[SixThreeLane]

I got a Scat crank as a part of a Coast High Performance 347 stroker kit a while back. It's going into a 100% street-driven car. I just got around to measuring all the journals on it, and I found something a little funny. Turns out the main journals start out, from the #1 journal, at 2.2482" - totally within spec. However, as you go down the line, the journals get incrementally larger. The #2 journal measures in the same as the first one, the #3 journal is about .0001" larger, the fourth one is about .0003" larger, and the final #5 journal is about .0005" larger, coming in at 2.2487". So the mains are microscopically bell-shaped, for whatever reason.

Now the thing is, even though .0005" difference seems like a lot, these journals are all technically within spec. According to Ford literature, they can be anywhere from 2.2482" - 2.2490", although I'd expect more precision from a "performance" aftermarket crank, personally. Now, my main housing bores, at the moment, are still in stock condition, and measure in at roughly 2.4412" - the very smallest they can be and still remain within spec (specs are 2.4412"-2.4420".) I plastigauged the crankshaft installed in the block today, and this whole setup left me with a measly .0015" clearance on one end, working its way down to .001" on the opposite end.

It's my understanding that in a best case scenario you really need to have about .0020"-.0025" oil clearance on your mains for these engines, is this right? Of course, if you go by the book, the clearances can go from .0005" to .0024". So by that measure, I'm still well within specs, but I'd prefer to be closer to .002" than I am right now. Since I'm going to be getting the block align honed anyway, should I just tell the machine shop to open up the main housing bore about .0006" or so from what it is now, and give me a little extra room to play with? Should I go out and try to find some different bearings with looser clearances? Should I try to send the crank back and have them regrind/polish it for me? I really don't want a crank that's ground .010 under, but I understand that's often standard procedure in these cases. And they can't just send me a new crank, because the whole rotating assembly is already balanced, and they'd have to redo everything. Not to mention I'm short on time already.

Trying to figure out the best course of action here...

 

[PSIG]

Not to worry. You could do any option you listed, but I would simply grab a set of HX bearings and swap all except 1/2 of #1. That will give you .002 to .0023 across all of them. That would be just fine. Although it's a street engine, it will be used at race loads occasionally, but not for long periods like an endurance engine. So, that clearance will be good for any rpm you might want to hit. Get your side clearances right for oil control, run a standard pump at 50-60 psi hot, and you're good. If you're running a thin oil and can't make the pressure hot at revs, swap to a HV pump (not HP pump or heavier oil) to get it.

David

 

[SixThreeLane]

Much belated thanks for your answer!

I figure I may as well bump this thread to include a new question, since it also pertains to the exact same crankshaft...

How do you guys feel about crankshaft thrust surfaces on these cast Scat cranks? I was reading some (possibly unreliable) things that made me doubt the quality of the finish on them... saying they are a bit rough with machine marks when they should be smooth and polished looking. So I went out and checked mine...

I don't have anything to compare it to, but it seems slightly rough to me. I'm only worried because I read some horror stories about chewed up cranks and thrust bearings and such, and I'd really like to avoid a teardown in the near future! Can I just polish those surfaces up with some really high-grit sandpaper or something? Does it need to be re-machined there? Leave it and stop worrying about minute details? I wasn't aware until a couple days ago that thrust bearing failure was kind of an issue with SBF's.

It's pretty hard to believe Scat would be selling cranks like that if they're truly prone to failure.

 

[DanH]

the mains are no big issue , but that thrust sure the hell is . send it back

 

[Mroldfart2U]

the mains are no big issue , but that thrust sure the hell is . send it back

X-2 Six, send it back telling them the reason, maybe it got missed for the polishing due to it being friday a coffee/smoke break lunch break or something but its def not correct.....

By the looks of it it just got missed after the journal polishing, as you can see the marks left from that in the thrust area...

 

[SixThreeLane]

Man, you guys are killin' me! Haha!

I can't very well send it back at this point, it's been so long since I bought it and it's already been balanced.

I had my machinist tell me it was not an issue at all and he's never heard of any problems occurring as a result of the machining in that area. Ever. And he knew exactly what I was talking about when I asked him, just told me to make sure my thrust clearance was correct and I'll be fine.

Truth be told, it does not actually appear as though they just "missed" that area when polishing it. If you compare it to the other surfaces on the crank that are 90 degrees from the journals, it's clearly been smoothed out compared to them. Just not very much. The picture exaggerates the machine marks.

I'm going to take it by a place nearby that specializes in crankshafts only and see what their opinion of it is. They're supposed to be wizards when it comes to cranks, so I'll see what they have to say. I'm sure they can polish it up if they think it's necessary.

This thing has got to go together THIS WEEK, so I'm running out of time to nitpick about this stuff the way I love doing, LOL.

 

[SixThreeLane]

Just thought I'd give you guys an update... so after I took it to the second machinist, the guy who specializes in nothing but crankshafts, he too corroborated the first machinist's opinion that there was really nothing wrong with the thrust surface at all, and in fact most Eagle cranks he sees are even worse. So I'm just running with it, since these guys have way more experience than I do.

 

[edward68]

I bought a scat kit had short block assembled , had bad vibration it was a balanced kit, found crank bent enough to vibrate badly all the way up that it was a chore to drive at 100klm. tore engine down and had crank straightened and rebalanced