V8NOTE488, thread lubricants, V8 Workshop Notes, V8 Register, MG Car Club

488 Thread lubricants Over the years the topic of thread lubricants has been of interest to V8 members because coating the threads will help avoid problems in the future with bimetallic corrosion which can result in the bolt corroding solid in the aluminium V8 block. A recent query on the V8BB sought fellow members' advice on the use of Loctite on the exhaust manifold bolts and whether an alternative might be better. (June 15) Our very first V8 Workshop Note in April 1979 from John Dupont on removing and refitting the water pump mentioned "the four long bolts must be coated with 3M EC776 thread lubricant - Rover part number 605764. Failure to do so may mean the next time you come to change the water pump, the long steel bolts may be impossible to remove as they will be corroded solid in the aluminium block. Only coat one bolt at a time and put it in the pump and block with the coating wet. Do not let the lubricant air dry. Finally torque up the bolts." V8NOTE1 The recent query came from Mark Ratcliff in Bergen Norway who asked "when replacing the exhaust manifolds, the workshop manual calls for a spot of Loctite 40 on the bolts but I cannot find it on Loctite's website. What is it - thread lock, sealant of a coating to stop bimetallic corrosion? What can I use instead?" Tony Lake responded saying "I talked to one of the Loctite technical people I used to see when I was working - Tony had a career with a leading diesel engine manufacturer. He explained that RC40 has been replaced by 640. RC40 /640 is a slow cure high strength high temperature retainer. When I explained that the fastener is a bolt rather than a stud and into aluminium he feels that a better choice would be Nutlock 243 which is a medium cure moderate strength retainer containing a lubricant. The risk with a high strength retainer in aluminium is that a thread might strip on disassembly if the prevailing torque is high enough. Loctite 243 There are occasional bulletin board comments about exhaust manifolds which leak and they are probably not clamped properly. My experience is that the manifold bolts can be quite tight, for a number of reasons - crud in the thread, which may be old Loctite or corrosion, or a foul on the manifold mounting holes themselves. A tight bolt will absorb some or	even all of the recommended tightening torque resulting in reduced bolt stretch, low clamping force and a potential exhaust gas leak path. The last cylinder heads I rebuilt had tight threads so I ran a plug tap right to the bottom of each exhaust and intake manifold bolt hole and was quite surprised by how much rubbish I cleaned out. I find that anything, other than a lubricant, in a bolt hole which can interfere with free running assembly is a disadvantage, so I use oil or grease on threads, washers and under the bolt head to reduce friction, maximise bolt stretch and clamping force. Any manifold bolt should be able to run up finger tight to the washer face. Loctite won't enhance low clamping force particularly in a demanding application like the exhaust manifold. Mark Ratcliff commented that he was thinking of using some stainless steel allen head bolts from Clive Wheatley mgv8parts for the reassembly as Clive says they don't need thread lock/lubricant. Many members might be aware of the bimetallic corrosion concern with an aluminium block as we have in the Rover V8 engine and would like to know a little more of the purpose of thread lubricants, how they can avoid bimetallic corrosion and the particular challenges with a bolt and thread in a demanding environment like an exhaust manifold? Tony Lake offered the following useful notes. What is the aim and purpose of the thread retainer and/or lubricant? A properly designed assembly using threaded fasteners must have sufficient clamping force to withstand all the loads applied to it without coming loose. The worst case in an engine is that the bolts that keeps the con rod and cap together. They are subject to cyclic tensile loads that try to separate the rod and cap. If the clamping force is insufficient no amount of thread retainer will keep the two parts together. However the lubricant used during assembly has a profound effect because the lubricant and the quality of the surface finish on mating parts and in the screw threads determine the friction to be overcome during tightening and therefore the final clamping load that keeps the assembly together. Low friction means least torsion in the bolt and maximum stretch. I have never solved a loose bolt problem with a liquid thread retainer in a high load application. Thread retainer can help in light duty or passive situations where low	cost means tight tolerances are unaffordable and a liquid thread retainer gives extra security. How do various thread treatments help avoid a steel bolt in an aluminium tread locking up over time? In my experience the use of oil or grease prevents corrosion between an aluminium thread and a steel bolt. Liquid thread sealants may do this as well, I don’t have any experience with them. I guess MG or Rover allowed dry assembly on a lot of engine fasteners. The worst I have found are the bolts holding the engine mounts onto the block. They broke when I undid them during my engine rebuild, although by then they had been in place for 30 plus years. What particular challenges are there with a bolt and thread in a demanding environment like an exhaust manifold? The exhaust manifold is subject to a slow thermal cycle. In the V8 case the clamping force changes from high when cold to quite low when hot. This is because the rate of expansion of aluminium is about twice that of steel which allows thread contact and therefore clamping force to relax when hot. Aluminium is a less strong material than steel so care has to be taken when tightening a bolt, it is easy to strip a thread. In the V8 case good lubrication and free running bolts helps reduce friction and maximise bolt stretch. The coarse UNC thread must be used in aluminium with at least 2 X bolt diameter engagement to avoid stripping. A compliant gasket is required to seal the joint but it takes a permanent set once bolt load has been applied and contributes to clamping force reduction over time. The heating and cooling cycle stress relieves both cast iron and fabricated steel manifolds and inevitably some distortion takes place. The old gasket may then not accommodate the flatness error. If distortion is severe enough surface grinding will restore flatness, distortion is unlikely to occur again. After a manifold refit with new gaskets it is vital to get the engine hot several times and then re-torque all the bolts otherwise a leak is inevitable. Tony Lake then added "having written this I thought I should take my own advice and check the manifold bolts. The first bolt I checked was finger tight, its mate on the same flange was OK. Several others were not very tight, so I’ve been round both banks and nipped them all up. I suspect manifold bolts are a neglected maintenance item for a lot of owners."

V8 information and support See our popular V8NOTES listing, information gateways, top tips, spares and services specialists and spares for sale and wanted.	V8 Workshop Notes Information Gateways Top tips for new V8 enthusiasts	Technical topics Spares & services specialists V8 spares for sale and wanted