretch at a given torque is pretty much the same. The higher grade bolt just has the ability to stretch more, and generate more clamping force.

I do agree with 2cams70 about choosing known quality fasteners. I've received some really poor quality fasteners from some sources I considered reputable. I have an old fastener collection that I check before sourcing new.

[2cams70]

Rohan explained that regardless of the bolt spec the amount of stretch at a given torque is pretty much the same. The higher grade bolt just has the ability to stretch more, and generate more clamping force.

In other words if you tighten a high grade fastener to a figure less than the specified figure for the fastener itself it may not generate enough clamping force to prevent itself coming loose.

[rgh0]

If you use a grade of bolt that has a higher torque specification than the actual torque you are tightening it to it has a greater likelihood of coming loose because it isn’t being stretched enough. I really don’t understand why you seem to find it so difficult to just agree or disagree with that statement Rohan. I don’t need any more corporate type gobbledegook!

If the rest of you think it’s always a good idea to throw away your original uk made GKN bolts and replace them with shiny chrome plated Chinese made grade 10.9 labelled Allen screw go right ahead. Please send me a PM so I can collect all your old bolts though!

To keep it simple .... I disagree

I try to keep my engineering disussions to terms and concepts that the average member of LotusElan.net can understand. If you see this as corporate type gobbledegook you dont need to read my posts. If you dont understand the difference between youngs modulus and the the yield point of different steels and how they affect allowable torque and stretch I cant help that.

cheers
Rohan

[snowyelan]

Rohan explained that regardless of the bolt spec the amount of stretch at a given torque is pretty much the same. The higher grade bolt just has the ability to stretch more, and generate more clamping force.

In other words if you tighten a high grade fastener to a figure less than the specified figure for the fastener itself it may not generate enough clamping force to prevent itself coming loose.

I think you have misunderstood what I meant. The clamping force between the grades increases at the same stretch rate, so its apples to apples until the low grade runs out of strength.

[EricB]

Thanks for the healthy discussion. Many good points for me.