[dartravens]

so i got a new project its a 78 k5 got a th350 tans rebuilt i did it my self stock stall converter np203 behind it however the engine came as a shock to me ive never torn down any thing but a slant 6 till this i assumed i havea 350 so most of my parts went to that turned out after scrapibng the gunk off motor was a 817 casting 400 2 freeze 2 bolt mains so due to neglect i decided id build it up a bit

parts list ive got to work with so far
carb edlebrock 1406 600 to 699 cfm{changed to manual choke}
intake edlebrock performer dual plane {came with truck}
heads were toast warped so i found a nice set of rebuilt
993 casting gm goodwrench heads with the larger valves 2.02"/1.60" 75cc
summit 1103 cam and new lifters and springs
double roller timing chain{almost gear drived it budy convinced me not to}
400 block was original to the blazer shocking to me the cylinders and rings checked out no machining neccesary standard size bearings bought a new set retaining stock pistons they looked clean new rod bearings
new scat crank shaft [better counter weight design lighter] {original crank had damp bolt broke off on it and was half done with heli coils ... not standing for that}
extended oil pan
comes out thrue a pair of hooker comp headers long tube
2.5 pipe
to thrush glasspacks dumping out just before rear axle
dizzy is a summit SUM-850001R
all apart right now but test fit seems ok. now while apart any suggestions concerns i should look for primary use is a camp vehicle maybe some mild off roading live in arizona after all.

 

[ericnova72]

I only see one thing that could really trip you up...SCAT does not make a 400 crankshaft that will fit with the stock short 5.565" long connecting rods. They have added more material to the internal counterweights for better balance and less crankshaft harmonic vibration, but this increase in counterweight diameter means the shortest connecting rod you can use is a 5.7" long, the stock length for all other SBC engines.

Trying to use a SCAT crank with the short stock 5.565" long 400 connecting rods means that when the piston is pulled down the bore nearing Bottom Dead Center, the larger crank counterweights will crash HARD into the piston skirt and wrist pin boss on the piston....in fact the crankshaft will not be able to even rotate a full 360°, you'll never even be able to rotate it enough to get all 8 pistons and rods back into the block.

Eagle and RPM International are the only two aftermarket companies that make a stock replacement 400 crankshaft for use with the short 400 rod, as far as I know.

If you want to use the SCAT crankshaft, you're going to have to purchase a set of 5.7" long large journal rods and a set of pistons with the correct wrist pin height placement(called "compression distance") to match that rods length and crankshaft stroke.

If the stock crank nose can be fixed, either by proper installation of a helicoil insert for the stock 7/16"-20 TPI thread or drilling oversize and tapping to 1/2"-20 TPI thread....and there is nothing wrong with the rod and main journal surfaces other than needing a polish or re-grind, then that would be the lowest cost way to go.
Nothing wrong with a properly done helicoil...since the coil is steel wire it is stronger than the original cast crank material, done correctly the hole is stronger when helicoiled than it was just direct thread cut into the cast crank nose. This is the same reason many race aluminum heads have helicoils in the rocker stud, intake face, and exhaust manifold bolt holes.

With all that said, once you get you crank deal figured out, the rest of your plan is good, should make a nice strong torque engine.
Your buddy is a god one to steer you away from the gear drive, you'd grow to hate it.

 

[64nailhead]

Wow, nice score. Finding 400's in as good of shape as you're describing is not an everyday occurrence. I'm not a big fan of the 993's, but for a heavy vehicle with a cam that wants to make a lot of low rpm grunt, then they'll work out fine. Just don't expect to be spinning this to 6K with any authority with those heads on there.

Pay close attention to the small details, i.e. bearing clearances, piston to deck height, etc., and you should have a real decent, torquey motor.

X2 getting your crank repaired. No need for a new crank due to the balancer bolt. The helicoil repair was be been done 100,000 times.

Good luck

 

[dartravens]

herss the crank model

https://www.summitracing.com/parts/sca-910400

 

[dartravens]

https://www.summitracing.com/parts/sca-910400 was the crank

 

[dartravens]

local machine shop says hole drilled to big already theyed have to extract fill and redrill then they claimed it would need a rebalance after that was close to 290 bucks new crank as 209

 

[ericnova72]

local machine shop says hole drilled to big already theyed have to extract fill and redrill then they claimed it would need a rebalance after that was close to 290 bucks new crank as 209

They would only have to fill and redrill to take it back to 7/16-20 thread, if it is too big to fit the 7/16-20 helicoil then just drill it for a 1/2-20 thread, if it is too wallowed out to manage that then drill it a little bigger yet and fit it with a 1/2-20 helicoil and get a new 1/2" bolt.

You need to find a different shop if they fed you that line of bull****, fixing the bolt hole in the end of the crank changes balance not one bit....the hole is on the very crank centerline for christ's sake....that's like saying you have to rebalance a tire because you replaced the wheel bearings. :nono:
Shop is trying to scam you.

Okay, with that rant out of the way...if you follow the Summit link you provided, and click on the "applications" tab, it shows right there the crank is for 5.7" rods.
I've built more than a few 400's/406's and 383 strokers using SCAT cranks, so I know about the cranks....the SCAT crank counterweights will not clear the pistons if you try to use the 5.565" rods....if you don't believe me go to SCAT's web site and look up the part #, or contact them directly by phone, they will tell you.
Crankshafts - SCAT Crankshafts

If you do just want to buy a replacement crank, it will have to be Eagle or RPM International.

.

 

[dallam11]

Before I trashed the 383 in my Camaro, it had 993 heads on it. I remember the pistons were cheap cast flat tops which made the compression a nice 9 point something to one. I could run middle grade gas but not low test. The engine had a mild but noticably lumpy idle and pulled like a frieght train. I remember a few time running the tach a bit past 6K and it never did slackin up on the acceleration. Yes, I ported those 993s and added the big valves but the motor seemed good to me. Don't underestimate their potential! Good luck

 

[dartravens]

well crap

looked back at crank and sure enough sent it back felt like a idiot and ordered a matched regrind set from another manufacture that came with bearings was only ground .010 under so wasnt crazy only set me back with bearings about 220.00 had over size bearings to match thanks for the advice much appreciated didnt even open the box just sent crank back

 

[dartravens]

would love to find a mid 70s camaro to drop this thing in would be tire smoke heaven

 

[ericnova72]

looked back at crank and sure enough sent it back felt like a idiot and ordered a matched regrind set from another manufacture that came with bearings was only ground .010 under so wasnt crazy only set me back with bearings about 220.00 had over size bearings to match thanks for the advice much appreciated didnt even open the box just sent crank back

Hey, at least it was caught before you had it oiled up, in the block on the main bearings, and 2 or 3 rods and pistons hung on it and found out it would not rotate enough to install the next pair. ain:

:thumbup:

 

[dartravens]

yah no joke there heres the block as it sits lol needs cleaned, painted and to finish dissasembly not neccesarely in that order https://ibb.co/cjzYG7

 

[dartravens]

fun

blocks in hot tank now new cam bearings going in after at this rate starting from scratch
send pics as it happens