BP-4W Build? Thoughts?
 

Hey guys haven't been on here in a while. Sold my 95 na a few years back to much rust. A few months ago picked up a 99 nb and she's healthy and up to date on maintenance. Just couldn't stay away.
Anyway was browsing my local Craigslist and came across a bp4w engine, flywheels, clutches, and a 10AE dash with airbag all for 100 bucks.
The motor spun #4 rod bearing and I think it might be salvageable but I don't know yet. The head looks clean but the guy said it has the "exhintake" cam and I don't know if its right has the 197.5* hole drilled in the cam gear but I am not sure if its correct
Anyway I plan to build the motor and head up for boost and really want it to be able to rev out to 8k. Looking to pick your brains for ideas on the build. I've got an idea, again just want to hear your thoughts on it. Anything is appriciated
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First you should figure out why it spun a bearing.

The bottom end stunk of burnt oil like really old burnt oil I'm thinking it ran low on oil was gonna replace the pick up and get a main bearing support plate from a newer motor and get a boundary pump

That crank looks rough. Take it to a machine shop and see if they think it's salvageable, or pick a better condition used one on Craigslist /scrap yard.

​​​​​​You're sitting on a lot of parts that are going to be replaced for a "build "

I know the pistons rods and bearing are going and the clutches, but you don't think its worth working the crank to run oversize rod bearings?

Get a second opinion from the machine shop. Me personally I'd be hunting down another crank. But if they can polish it and it hasn't breached the nitride depth, it might be feasible. Again, see what the builders say

I second the finding a second crank. You have to pay to get it ground down and polished, then you pay again when you get it balanced. By the time you've done that, you could have just gotten a good (used) crank in the first place.

I found out the hard way that, when sourcing oversized bearings (for the undersized crank), you run into problems if you need to have the option to have slightly odd bearing sizes (1mil under or over). Especially if you have your blocks mains bored/honed out and need slightly oversized (1mil) bearings. You can't get both oversized (1mil) and 20mil undersized at the same time.

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Taking 0.001 off the rod journals would need rebalance?
So you are saying all the journals need to be the same size?

Generally you buy bearings of the same dimension in a set. If it's all oversized or all standard depends on the wear and re-polish on the crank over time. Maybe you can buy a single oversized bearing? But that just seems half assed and you can find a decent miata crank or a lighter protégé cast crank relatively easily.

I never knew about the over/under limitations. I guess those who ever run billet mains and have it line bored, have otherwise pristine cranks.

Well what I was wondering is if I could just have that journal worked and then have the other rod journals match and not even mess with the main but I don't know if that's possible or I'm just dreaming
you said the protege crank is lighter? By how much?

That's completely possible. A good machine shop can work wonders. But those of us out here in internetland are pretty limited in the advice we can give you as compared to a competent machinist who is holding the crank in their hands.

Any time you touch the rod journals, you re-balance the crank.

And, no, the journals don't all have to be the same. You can always mix-n-match bearings from different sets. It's just that if you do, some future engine-builder will find you what you did the hard way, and hire a witch to place a curse on you. Now, admittedly, there are a lot of incompetent witches out there, but is that a chance you want to take?

I wouldn't personally switch to a Protege crankshaft. No personal experience, just a gut feeling that there's a reason you rarely hear about people breaking stock Miata crankshafts even in engines making 2-3x the original torque.

Yeah that was more or less a last-ditch effort suggestion. Miata cranks are forged Protege cranks or cast something something counterweights do a search

Ok so I'll just take it down to my local machine shop, my buddy has a build b16 ek hatch that had the crank worked from some shop down here so I'll just go in and see what the machinist has to say. Hopefully something good if not I'll start look in for other cranks.

Thanks for all the advice :bigtu:

My crank needed to be ground and I ended up getting a new one off her for 200 I think. Treasure coast maitas also has them pretty cheap. Then to balance my pistons rods and crank was $100-150 if i remember right.

I still have no idea if his exhintake cam swap is correct don't know if you guys have any clue what it should look like.

Well I might be in the market for a crank haha. Just need to see if this one is salvageable compared to the price of a good crank.

Dig up a thread that tells you how to appropriately clock the cam gear, then make sure the two front lobes of the intake and exhaust cams are pointing away from each other during install at TDC.

Ok thank you I will check when I get home and do some more digging.

Can anyone verify that my cams are actually intake cams? The guy said they were exhaust cams. https://cimg1.ibsrv.net/gimg/www.mia...14bbfa17ed.jpg
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