Con rod bolts torque value

[AndrewSlater]

So my engine parts returned from machining before Christmas and have been cleaning them up until now.

I am not at the stage where I can trial fit my new Con rod bearings but am struggling to find details the torque spec for the rod bolts.
The Dempsey book says 20Nm + qtr turn + loctite, but have read that later bolts take a different figure.

Can anybody point me I the right direction?

My bolts and nuts are the standard ones sold by Rosepassion.
The car is a 74 2.7.
As always any help / suggestions much appreciated.

[AndrewSlater]

Forgot to say my Porsche tech spec book 1st edition 1974 says 5 to 5.5 mkp which is roughly 50 - 55 Nm. Seems wildly different to Wayne book but no mention of an additional quarter turn.

[911hillclimber]

Might help;

I used Dempsey's book and values when doing my hill climb 3.2, stock rod bolts.

These rod bolts are the stretch type, single use only so followed the Dempsey book carefully.

All ok after MANY trips to the red line limiter and WOT nearly all the time when engine cold/ok/hot.

If you need more assurance, then have a look at the Bruce Anderson build section or a quick call to Mike at MB Engineering? Bet mike is there right now in the shop.
Very User-Friendly.

[210bhp]

Andrew

The little spec book for the year before (1973) confirms the same 5 to 5.5 mkp for 2.4 and 2.7 litre.
Just in case you thought you had a misprint in your 74 book.

Regards
Mike

[Lightweight_911]

From memory I think the bolt specifications changed during the intervening years & the recommended torque figures were reduced.

What number is on the bolt heads ? Latest ones are (i think) 12.9

[AndrewSlater]

Thanks for the responses everyone.
I would probably have torqued my rods and moved on by now but my torque wrench only goes down to 27Nm so I stopped and read up a bit more - which is where I got confused.

My new rod bolts are marked 12.9 ( as are the ones I removed that are circa 1980s ).

The little spec book says 5-5.5mkp (36.2 -39.8 lb ft ) [ 49Nm - 54Nm ]
The Wayne Dempsey book states 20Nm + Loctite + quarter turn. (14.75 lb ft) ~2.05mkp
(Wayne also says the old Porsche spec was 50Nm with no Loctite just oiled)
Bruce Andersons books shows (37 - 40.5 lb ft ) - similar to the little spec book.

So the question is which way to go ?

20Nm + Loctite + quarter turn
or
50Nm just oiled

Again any final thoughts much appreciated

[AndrewSlater]

To add to my confusion.

Could it be that the lower torque setting is for the later 9mm bolts and the higher figure for the earlier 10mm bolts?

[Lightweight_911]

I'd be inclined to speak someone like Neil Bainbridge & ask his advice.

[fetuhoe]

I agree that the subject of fasteners in 911 engines has become very confusing and the mythology that has been developed over the years continues to increase and add to the cost of building engines.

I cannot agree with Wayne Dempsey's comments that 911 Rod bolts are 'stretch' type fasteners and that they can only be tightened once.

There is nothing in any Workshop Manual to suggest that this is the case and I have studied the texts in great detail from Volume I right through to the 1984/89 supplement.

Stretch bolts are never tightened to a simple torque figure which a strong indication that the bolts are elastic in nature.

It is true to say that the NUTS being used are defined by Porsche as malleable and this means that they will deform and should be replaced.

Very different to replacing bolts.

Con Rod bolts are always designed to have a preload which exceeds the maximum alternating stress that the bolt sees in service and this means that bolts can be rated for 'infinite life' as they never see any fatigue loading.

It has become custom and practice to replace these bolts at every engine rebuild and as a precaution but IMHO it is just not necessary although the nuts should be changed.

The torque settings needed are straightforward. 50/55Nm with a lightly oiled bolt and nut.

The later manual revised this to 20Nm followed by a 90 degree turn. This should result in an almost identical preload and is NOT very different to the simple torque figure but is likely to produce a much more consistent result.

The real problem with Wayne's suggestion is that he will only tighten a new fastener once and it is quite likely that if this advice is followed the preload applied will be insufficient.

The reason for this is that new fasteners tend to have slightly rough surfaces and this increase the thread friction or 'nut factor' which reduces the preload. If this falls below a critical level the bolt may them be subjected to fatigue loading and failure will almost certainly occur. This aspect of con rod bolt behaviour is very well established.

The only issue with con rod bolts stretching is when they are overtightened to the point that they can no longer support the require prelad or if the preload is so high it deforms the rod.

If you use an ARP Bolt to replace an original Porsche rod bolt you will almost certainly have to re-size.

The torque to preload relationship is likely to be very inconsistent across a batch of fasteners and the best practice is to 'burnish' new threads by tightening them to torque 2-3 times before final checking.

I would use the old nuts for this procedure and replace them for the final tightening. This still means some error may exist due to the friction in the nut but if you adopt the 20Nm + 90 degrees this will reduce.

ARP for example recommend the use of stretch gauges for tightening rod bolts and this method eliminates any issues of thread friction and is by far the best idea.

I would never Loctite rod bolts, if they are tightened correctly it is just not required and adds another variable.

[AndrewSlater]

Thanks for the advice Chris.

There is definitely lots of conflicting information on this subject across the web.
Bruce Andersons' book also states the bolts are malleable and are of the stretch variety - and says that they can only be torqued up the once or they will be ruined. Wayne Dempsey's book gives the same picture.

When you say - "best practice is to 'burnish' new threads by tightening them to torque 2-3 times before final checking"
What is the final check? Or are they then good to use ( with new nuts )?

[911hillclimber]

When I did my 2.4 E/S engine 20 years ago I used the Peter Morgan book. That engine used new rod bolts and nuts, no loctite, simple torque and lasted well.
The 3 rebuilds I've done since are all on a 3.2 which uses smaller bolts.
I followed Anderson on the first, new bolts etc.
I used Dempsey on the other 2 rebuilds but no locktite, just a drop of engine oil in the nuts.

All these rebuilds have been successful in that they all tolerated hill climb use, the 2.4 did about 30K before I sold it to Majordad on here before DDK existed!

[Gary71]

Torque + angle is the only method for repeatability on anything important. Although I have been drinking since midday so probably best to ignore my advice!

[fetuhoe]

The early 2.0 litre engines used a relatively short bolt of 50mm length and these bolts could stretch during tightening as they had insufficient strength to produce the relevant preload without yielding.

To describe this as a 'stretch' bolt in a modern sense is a little wide of the mark as they were not designed to produce a controlled preload in the may that a modern head bolt is used.

When the rods were upgraded in the 2.2 litre and later engines the rod bolts changed significantly in terms of length and also material.

If you measure the length of these bolts carefully before and after tightening they do not change length so I believe that will be good to re-use. We have checked this in the past but I haven't recorded the information.

I have a set of used 2.4/2.7 rod bolts in the workshop at the moment and I will try to find time to tighten them up and make some measurements.