tube80z Posted December 1, 2007 Share Posted December 1, 2007 A friend has my old race motor and it started getting what sounded like a nasty rod knock. He took it apart expecting to find a broken crank. I've never seen one break like this and it was very close to coming all the way apart. This engine had one season under me 7500 RPM limit and a few events for him at a lower 7K limit. Just goes to show that these parts are getting older and racing is hard on things. Cary Quote Link to comment Share on other sites More sharing options...
cygnusx1 Posted December 1, 2007 Share Posted December 1, 2007 Wow, good call on pulling the motor apart and saving it from total destruction! Quote Link to comment Share on other sites More sharing options...
Tony D Posted December 2, 2007 Share Posted December 2, 2007 Were those standard sized journals, or 0.010" undersized? I have seen that kind of breakage 15 years ago in Japan in racing cranks, I don't think the parts age had as much to do with it as the conditions it was placed in. I have seen undersized crank journals break like this because of the improper grinding of the radius (or lack thereof). Was this crank magnafluxed before being put into racing service? Quote Link to comment Share on other sites More sharing options...
tube80z Posted December 2, 2007 Author Share Posted December 2, 2007 Magnafluxed and all bearings are standard. Maybe a fluke but still impressive to see. Even more impressive when you think it was still revving to about 6K for a bit after this. Quote Link to comment Share on other sites More sharing options...
katman Posted December 4, 2007 Share Posted December 4, 2007 Who prepped the crank? Real race motor cranks have the oil hole chamfers in the journal shaped different than that. I can also see tool marks in the chamfer. So I suspect the journal fillets weren't shot peened and micro polished either. This shouldn't have happened after a season below 7500, even with a junkyard crank to start. I suspect improper prep and no heavy metal balancing. Define "race motor". Good catch though. Quote Link to comment Share on other sites More sharing options...
1 fast z Posted December 4, 2007 Share Posted December 4, 2007 A raced prep crank and it still has the old press in plugs? Any competent builder would of had threaded plugs in there for cleaning. Quote Link to comment Share on other sites More sharing options...
katman Posted December 4, 2007 Share Posted December 4, 2007 A raced prep crank and it still has the old press in plugs? Any competent builder would of had threaded plugs in there for cleaning. Actually, Don Potter (one of the premier Datsun race engine builders back in the day) used to put the old press in plugs back in because he said if another builder ever redid one of his motors and the owner tried to bring it back to Don (which happened), he'd know it had been worked on because "all race engine builders put allen set screws back in the oil holes"! He maintained that modus operandi until we lost one of his soft plugs once (caught it with slight oil P drop, no damage done), which he blamed on bad plug metallurgy. Now, if he ever actually finishes a motor (doubtful), it has allen screws. Quote Link to comment Share on other sites More sharing options...
Flexicoker Posted December 4, 2007 Share Posted December 4, 2007 I'd rather press in a ball bearing and peen the edges of the holes over. Threads create some pretty huge stress concentrations. Quote Link to comment Share on other sites More sharing options...
1 fast z Posted December 6, 2007 Share Posted December 6, 2007 I'd rather press in a ball bearing and peen the edges of the holes over. Threads create some pretty huge stress concentrations. Maybe you should press in dowells for your harmonic balencer and flywheel bolts also, and get rid of your threads, so you dont create stresses. Quote Link to comment Share on other sites More sharing options...
johnc Posted December 6, 2007 Share Posted December 6, 2007 Can you guys tell where the crack started? I assume somewhere near the journal, but it kinda looks like it started out on the throw. Quote Link to comment Share on other sites More sharing options...
Flexicoker Posted December 6, 2007 Share Posted December 6, 2007 Maybe you should press in dowells for your harmonic balencer and flywheel bolts also, and get rid of your threads, so you dont create stresses. Good idea! Then I can keep them togethor with JB Weld! You really insist on having the last word all the time don't you? Unless you have experience with the method I recommended failing, stop talking before you make the rest of us stupider. Quote Link to comment Share on other sites More sharing options...
Tony D Posted December 7, 2007 Share Posted December 7, 2007 Anybody familiar with SPG Dowelling Jigs for Crankshafts? 8 dowels, offset, with a big gland nut in the center like a knock-off wheel spinner. VW has em... Many prepped Datsun engines have dowels in the flywheel (besides the LD28) to add some shear resistance, as well as positively locating the flywheel in the EXACT same place every time it's removed. Critical when you want that expensive balance job to actually be repeatable. I was looking at the photos, and it looks like something in the journal propogated outward, but it's very hard to tell from the photos---or at least on my computer it is the way it opens the photos. It's an interesting looking break, that is for sure. Blast from the past: Don Potter, the "Curmudgeon" LOL Say it in a loving way. I'm positively radiant that someone remembers someone meaner than me (when he wanted to be) ROMAFLOL!!!!! He did nice distributor work. E88 Heads as well. Is he still up in Milpitas anybody know? What was it DLP Engineering? Quote Link to comment Share on other sites More sharing options...
1 fast z Posted December 7, 2007 Share Posted December 7, 2007 Magnifluxing, isnt even enough, you NEED to Xray the critcal components, to ensure a crack is not moving from the inside out. Yes, you need to always dowell your flywheel, Ill look up my last to stroker build picturs and show how I did it. Quote Link to comment Share on other sites More sharing options...
HowlerMonkey Posted December 7, 2007 Share Posted December 7, 2007 I've seen something similar and it was from a slight bit of water ingestion. It was also on number six like most L28 hydrolock victims. Quote Link to comment Share on other sites More sharing options...
tube80z Posted December 8, 2007 Author Share Posted December 8, 2007 Who prepped the crank? Real race motor cranks have the oil hole chamfers in the journal shaped different than that. I can also see tool marks in the chamfer. So I suspect the journal fillets weren't shot peened and micro polished either. This shouldn't have happened after a season below 7500, even with a junkyard crank to start. I suspect improper prep and no heavy metal balancing. Define "race motor". Good catch though. The crank was cleaned, checked for size, magnafluxed, and all the parts were checked for balance and found to be close after a few small drills. Race motor in this case is really just an autox car and you could have said hot street or something similar. It's not prepped to real race car levels. That said I've never had a problem with them coming apart like this ever and was sorta surprised. I should also mention this crank was put in to replace a crank that was killed then I had a balancer come apart and spun on the nose of the crank. I had a race the next week and needed to get the car together. So this was a rush job at best and what little was done was all my fault. Cary Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.