Assembled my stroker 381 cu in today (3.75 crank and 4.020 bore).

Installed my GMPP 847 camshaft (296/304 with .575/595 lift) and the engine barely turned over in two places. Obviously it was impacting the camshaft by the tiniest bit.

I'm using 5.7 H beam SCAT rods and their SRP flattops...that my local machinist balanced.

Seeing as the engine is assembled and I don't feel like (or have time for) disassembly, rods ground around the nut portion, rebalanced, and reassembled.

My other option(s) are get the same camshaft in reduced base circle sizing (it is made by Crane for Chevy I came to found out) or what?

By the way, the machinist did an outstanding job on the min decking request...they are straight and equal which leaves my pistons .016 "in the hole", with my flat tops, and small chambers...equates to 11.9 to 1 compression!

I think it will work...with a big cam. :D

 

Sounds like you know your 2 options. I opted to have the rods ground, but I found out before the assembly.

 

GET A SMALL BASE CIRCLE CAM AND REMEMBER NEXT TIME USE AN I BEAM ROD FOR MORE CLEARANCE :cheers:

 

My son and I built a 383 stroker motor for his '87 last year. We bought the crank, rods, and pistons from Scat. Heads are AFR 190s. I had Comp Cams grind one of their recommended sticks on a small base circle-no extra charge, and no interference problems. Last dyno run was 450 hp at wheels and 465 lb-ft torque. Ed Wright is still sorting out the chip for us-recently put on full length headers-the Accel-Lingenfelter intake was working too well against the stock exhaust headers.

 

My recommendation is to first find out exactly where the interference is; then try to measure how much clearance you'd need.

It's possible that even a small base circle camshaft won't give you the clearance you need; the interference could be that tight.

Sorry to say it, but your worse fears may well be realized.

Jake

 

JAKE I REALLY DOUBT A SMALL BASE CIRCLE CAM WONT HELP HIM OUT I USE A SMALL BASE CIRCLE CAM IN EVERYTHING THAT IS NOT FACTORY STOCK AND NEVER HAD A PROBLEM NOT TO MENTION THE TRUCK SIZE CLEARANCE YOU WILL HAVE THEN :cheers:

 

When you go to the doctor complaining of a chest pain he doesn't immediately call the operating room and break out the chest cracker to do open hear surgery.

You first run some tests and do an examination; same principle here.

Before opting to order a new camshaft and tear into the engine one really needs to know what you're dealing with; it's a basic rule of engine mechanics.

Be smart and do it right.

Jake

 

i don't think the small amount of removal on the rods will really change the balance. i know of many engines that were clearanced after balancing, without problems or vibrations or bearing wear.

what does surprise me is that you're lacking clearance. my 3.875" stroke crank cleared my cam motion cam which was on a 1.080" base circle, with .580" lift, which i thought was pretty close to what's used on the 847 cam.

can you see where there's a problem? are you sure it's not touching somewhere else, or that some other aspect (like cam bearings or a bent core) is causing a tight spot? or maybe those scat rods are really big in that area as compared to the eagles i'm using.

btw, you mention grinding around the "nut" area. don't those rods use bolts? also, the area needing clearance is on the opposite side of the journal from the bolt head.

-michael

 

Are you sure it's the rods hitting the cam and not the rod bolts? There are diff bolt heads which give more clearance on strokers; my Eagle ESP came with the ARP capscrews. You really shouldn't have a prob with only a 3.75" crank; although that's a pretty high lift cam.

 

http://forums.corvetteforum.com/zerothread?id=776168

 

Always assemble and check before having it balanced! :nono:

I too have a 3.75 and eagle H-beam rods and I couldn't get the rods to hit no matter where the cam was in relation to the rods or bolts. Make sure you find out EXACTLY where the interference is before continuing or you may be making more work for yourself. Maybe put a light coat of moly or clay on the lobe to see if there is impact. Good luck!

 

Well, thanks for all the assistance. Was out of town for few days...didn't mean to leave people hanging.

To assemble and check everything at each stage would take months when dealing with a machine shop if you want to do everything yourself-like I do. I skipped many steps serious engine builders would complete. Time and money and caution all part of the decision.

The SCAT rods are indeed H-beams (my preference compared to I beams) that do have a 8740 ARP capscrew-my apologies for stating the nut portion of the beam-just trying to clue everyone in with a simple picture.

The rods do impact the camshaft in two places-during assembly and then again after I wrapped the camshaft in masking tape and spun it over to see the cuts-two lobes and two rods right at the area where a nut would normally be located on a bolt and nut arrangement.

I investigated the reduced base circle, but Crane could only guarantee 1.5 to 2 weeks for a camshaft and then gave me lots of doubt that my CC R-type lifters would work along with requirement for shorter pushrods. So I gave up on that idea and will be hauling the rods/pistons to the machinist for some minor grinding. No rebalance will be accomplished. This will put the engine into an overbalance condition which I was assured is not the end of the world-some folks even do this as standard operating procedure.

Thanks again. Will post when this thing runs...in about a week???

:)

Also Cork...loose the capitals...that is yelling...no need to yell around here! :)

 

Good call on keeping the larger base circle, its friendlier on valve train parts.. and no need to rebalance(as you said).


Another thing, you mention .016 in the hole.. I hope you are running a very tight gaskets (like a cometic) that keeps you around .040 quench.. this is critical to running high compression safely (i've had 12-12.5:1 motors that run on pump gas every day with out issues.. but quench was ketp to .035-.040)



[Modified by JordonMusser, 11:12 PM 3/19/2004]

 

can someone define what a small base cam is for me?

I keep hearing about it, but I dont understand how it will allow clearence, and keep the same lift/duration numbers????
:confused:

 

I would start by first checking whats hitting what. The block usually needs to be clearanced for a 400 throw crank. You didnt say it was done- so this may be the cause. Check the area at the bottom of the bores and along the pan rail. You do have a decent lift cam but its not common for a stock 400 stroke crank to hit the cam.

Check to see if the rods you have a specifically for strokers. Some aftermarket rods are specific on their purpose. Caps screw style rods generally have more clearance than rods the use " a bolt and a nut".

 

Easy.

The cam bearing journals are the same size (normal sized cam bearings), but everything else is smaller in diameter.

The heel of the camshaft is for instance 2.01 inches in diameter on a Crane model I have, if you order a reduced base circle/small base circle then one size they offered was 1.04 inches in diameter. So the lifter would effectively drop ~.5 inches in the bore (diameter is twice radius, etc). This is where Crane cautioned about lifters not designed for reduced base circles as it would drop and fall below the 'handcuff' guides that the OEM blocks employ. This also means new longer pushrods as well, although that may be required as I have yet to assemble and check the length (I've decked the block and the heads and using a different head gasket-all enter into the equation of correct length of pushrod.

And to answer your question...the lobe still rises .359 inches off of the reduced/small base circle so the rocker arm multiplies it by 1.6 in my case to end up with the same lift at the valve. My intake valve lift is .5744. So lift will be the same, but duration will actually be slightly shorter due to less area under the curve...makes sense if you think about it-don't know how to describe that one...:)

Besides grinding the rods means I don't have to buy new parts-lifters, camshaft, pushrods...stuff I've already got. :)

 

More metal is ground off the back side of the cam lobe, reducing the base circle. To keep the lift/duration/acceleration the same, corresponding metal must be cut off all other surfaces as well. This reduces the diameter of the lobe, allowing more clearance, while retaining the OEM profile. Longer pushrods usually required.

In the good old days, places like JC Whitney offered "3/4 Race Cams" and "Full Race Cams" where a grinder simply ground off the back side of the lobe and the top of the lobe, giving extra duration without increasing lift. Or he could also leave material on the top of the lobe, giving more lift as well.

...It was a golden age of hot rodding... :D

...(Sometimes they actually hardened them afterward)... :crazy:

 

Now you know why the serious engine builders don't skip any steps..... ;)

Larry
code5coupe
:seeya


[Modified by rocco16, 1:39 PM 3/20/2004]

 

jake i order small base circle cams as standard issues on any stroker motor i build but that might be why i personally never saw a clearance issue neither always thinking ahead i guess :D :cheers:

 

Thanks rocco16 for stating the obvious and rubbing it in.

:rolleyes: