MG-Cars.info
MG MGB Technical do I need a new con rod? BBS discussion at MG-Cars.info
MG-Cars.info
Welcome to our Site for MG, Triumph and Austin-Healey Car Information.
Parts
MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.
|
|
MG MGB Technical - do I need a new con rod?
| I've been rebuilding the engine on my '68 roadster over the winter and had all the big ends and mains torqued down ok. I needed to check one of the big ends and removed the cap and then replaced it again. When I came to torque the bolts up again I found that I could not get the wrench to click. There seemed to be too much movement. It just didn't feel right. The lock tab was also distorting as the end under the bolt head turned. Now I'm wondering if the thread has started to strip or the bolt has stretched. Either way I'm not confident with the situation and am wondering if I should get a second hand conrod. My local MG place has just stripped 10 engines for spares so I can pop down there with my old con rod and a set of scales to check for weight match. Are there any disadvantages to this? Also how can I tell if its the bolt stretching or the thread stripping? Or if it's neither of these and just me not having enough grunt to tighten the bolt properly!! Forgot to say this is an angled cap with circlip gudgeon pins. |
| Steve Church |
| Hi Steve If the bolt threads are stretching it will show if you hold a straight edge to the threads, there tends to be a dip where the bolt is stretching. Peter |
| Peter Burgess Tuning |
| If the lock tab is squishing out, then your torque readings will be squirrely until it is gone entirely. NEW locktabs always! FRM |
| FR Millmore |
| FRM, The locktabs are not so much squishing out as turning with the bolt head thus 'buckling' the strap. Point taken though so will get some more locktabs. Certainly cheaper than another rod. |
| Steve Church |
| Steve Could also be caused by the clicker in your torque wrench sticking as they do occasionally -- If this is the reason---------- the bolt has been strained , don't risk it New bigend bolt required + locktab Willy |
| William Revit |
| Not sure if it's that Willy. I tried one of those torsion bar torque indicators. I recon I got to about 60 ft lbs instead of the 70. |
| Steve Church |
| What happens when you press on to torque value? I guess you have not got there because you have been arrested by the distortion in the locktab. I have seen a number of locktabs distorted in this way on opened engines. Left owners wondering whether these were a contributor to wear/failure of engine bearings. Guess there is no way of knowing post the event! If bolt threads are buggered it should be obvious. If receiving threads are buggered it is not a difficult task to remove the rods and inspect. I'd suspect locktabs are distorting caused by soft metal composition. Perhaps try a different source for locktabs? An engine rebuilder perhaps as a supply source? Might be nothing more than the locktabs, having seated accurately on the first assembly, are playing up on what is, for this rod, a rebuild. |
| Roger T |
| The lock tabs worked ok when they were used fresh. Guess I was pushing it to reuse one. These were from the MGOC. |
| Steve Church |
| Steve - My action would be to replace the bolt and the lock tab and check that the new bolt will thread easily all the way trough the rod threads, without any binding. Throwing a rod at speed can definitely ruin your day, not to mention the engine (I know from experience after throwing a rod in our TD at 4200 rpm). Cheers - Dave |
| David DuBois |
| I've looked at the bolts this evening. They appear undamaged, no waisting and no visible distortion of the thread (under an eyeglass). They thread through the rod threads with no problem. The lock tabs on the other hand are not that good. I should have some new tabs tomorrow. If the original bolts don't torque up I shall go for new bolts or a replacement con rod. |
| Steve Church |
| "I recon I got to about 60 ft lbs instead of the 70." WSM: "big end bolts 35 to 40 lb ft." Does anybody read before they post? Or take engines apart? It does all tell you that those old bolts and rods are really good! FRM |
| FR Millmore |
| FRM, My apologies. Do I read, yes I do and I ask questions when I'm unsure. Do I take engines apart. No, I've never taken one apart to this extent (but I have rebuilt most of the rest of the car). I torqued all the mains to 70 and the big ends to 37. I then went back and rechecked the mains and the left the torque wrench set to 70. Next day I needed to recheck one rod bearing and assumed the wrench was set at what I had thought to be its last use (mistakenly assumed to be the big ends). Thus the mix up. I must admit to being a bit embarassed. You are right about original materials. If I had done this with replacement bolts I would have had a sheared bolt. A lesson learnt I feel. I will probably replace the conrod and bolts with one from an original engine. |
| Steve Church |
| Steve - Figured it was something like that. First rule: Always check the spec and wrench setting even if you "know" what it is! (or especially if you "know" what it is!) And if it feels wrong, check again. I always check torque wrench settings both before and after tightening. I was mostly surprised by the other posters who should have picked it up from what you wrote - seems to happen a lot. Bolt and rod are likely perfectly OK, if there is no discernible thread pitch error under magnification. I've seen quite a few that were about in that range, with no ill effect. These bolts are extremely tough, which greatly reduces the likelihood of breakage, in contrast to higher strength bolts which may be brittle. But I certainly understand your caution re replacing them. If you decide that the old parts are scrap, you could do a forensic sacrifice and torque to failure, with the result reported for posterity. In general, the thread engagement in these rods is such that the bolt will break long before the threads strip. I have dismantled engines with very noticeable bolt stretch but no thread failure in the rod. Of course, you then have the problem of repro bolts. FRM |
| FR Millmore |
| I'm not so sure about this. Quite frankly if the rod bolt has been torqued to 70ft/lbs then both the bolt and the rod thread have been stretched. I'd be inclined to replace the rod and bolts to be on the safe side. As Dave says, not a pleasant thing to throw a rod and replacing the rod is cheap insurance. |
| Iain MacKintosh |
| I agree with Iain The bolt is history - engine failure without doubt I reckon the rod itself will be fine though Steve A new set of bigend bolts should always be on the rebuild list anyway --but thats up to you If you are 100% sure that that is the only bolt you over tweeked you could just replace that one and continue on if you're on a budget I suppose If there is any doubt in your mind that you have overdone any others I would replace the lot Shortcutting on things like bigend bolts is a bit of false economy --- in my opinion Willy |
| William Revit |
| I'm sure I have not overdone any other bolts. I remember checking the WSM and setting the torque wrench. I had reason to remove and retighten this particular big end so am confident of the others. My concern about new bolts is that they may not have the same weight. I took a lot of trouble getting the rods to balance within a gram. I may try to get a couple of original bolts. My local MG place has just broken 10 engines. They reckon that the bolts will be ok but as has been said it would be false economy. |
| Steve Church |
| Steve If your concern is weight - simply weigh the bolts and compare----- My personal opinion-- I always replace bigend bolts during rebuild unless it's a known engine to me where I can be confident that all is as it should be The problem you face is other people------- Getting bolts out of another engine unknown to you is risky as the same thing that has happened to you could well have happened to them but nothing done to fix it up, you could well be getting worse than what you already have. Overall the price of a new set of bolts is cheap compared to the cost of a blowup In my case there is no choice, I do everything I possibly can to eliminate the risk of failure ------- new bolts Willy |
| William Revit |
| Will, I took advice from a few people regarding bolt reuse when I got to rebuilding the bottom end (including MGOC technical dept). Whilst I appreciate a lot of owners have bottomless pockets when it comes to our cars, mine are not. If I replaced all the bolts and studs on the engine I would be adding over a third to the rebuild cost (I only took the engine out to replace some oil seals...I have now replaced crank, cylinder head, pistons, cam followers and push rods and had a cylinder rebore!). I appreciate your comments on when in doubt, and I have some reservations about these particular bolts, which is why I shall be replacing with new. I shall take a chance on whether the quality of the new parts is as good as the originals. |
| Steve Church |
| Good move Steve The way I look at it is if you use old headstuds, that's ok as the worst that can happen is one breaks during tensioning and can be replaced or if it breaks later on it might cost a headgasket--no big deal there But bigends are a different thing as they are in the rotating section of your engine and if one lets go it's goodnight engine. I've used bolts from various manufacturers and tend to go for more expensive ones the further up the rev range the engine is going to go Any new replacement bolt has to be better than the unknown Good luck with your rebuild - unfortunate that you tweeked that bolt but with what you have spent on your engine already a new set of bigend bolts shouldn't add too much to the pain One thing - If you replace the whole 8 bolts as a set they should be almost identical weight meaning you don't have to worry about any balance problems over and out Willy |
| William Revit |
| Just to finish off the story, I have now fitted a pair of replacement big end bolts from Brown and Gammon. They are the same weight as the originals to within 1 gram (my scales only resolve to 1 gram). And torqued them up to correct value this time. I was feeling quite confident about this until I just read the thread about Moss parts - con rod bolts specifically. I hope they are not all sourced from the same place. I'm a little uneasy now! |
| Steve Church |
| Steve, You have made a decision for reasons that made sense to you at that time. Brown and Gammon would give me a considerable degree of confidence. Stop reading. Drive it. If a problem develops, deal with it. We can never know the future. We can only live in the moment! Regards Roger |
| Roger T |
This thread was discussed between 15/02/2012 and 22/02/2012
MG MGB Technical index
This thread is from the archives. Join the live MG MGB Technical BBS now