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ARP main cap bolt torque?

PostPosted: Fri May 17, 2019 9:33 am
by vxah
I have steel main caps on my 681F block with ARP bolts not studs and nuts, anyone have a torque figure for these? I have had a search but can only find the stud kits listed. There is a listing for general 7/16" ARP bolt torque but it shows three figures depending on tensile strength which of course I don't know.
Not sure when this block was originally put together but it could have been 10 years ago?

Re: ARP main cap bolt torque?

PostPosted: Fri May 17, 2019 11:57 am
by englishmaninwales
I fitted ARPs mains, but to earlier round caps.
ARP E-6372-526-kit 7/16” diameter 200,000 PSI bolts.
Torque 80 ft lbs lubricated with 3 equal stages.

But not sure sure if this is applicable to your steel main caps / later engine.
Malcolm.

Re: ARP main cap bolt torque?

PostPosted: Fri May 17, 2019 1:13 pm
by vxah
Thanks Malcolm, that would put it midway in the listing 70-80 0r 90! I searched for the number you gave but came up empty? I will be using ARP ultra torque lube.

Re: ARP main cap bolt torque?

PostPosted: Fri May 17, 2019 2:49 pm
by 2cams70
80ft/lb is way too much for the main cap bolts especially if you are using ARP lube. Just use the standard figure for torque - 55ft/lb. I asked the same question of Tony Ingram whom I bought my ARP bolts from and this was the figure he recommended. Makes good sense to me. Ideally you need to find the torque used when the caps were fitted for line boring because this will be the figure at which the tunnels are most round. The threads in the block were never designed for 80 ft/lb torque.

Re: ARP main cap bolt torque?

PostPosted: Fri May 17, 2019 5:24 pm
by englishmaninwales
Sorry, appears I’ve given you duff information. Just quoted the leaflet supplied with the (engine specific) bolts.
Malcolm

Re: ARP main cap bolt torque?

PostPosted: Fri May 17, 2019 6:27 pm
by Bud English
englishmaninwales wrote:Sorry, appears I’ve given you duff information. Just quoted the leaflet supplied with the (engine specific) bolts.
Malcolm

Gotta watch that leaflet supplied info. Not giving it a second thought, I followed the Fidanza supplied leaflet and I torqued my new clutch to flywheel setup to 30 ft/lb. Third time through the stepped up tightening sequence, at about 28 ft/lbs, the threads in the new aluminum flywheel let go. Of course by the time the first one at that stage stripped, all of the others were already over torqued. Not one of my better days.

Re: ARP main cap bolt torque?

PostPosted: Fri May 17, 2019 7:01 pm
by englishmaninwales
Hmmm...that’s a worry. Not wishing to hijack the thread, but what should I do? This was 3 years / 10k road miles ago. Pull the engine, and re - torque or just leave alone? Any way of knowing if the block threads are damaged?
Malcolm

Re: ARP main cap bolt torque?

PostPosted: Fri May 17, 2019 7:42 pm
by vxah
So Malcolm, it didn’t tear the threads out then and hasn’t now.. think I would leave well alone? Probably won’t do mine up that tight though! Did you use arp lube on them? Might not be so tight if not?
I’m guessing 55 ft/lbs with ultra torque will be more than factory build with what ever they used?
I have a Fidanza flywheel also so I had better be careful!

Re: ARP main cap bolt torque?

PostPosted: Sat May 18, 2019 6:52 pm
by Esprit2
I don't have a torque spec answer for your specific application, but...

ARP typically requires the use of their own thread lubricant, the lube has evolved through the years, and their torque spec varies with the lube used. The fasteners remained the same, only the lube changed, and the torque specs changed with each lube change (ie, coefficient of friction affects torque required). If you don't have the original instrructions and/or thread lube, then contact ARP, order the current thread lube, and inquire about the torque to use with that bolt size & lube.

As an example (this is NOT for a Twin Cam):
Lotus 907 ARP Head Studs & Nuts

Way back when...
120 Lb-Ft with motor oil (?? doesn't specify mineral oil or synthetic oil -- outdated)

Replaced by...
90 Lb-Ft with old "ARP Ultra-Torque Fastener Assembly Lubricant" (outdated.)

Currently...
110 Lb-Ft with new "ARP Ultra-Torque Fastener Assembly Lubricant".

The ARP fasteners never changed, only the lube changed... and with it, the torque spec changed.

For anything as critical as the main bearing caps, call ARP's Tech Service line, and ask.

Regards,
Tim Engel

Re: ARP main cap bolt torque?

PostPosted: Sun May 19, 2019 2:20 am
by 2cams70
The pre and post crossflow Ford blocks have slightly different torque specifications according to the Ford manuals:
Pre crossflow (which includes Lotus Twin Cam): 55 to 60 ft/lb
Post crossflow: 65 to 70 ft/lb.

I'm not sure why they are different. The Tony Ingram figure of 55 ft/lb was set on the lower range considering the use of ARP lube (the latest spec.) on the threads. My tunnels were bored with cap bolts at 55 ft/lb.

To be honest I find the ARP literature on their "Ultra lube" to be a bit strange. The claim seems to be that it's purpose is to provide more "consistent" torque readings with little or no tightening and untightening cycles required to initially seat the threads. It mentions nothing about it changing the actual torque figure and in fact when you look at the charts they publish it seems that the ultimate torque figure is the same as when oil only is used (only more consistent). Judging by the colour of the stuff it almost certainly has a component of Moly in it which is well known to reduce the tightening torque compared to straight oil.

Whilst ARP may be experts in fastener technology I doubt that they have expertise in every single engine design and application that is out in the market and the Lotus Twin Cam is a pretty obscure engine in the whole scheme of things in the USA.

Re: ARP main cap bolt torque?

PostPosted: Sun May 19, 2019 4:37 am
by StressCraxx
I have used the ARP bolts and their lube for years on conrod bolts and main cap bolts. I have found their torque specs on conrod bolts to be consistent with their recommended stretch specs measured with a micrometer on my twincam or my FF Kent race motors.

I have broken two cast iron cranks on FF motors due to fatigue failures between the #4 main bearing and the #3 crankpin.. The rod bolts and the main cap bolts did not fail.

I would not hesitate to use the torque values they recommend. It's all about the prestress stretch of the fastener and the margin over the cyclic loads.

Regards,
Dan Wise

Re: ARP main cap bolt torque?

PostPosted: Sun May 19, 2019 7:50 am
by englishmaninwales
Ok, so I guess this was always going to split the opinion.....Any more contributions please? (Where are Rohan and Nick when they are needed? :D )
Malcolm

Re: ARP main cap bolt torque?

PostPosted: Sun May 19, 2019 10:30 am
by 2cams70
The main bearing bolts are not highly stressed compared to the conrod bolts. They aren't subject to high inertial loads like the conrod bolts. There's 10 of them compared to 8 conrod bolts of much narrower diameter. The chances are that even if the main bolt tightening torque is incorrect things will still hold together unless they are drastically wrong. Whilst this doesn't negate the need to find the correct figure it puts things in perspective somewhat. Conrod bolts are easy to fasten correctly. You can measure the stretch and the female side is threaded into something (the conrod cap) with simple geometry that is similar and therefore predictable across many engine designs.

Re: ARP main cap bolt torque?

PostPosted: Sun May 19, 2019 11:43 am
by rgh0
I use hex socket Unbrako bolts in my main caps. The weakest part of this bolted join is the cast iron block threads so I torque to the recommended Lotus torque specs but use the ARP lube so the bolt tension is a bit higher than standard. Never had a problem with the main caps as the bolts are big compared to the loads they see and the metal around the centre main is a little thin which is where the blocks tend to crack at the bottom of the bore if your going to have a failure of the crank main caps.

If main cap bolting was an issue I would go the studs and nuts so you are not turning a bolt in the cast iron block but this has never been an issue

img_6317.jpg and


cheers
Rohan

Re: ARP main cap bolt torque?

PostPosted: Sun May 19, 2019 11:51 am
by vxah
I guess I’ll tighten them to what seems and feels right, probably 60 ft/lbs. I think one tends to get a reasonable feel for tightening after 40+ years.
That said all the modern angle tightening these days often makes it feel like you are wrenching bolts to tight, even though they are one use bolts. Do ARP offer any angle tightening bolt sets?