| Author | Message | ||
     Martin Cutler Prolific User Username: martin_cutler Post Number: 185 Registered: 7-2007 |
Hi guys, Progress has been very slow on the R Type engine rebuild, but I now have all the broken studs successfully removed from the block, with no damaged threads. Quite a task. I am now ready to have made or buy a set of studs. Questions are: If I get them made, what sort of material? Can I go with a harder material and waist the studs? What about reusing the old rusty pitted studs? Cheers Marty | ||
| NORMAN GEESON Unregistered guest Posted From: 86.25.208.191 |
Martin The later 4.5 ltr engine head studs are made from either EN 24 or EN 110, (data from drawings). I think the very early studs on 4.25 ltr engines were made from EN16T and it is likely that the material alternatives were only introduced due to supply issues. I know that EN24T in bar form, in England is basically unobtainable.Either EN16T,EN24T or EN110 should suffice for the studs. Don't attempt to waist the studs, nor take them over 35ft/lbs maximum, there is no need. The trick to gasket sealing here is to get the right head gasket, not to keep stretching the studs and plucking the block surface everlasting upwards. Ristes of Nottingham England have spent much time and money on developing the gaskets with a modern filler. These gaskets tighten down fairly solid and do not keep collapsing and needing constant retorquing. Usual disclaimers, I have no connection with Ristes, but have first hand experience of their 'modern' R Type gaskets. (Message approved by david_gore) | ||
| David Gore Moderator Username: david_gore Post Number: 1085 Registered: 4-2003 |
Norman, I suspect you are right regarding the substitution for commercial availablity rather than technical reasons. The British EN series of steel alloys especially the Nickel/Chromium/Molybdenum EN25/26 grades gave us some of the most versatile materials for arduous engineering applications. The US SAE/ASTM specifications were not as comprehensive and there was a strong American tendency to select materials on cost rather than performance - EN24 = SAE4340 was top-of-the range specification and En16 [SAE4130] was the general purpose high tensile alloy steel. EN110 is intermediate between SAE4130 and SAE 4340. All these alloys heat treated to the same tensile range are suitable - you need the heat treatment code from the drawing to determine what strength is required for ordering. My preference would be EN24 and a quick check has revealed it is still available in the UK in condition T, see the following links: http://www.westyorkssteel.com/en24.html http://www.kvsteel.co.uk/steel/EN24T.html High Nickel/Chromium/Molybdenum steels - Britain's priceless gift to the engineering world and this metallurgist's universal fix-it for a wide range of technical problems when these steels were commercially available from my employer in Australia. Unfortunately, economic factors mean these alloys are gradually disappearing from the market due to accountants rather than engineers having the final say on purchasing policy. | ||
| Martin Cutler Prolific User Username: martin_cutler Post Number: 186 Registered: 7-2007 |
Thanks guys. 3/8"BSF 20 tpi. I have bought an original type copper/asbestos head gasket, but a monotorque type might be the better way to go. I think we have had this discussion before. Block is nice & flat, no raised areas around the stud thread holes. Cheers Martin | ||
| Leho Proos New User Username: lehoproos Post Number: 6 Registered: 8-2006 |
Martin As used above, 'waist' of a stud I understand to mean a reduction of the diameter of the unthreaded length of the stud to the thread root diameter. Waisting of cylinder head studs (or bolts) is done to increase the maximum energy load capacity. Dependent on the actual threaded and unthreaded lengths, a waisted stud will absorb a 20% to 30% increase in energy load for the same maximum tensile stress. Leho | ||
| David Gore Moderator Username: david_gore Post Number: 1086 Registered: 4-2003 |
I have always understood the incorporation of a groove or waist on the stem of a bolt/stud/shaft was a stress relieving measure to minimise fatigue cracking under cyclic stressing/bending loads in service. The most common location was immediately under the head where residual maching marks could act as stress concentrators and generate fatigue cracks. The second most common location was the transition from threaded to non-threaded sections of the fastener. The determining factor for the location was always the way the stress loadings were transferred to the fastener and its support in the surrounding struture. I was not aware of its use to increase energy load capacity - a classic case of engineers and metallurgists encountering different problems and deriving a common solution. The engineering factor would have been reducing the notch sensitivity of the fastener in service by allowing the bolt to relieve stresses by flexing rather than remaining rigid and being susceptible to machining grooves acting as stress concentrators to initiate cracks when cyclic loads were applied in service. | ||
| Randy Roberson Experienced User Username: wascator Post Number: 42 Registered: 5-2009 |
Does not the waisting allow the stud to stretch more per unit load, so it will stay tight through a greater range of movement of the joint, as when the gasket settles? | ||
| David Gore Moderator Username: david_gore Post Number: 1087 Registered: 4-2003 |
No - the ability of any material to move elastically [i.e. return to its original dimensions after stressing] is determined by its Young's Modulus which allows calculation of the maximum stress that can be applied to a fastener without causing permanent deformation. Reducing the cross-section of the fastener reduces the maximum stress that the fastener can withstand compared to one with a greater cross-sectional area. The way in which the load is applied is also relevant, cyclic stressing and static stressing have significantly different consequences for a given fastener. The main reason for incorporating waists/grooves on a fastener would be to improve resistance to fatigue cracking in service when cyclic loads are present. You can imagine the number of cycles that arise in an internal combustion engine from the compression and ignition strokes for each cylinder and these have to be constrained by the studs holding the head on the engine block and last for many many hours of operation before they are likely to be replaced [if ever]. | ||
| Jeff Young Frequent User Username: jeyjey Post Number: 73 Registered: 10-2010 |
My understanding is that waisting is primarily to remove sharp corners which act as stress raisers. But on that note, even more important for the prevention of stress/fatique cracking is to have the threads rolled rather than cut. If it were me, I'd want a rolled & waisted stud, but I'd probably settle for just a rolled one if I had to. Jeff. | ||
| Leho Proos New User Username: lehoproos Post Number: 7 Registered: 8-2006 |
David wrote: "Reducing the cross-section of the fastener reduces the maximum stress that the fastener can withstand compared to one with a greater cross-sectional area." Just adding a clarification - Waisting reduces the diameter of the unthreaded length of the stud to the root diameter of the thread. So a waisted stud and an unwaisted stud can withstand the same maximum stress. Leho | ||
| Randy Roberson Experienced User Username: wascator Post Number: 43 Registered: 5-2009 |
Waisting the stud also reduces its stiffness. This improves fatigue resistance, but also it stretches more per unit load, which beneficially changes the relationship between the joint stiffness and fastener stiffness. A less-stiff fastener with more preload, will stay tight better. We were both correct. | ||
| Jeff Young Frequent User Username: jeyjey Post Number: 74 Registered: 10-2010 |
I thought the locking of a torqued fastener was due to deformation of the threads (ie: stretching in the threaded area). It wouldn't seem (from a layman's perspective) that stretching in the non-threaded area would have any effect on that? Jeff. | ||
| Randy Roberson Experienced User Username: wascator Post Number: 44 Registered: 5-2009 |
Locking of the fastener is not the issue here; rather the joint can get loose (i.e. lose most or all clamping force, as on the gasket)if the gasket gives more than the fastener is stretched. Say, for example, the gasket in a finished joint (that is, with fasteners fully preloaded) gives over time in the amount of 0.005 inch, and the fasteners are stretched 0.004 inch when initially tightened. It is obvious that, once the fastener loses its stretch, it is no longer clamping the joint. If it had been stretched 0.008 inch initially, it will remain tight. In this case, the fastener has not loosened by turning, just by loss of preload through joint movement. Think of the tightened bolt as a spring: as long as the spring is stretched or otherwise deformed, it is pushing or pulling back; if it is not deformed anymore, it does not push nor pull. Many times a smaller bolt will hold a joint better than a larger one because it is stretched more per unit of preload so it will stay tight over a wider range of joint movement. The locking you are referring to is the prevention of the fastener from turning in the direction of loosening, causing gradual loss of preload; friction in the loaded threads, etc. | ||
| Jeff Young Frequent User Username: jeyjey Post Number: 75 Registered: 10-2010 |
Ahhh... I failed to take into account the compressible gasket. Thanks, Jeff. | ||
| David Gore Moderator Username: david_gore Post Number: 1088 Registered: 4-2003 |
Some additional information - a rolled thread is preferable to a machined thread in all applications where fatigue is a potential problem; the rolling process generates residual compressive stresses below the surface which act against the tensile loadings thus reducing the actual tensile stress in the metal which improves fatigue resistance. The usual practice with manufacture of replacement fasteners after a failure was to machine the waist/groove to a diameter slightly less than the root diameter of the thread/bolt shank depending on the location of the crack and knurl the groove with a smooth roller to introduce as much compressive stress as possible given the lathe available and tensile strength of the alloy used. This reduction in diameter was specifically intended to ensure maximum stresses were generated in the section of the fastener in the best possible condition to resist them. | ||
| Randy Roberson Experienced User Username: wascator Post Number: 45 Registered: 5-2009 |
I agree with David. Then the thread root is no longer the highest-stressed part, the waisted diameter is. My employer have some Wartsila engines which use the most beautiful waisted studs with rolled threads, all nuts hydraulically tensioned. They are really engineering works of art. |
