Are 1mm over rod bearings acceptable for 300 whp?
 

I’m looking for 1mm over rod bearings or else I’ve got scrap my crank. Has anyone had experience with 1mm over bearings? I didn’t think it would be a huge issue machining it down that much, but does anyone with more knowledge have any opinions? My machinist seems ok with 1mm over bearings with my power goals of < 300 wheel horsepower. The bearings in question are the CR4105AM10 King AM series bearings. They’re available from summit racing I believe. It would be mixed duty driving some track duty but 90% street. I wanted the XP series bearings but they’re not available 1mm over so that’s how I’ve arrived at these bearings. Any thoughts or opinions would be very appreciated.

You should be fine with 1mm over bearings, however used cranks are cheap.. you could probably buy a used one and have it polished for what your going to spend having your crank turned and not have smaller journals.

-Mike

1mm is too much. It won't clean at .25mm-.5mm? If you have a good crank grinder, I would get a used crank that is worn for cheap and have it ground -.25mm. Factory cranks look like they were ground by drunk flunkies from other manufactures.

Yeah, makes me wonder if you or your machinist transcribed the units incorrect. Typical crank grinds start at .25mm, or .010", then step to .50mm or .020". After that I'd scrap the crank, I had no idea larger oversized bearings even existed.

it was already ground down .25mm from the previous owner and it had spun the first rod bearing pretty bad. Was hoping to get away with .75 OEM bearings or something of the sort but he said the wear was going to need .75 off from where it currently sits.

OEM bearings come as large as .75mm over. What’s another .25mm? I’m not shooting for the most astronomical power numbers either. Just about 90% of what my 2560R can put out. I was more concerned about the type of bearings. If I went with king bearings I wanted the XP lineup but only the AM series seems in stock in 1mm over, but I feel like o remember seeing the XP available somewhere. Just can’t find them anymore. The AM series seems like it is for street applications according to king, where as the XP are for race applications. I plan on 90% street 10% autocross and light duty track stuff. Will have an oil cooler before taken on the track + boundary oil pump at 72 psi.

I was quoted $100 for a crankshaft grind by Custom Crank in Knoxville and I want to say Daniel Semon in NC was selling his cranks for $350.

You just need a std core crank that needs to be ground. I would think you could find one for $100.

Why do you want XP bearings? Despite my due diligence, I had nothing but problems on a few sets of ACL Race bearings, and one set of the King XPs. Just excessive metals in the oil analysis, that was confirmed with multiple tear downs. I finally put OEM rod and Mahle OE equivalent mains in, and oil analysis have never looked better.

But regardless, I don't know anyone that would recommend grinding 1mm oversized.

Well I was wanting OEM, then ACL race bearings, then the XP bearings in that order for preference. My understanding that OEM was best but ACL race bearings were the next best thing. I’ve read more into the AM bearings and it seems like the materiel is more forgiving than a race bearing/closer to OEM. I was just hoping to find a race bearings because in my mind that was more tailored towards my situation.

I know 1mm grind is relatively a lot, but why would they make bearings in this size if it weren’t ok? Also if .5mm grind is perfectly acceptable, what’s another .5mm realistically going to do? It’s not like I’m shooting for the most ambitious power goals. Are there any proven problems grinding it down this far or is it that it just isn’t common. People make 500+ wheel HP on the stock crank. I don’t think 1mm off is gonna cause problems at like 280 whp. In terms of area of the circle its only a difference of 4.4%

Unless there’s true empirical evidence as to why 1mm won’t work, I’m thinking the AM series rod bearings and OEM everything else. If it blows up I’ll buy you a beer and admit you were right lol

The hardness of the crank is only so deep and the moment of inertia/stiffness is reduced exponentially. It's much more then 4.4%. Although most cranks do bend at the connectors/weights and not the journals.

the nitride or whatever is typically only there for the first .25mm, yes. I’m simply stating the cross-sectional area is only 4.4% smaller, and while that doesn’t directly mean only 4.4% weaker, it does mean something. I’m not sure about it effecting the inertia that much, it’s a relatively small amount of material for a 30-40lb crank.

anyways I’ve messaged curly and another member looking into buying a crank off of one of them. Ill probably give the machine shop a call today and let him know not to start working on that crank. That probably concludes this thread. OEM .25mm or .50mm bearings here I come lol