JohnD Posted January 18, 2023 Share Posted January 18, 2023 Another in the long list of things I am ignorant of! I 'learn' that ARP the high performance engine parts company, say that conrods should be "Re-sized" before re-use. I had to look it up, and the process is based on the assumption that a used con-rod big end will be out of round. The face(s?) of the big end halves are ground down slightly, then bolted up and line bored back to the original round size. I'm, well, surprised! I have thought that only the bearing shells or the journals on the crank would be worn. The latter often need regrinding to restore roundness or uneven wear, but will run in new bearings of the correct oversize. They will sit in big ends that are completely unworn. The out-of-roundness that ARP refers to must be due to a 'set' caused by repeated stress, but such a set will only occur if the stress takes the part's material beyond its yield point. Which should not occur, even in 'ordinary' production engine parts. Or, does great age and long use affect the material? What does the Common Room of Sideways U. think? Mechanical Engineering Dept., unless Philosophy has a view? Link to comment Share on other sites More sharing options...
Martin Posted January 18, 2023 Share Posted January 18, 2023 When doing the GT6 engine I measured the con rods. Yes they are not exactly round. But I left it as it was and the Engine runs fine. Engine gets some bashing but for a full race engine, it may be different. For my race Polo we left it as well and never had any issue. Will have a look whether I find some notices from the measurements. Link to comment Share on other sites More sharing options...
spitNL Posted January 18, 2023 Share Posted January 18, 2023 It's not rhe eepeated stress or wera, the idea is that if you take a stock rod bolt out, and replace it with one that can clamp down “tighter”, it will distort the rod slightly and then lead to bearing failure. In order to avoid this, ARP recommends eesizing of the rod after the ARP bolt install. Link to comment Share on other sites More sharing options...
Nick Jones Posted January 18, 2023 Share Posted January 18, 2023 Yes, it is a thing. But… as with everything there are tolerances, so complete perfection is not required. When I did the Vitesse engine the rods were variously measured by two different engine machinist and one of the few things they agreed on was that the rods big-end bores were round enough. That said, in general, my philosophy has been that if the bearing that came out wasn’t clattered to death, the rod is probably ok. It’s worked so far. Link to comment Share on other sites More sharing options...
JohnD Posted January 19, 2023 Author Share Posted January 19, 2023 Thank you, all! I am better informed; so unless I buy ARP's costly parts I probably need not bother? It was just that I've read extensively on rebuilding and never come across it before. John Link to comment Share on other sites More sharing options...
Escadrille Ecosse Posted January 19, 2023 Share Posted January 19, 2023 8 hours ago, JohnD said: Thank you, all! I am better informed; so unless I buy ARP's costly parts I probably need not bother? It was just that I've read extensively on rebuilding and never come across it before. John John you would also most probably get away just fine if you did simply replace the std bolts with ARP ones. Not sure that the Triumph big end bolts are the 'weak link' in the engine anyway. Link to comment Share on other sites More sharing options...
JohnD Posted January 19, 2023 Author Share Posted January 19, 2023 And another thing - I'm on a learning roll. On Fb, there's a Ford Transit owners group, and I'm a member because of the TRansit. Someone had a fuelling problem and quoted their fuel pressure at 15680kPa (kilopascals) Now a pascal is a small unit of pressure, hence kilopascals, and I used to use them about gas mixtures, but I had to convert that value - to more than 2000psi! When people today lift their eyebrows at Lucas PI's 100psi, this seemed wildly excessive - but of course this is a diesel! Doh! John Link to comment Share on other sites More sharing options...
2.5piman Posted January 21, 2023 Share Posted January 21, 2023 Hello John, I have just rebuilt my Jaguar XK engine (1962) and it is advised to change the big end bolts. The only replacements available are ARP bolts. Like NIck, I look at the old shells and unless there is an uneven wear pattern I'm happy. Of course if the rod is re sized then the parallelism of the small and big end must also be checked after machining? One thing I did on the advise of a book I have is to check bearing shell crush. This entails fitting the bearings in their housings without the crank in place, torque the bolts down then slacken one. There needs to be a minimum of a four thou gap between the cap and block\cap and conecting rod. The mains were fine in my case but the big ends were barely two thou. The writer of the book is adamnt that this is a critical check. I obtained a second set of big end bearings and they were fine. I had never heard of this check before so thought you may be interested in it? Alec Link to comment Share on other sites More sharing options...
Nick Jones Posted January 21, 2023 Share Posted January 21, 2023 The crush is what supposedly stops the shells turning in the housing (not the tabs, many modern engines don’t have the tabs). So this would indeed make it critical. It is also sometimes given as a reason why shells that have been run shouldn’t be refitted if disturbed. Though I see not reason why not if they pass the test Alec describes (not that I feel I completely understand the test….) Link to comment Share on other sites More sharing options...
flatter4 Posted January 21, 2023 Share Posted January 21, 2023 Indeed the 'crush' on the shells keeps them in place (tags are only for assembly - especially important where it's necessary to align oil feed holes). Note you should also look at the back of the shells - any signs of fretting will be an indicator that the 'clamp' is not enough and the metal shell has been moving against the rod parent material. Unless you have some really good measuring kit (and measure at the right temperature) the appearance of the used parts is likely to be a better indicator of condition that a vernier picked up from the bottom of your tool box! I agree with the comments here - that 'resizing' is unnecessary. The only reason to do this is when someone has mixed the rod and cap halves up. You will then see out of round / steps and failed engines...... Link to comment Share on other sites More sharing options...
flatter4 Posted January 21, 2023 Share Posted January 21, 2023 On 1/19/2023 at 7:20 PM, JohnD said: And another thing - I'm on a learning roll. On Fb, there's a Ford Transit owners group, and I'm a member because of the TRansit. Someone had a fuelling problem and quoted their fuel pressure at 15680kPa (kilopascals) Now a pascal is a small unit of pressure, hence kilopascals, and I used to use them about gas mixtures, but I had to convert that value - to more than 2000psi! When people today lift their eyebrows at Lucas PI's 100psi, this seemed wildly excessive - but of course this is a diesel! Doh! John John, you need some more zeros...... modern common rail Diesels will run at over 2000 Bar = 29 000 PSI = 200 000 kPa If there's a leak in those systems it can do all sorts of damage: piecing skin, leak coming out as atomised - so ready to combust...... Even the pump - line nozzle systems will be typically 290 Bar at the injector nozzle (there is hydraulic amplification in the injector to raise the pumped pressure and lift the injector needle). Diesels are lovely things (espcially the industrial ones) - there's life in them yet! Link to comment Share on other sites More sharing options...
JohnD Posted January 21, 2023 Author Share Posted January 21, 2023 (edited) Thank you, flatter, interesting ideas/information. Piman, which book was this? Edited January 21, 2023 by JohnD Link to comment Share on other sites More sharing options...
2.5piman Posted January 22, 2023 Share Posted January 22, 2023 Hello John, Speedpro series, Veloce publishing: How to power tune XK 3.4, 3.8 and 4.2 litre engines by Des Hammill, ISBN: 978-1-845840-05-1 Alec Link to comment Share on other sites More sharing options...
Nick Jones Posted January 22, 2023 Share Posted January 22, 2023 Ah, Mr Hammill. Written lots of useful books Found this while poking about online. Only partially useful as it gives no specific figures, but explains the science a bit https://www.kingbearings.com/wp-content/uploads/2014/10/Design-of-crush-height-for-reliable-press-fit-of-high-performance-bearings.pdf Also this, though I'd not be trusting the absolute numbers as I'd think these are fairly engine-specific (or at least bearing diameter specific) https://mechanicaljungle.com/what-is-bearing-crush/ and a bit more here that refers back to the first King Bearings link. http://garage.grumpysperformance.com/index.php?threads/bearing-crush.10213/ 1 Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now