I have posted this question on the FE forum as well, but I want your opinions.

I checked my main bearing clearances using plastigage and came up with .001. Is this too tight? I am going to measure everything with a micrometer and bore gauge tomorrow to verify the measurements.

-Scouder

 

Way too tight man! You want at least .003 there. I don't know how many times you have used a plasti-gage so I have to ask how you are taking the measurements.

 

Dry bearings and crank. One piece of plastigage laying across the crank journal from radius to radius. Torque mains to 85lbs (recommended by Shelby) using moly lube on the studs.

-Scouder

 

I was 99.9% sure you had it down but I had to ask. I would call Keith Craft on that one also. He has done enough aluminum block to know the for sure answer. I bet he will chime in over at network 54. I know the aluminum probably expands more than the iron at operating temps but I can't see you not spinning a bearing with it that close.

 

Plasticgage is not too accurate for the most part. It wil ball park it for you.

I would suggest mic'n the shaft and at least using snap gauges to measure bore(if you dont have small bore gage). This will get you a true reading.

I agree, maybe the alum. expands more to open up after warmed up, but .001 is wayyy to tight. Usually .0027-.003 is good for performance motor.

 

Just an update. ( He says as the vein in his forehead swells)

Sure enough, the mics tell the tale. .0005 to .001 across the board. Crank measures fine at 2.7485. Bearings measure 2.7490ish. Will be discussing options with the machine shop tomorrow.

-Scouder

 

I have heard that when bearing clearances are too tight you can try a different set of bearings..Don't know if it is true..

 

I had a short conversation with the engine builder at Keith Craft today, and he said there is a solution using an undersize bearing set. I will be discussing it with him in more detail tomorrow. If I can get them over .0015 then I'll go ahead and run it.

-Scouder

 

That is true but if the bearings measure the same from all directions and they are where they are supposed to be then it isn't them. I think his crank is right on spec so it may be possible.

Did you measure at several differnent angles Brian? Did the bores measure perfectly round? I now trust the plasti-guage a little more than I used to. Thats all I used on mine. I think they are plenty good enough for all but big time race motors. I would say yours is on the verge of being one!

 

How about a crank shop to polish the journals? Might gain a .001 or so. Sounds way too tight for comfort even at .0015 to me.

Putt

 

Hey Guys,

The crank mics out at 2.7485 across the board. Which as near as I can tell is damn near perfect. If it were only one hole I would suspect a bearing, but all holes mic the same. I suspect that the main caps were not torqued to spec when the hone was done, resulting in a smaller hole at spec. I'm pretty sure at this point that the block is going back to the machine shop to be re-machined as opposed to messing with bearings. I need .0025 and thats about the only way I'm gonna get it. This motor on nitrous should make about 1000 sea level horsepower, and with that kind of push I don't need to be wondering if my mains are too tight. I'll probably have other things on my mind right about then!

-Scouder

 

I think you are making the right move, Scouder. As you know, once you are playing with the big boys (which you are at those hp levels) you can't play around with your clearances...especially when they are too slim as yours are...

 

Oh yeah. The plastigage. So far it has been accurate to within .0005 according to my measurements. Thats pretty darn accurate if you ask me.

-Scouder

 

Scouder,
I assume you have the crossbolts torqued while checking? Sizes for 1&5 should show as a bit different than 2-4 if you hadn't.

BUT, before you go to re-machine the block, you need to torque up the mains w/out bearings and take some measurements. Housing bores should be between 2.9412-2.9420". Also check to see how round the bores are, they should be within .0002, with .0005 at the max. You really should know where you are at BEFORE going any fiurther.

If the housing bores are too small, that would be the cause of the problem. The Shelby aluminum block is a ******* to line hone correctly. And if your machine shop is using the standard Sunnen stones in their mandrel, the machinist has had a real fight on his hands and the bores could well be off. We purchased the Sunnen stones specifically for line honing with aluminum when we started working with these blocks. Makes it a bunch easier, but most shops don't want to spend the money will just "force it through".

Also, what brand of bearings are you using? Clevite has been having a lot of problems with consistantcy and sizing. There are competition bearings that we routinely use that will open or close the clearances by about a .001 when we need to change the clearances. We do this quite often to get our targeted clearances.

BTW-you cannot polish the crank for more clearance. When trying to polish more than a few tenths, you cannot control material removal and will leave a jounal out of round. One can grind for more clearance, but you really need to cut .0012 in order to have enough material to make a true, quality grind. This will work in your case if you really have only .0005-.0010, but doesn't help when one is searching for that last .0005-been there.

Good luck,
Rob

 

Thanks for that polish info, Rob. I had heard conflicting stories so glad to have it cleared up. I knew someone would have the lo-down here!!

Putt