Hello everyone,

Building a 1.8 engine with a .25 undersized crank and a block that has been line honed with ARP main studs. I am getting from the machine shop that we are at .003" oil clearance on the mains. Under the Mazda spec of .004, but is this going to be too wide? From my research its not really where I want to be. Will be going for street/drag strip setup with a 300-350hp goal. The oil squirters will be deleted and I have a BE non-VVT pump. Rod clearance is .0022. I could get a hold of a standard crank and have more options to tailor clearance I guess? Just not entirely sure whats best.

 

I would personally not run a ground crank with those power goals. I’ve had bearing issues a couple of times. This was before running OE main bearings, which id also suggest.

im a few memosas in, but I thought mains were also .0015-.0025ish.

 

Thanks, this was also a concern of mine as well, but it is the only crank I have. I will probably look into finding a stock crank and getting the mains closer to .002-.0025 if that sounds more normal for power goals.

 

Well double check my math. .004 doesn’t sound like anything Mazda would recommend but maybe im
wrong. .004 sounds like: “your engine is fucked but it’ll probably be ok.”

 

Stock main clearance spec is .0007-.0014. .004 is the maximum clearance limit for rod and mains. Does seem huge lol







I guess my concern is with low hot idle oil pressure? I mean I will have the squirters blocked off which may help with a possible low idle pressure, but they wouldn't be open at idle anyway. IDK maybe I am overthinking

 

You may already know all this, but for the sake of completeness:

Journal bearing design is primarily a function of bearing surface area (width and diameter), rotating speed, load, oil viscosity, and oil pressure. If you increase the load, you need more pressure to maintain the minimum oil film thickness. If you increase the clearance, you reduce the pressure. My concern here is that you're talking about doing both of those things. Deleting the oil squirters will help offset the reduction in oil pressure, but we would need flow distribution data in order to estimate if that would be enough.

You're also adding a turbo, which will further reduce the pressure to the mains. You're also going to be running hotter oil than stock, which will further reduce the pressure to the mains AND directly impact the minimum oil film thickness requirement for a given load.

For a stock engine making 112 hp, 0.004" may be an acceptable worst case limit, but when you 2.5X the load on the bearings, add an oil consumer, and increase the temps, I would not consider those limits to be valid anymore.

 

Thats great, thanks for helping me wrap my head around this better. So, looks like I will be going to a standard crank and will try to tighten that up. Would .002" be a better target?

 

The ideal clearance is .002". I would not setup a new bottom end with more then .0025". The oil pump is directly driven off of the crank and they do not fare well with excessive clearance. For a performance build it is always best to have the crank ground. For turbo applications I prefer the 10mm thick oil pump from the VVT eng, Mazda used this on the non-vvt MSM. My recommendation for the power level your seeking is to run a good 5,10-40wt oil. Oil pressure is the supply and not relative to the load capacity since plain bearings rely on hydrodynamic lubrication.

 

Conventional wisdom on miataturbo.net seems to be to use a full sized crank, but FWIW all the cranks that Flyin' Miata used for built motors were ground down one increment (0.25mm, IIRC). I bought my crank and block from them originally, so that's what I have and it made 350 rwhp just fine. Track day car, not drag strip. ACL bearings, but I don't remember what the clearances were.

--Ian

 

Thank you everyone for the input, much appreciated.