I have always wanted a MINI and when the opportunity presented itself to pickup a mint 2010 JCW I jumped. Bought this from a friend of mine that originally picked it up to flip. Long story short, after encountering some issues I decided to have the engine pulled and sent to a machine shop for a rebuild. One of the best local tuners didn't want to touch an engine if they themselves were not going to install, but recommended another machine shop that also does their machining. This shop is more into domestic race engines. Anyway, I delivered all the parts to them and they are starting the assembly now. Today they called saying the tolerances weren't close enough on the main and rod bearings I provided. I went with King in standard size. They said they would like .001 (.026mm) undersized. They said the part numbers they cross referenced were CR4633XP026 for the rod bearings and MB5758XP026 for the main. The only place that I see undersized bearings referenced is at Summit Racing, but when I look at ECS Tuning, Detroit Tuned, Sneed4 Speed they all reference a + or oversized. The guy at the machine shop was trying to explain to me but I was a bit lost. The two above mentioned bearing sets are not available until June.... When I look at ACL catalog for bearings I don't see any listed as undersized. What am I missing? They haven't done anything to the crank, it was in fine shape. I went with CR pistons bored over to 77.5mm and supertech connecting rods.

 

These are what we have:

https://www.ecstuning.com/Mini-2010-...shaft/Bearing/

Maybe they have undersized special order or something?

 

Mini uses a color code system for crank mains, both upper and lower, based on codes found on the crank and upper half of the crankcase. Unfortunately, I don't have a breakdown of the size differences for the five different colors. If you want details of the system, send me an email address or some way to send you a PDF attachment, I don't want to publish this info online.

I successfully used rod bearings supplied by the rod mfgr, CP - Carrillo, but also didn't take measurements --- blind luck for a DIY'r.

 

Thanks for the replies. I had a suspicion this was a lost in translation issue, and it was. The machine shop insisted they needed an undersized bearing of exactly .0026mm. Most bearing I was seeing referenced a + or oversize. Technically both are correct, it has to do with how metrics are referenced in tolerances. I called King directly and they explained, also, they happened to have two sets left in stock. They told me to order through Summit and they were mine. Beats having to wait until June.

 

I think the confusion is when the machine shop said "undersize" he meant the crank was, and that the bearings needed to be oversized.
But the terminology for the bearing being an "undersize", was the bearing needs to be for an undersize crank. (due to wear)
So the bearing has +.026mm of material to make up for the crank wear.
They didn't tough the crank, but the journals wear with the bearings.

Regular bearings would have resulted in very low oil pressure, and premature wear in the long run.