humanz 3 Posted April 11, 2008 I noticed Pug Racing on ebay is selling some competition Big End bearings in .3, .5 and .8 undersizes. Since I'm overhauling my engine, I've been told to buy some big end bearings. I wanted to know what would be a decent undersize to get for an occasional track day/ fast road car, but was told it depends on my crankshaft condition. I know hardly anything about engines. How does crankshaft condition determine which undersize to get? What would be a good size to get if my crankshaft is in a fair condition? How would the sizing change if my crankshaft was in an excellent condition? I don't know what condition my crankshaft is in since my mechanic hasn't pulled the engine down yet. Can I take a chance and buy .3 undersized big end bearings? Is the undersize too small to make a large difference? Share this post Link to post Share on other sites
Tom Fenton 1,542 Posted April 11, 2008 The size tolerance of the crankshaft determines what bearings you need. If the crank has never been ground then it needs to be measured properly for wear. If it is worn then you would have it ground to the first undersize and buy the appropriate bearing shells. This is 100% a case of precision measuring and selection of components required, NOT "mix and match". Share this post Link to post Share on other sites
jord294 92 Posted April 11, 2008 having just taken my crank for regrind, i was told the maximum that can be taken off is .3. but the guy seems to think it could be as much as .5, but not sure as to whether he could stretch to .8 Share this post Link to post Share on other sites
Miles 331 1 Cars Posted April 11, 2008 You need to get the size from your machine shop, Std size is best, then you start grinding as required but not straight down to .8, You must use the right size matched to your crank, nothing else will do at all or you will get the death rattle. Share this post Link to post Share on other sites
humanz 3 Posted April 11, 2008 You need to get the size from your machine shop, Std size is best, then you start grinding as required but not straight down to .8, You must use the right size matched to your crank, nothing else will do at all or you will get the death rattle. Ok, so it isnt simply a case of buying the bearings and installing them. I would need to have a shop machine them down to those undersizes(.3,.5 or .8). And if I did buy the bearings already ground, there would be some rattling? Share this post Link to post Share on other sites
skinner2k3 3 Posted April 16, 2008 You would use the bearings as supplied. The crank must be checked and if required ground or polished to suit .3 .5 or .8 Share this post Link to post Share on other sites
