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very tight new bbc- HELP

2K views 9 replies 8 participants last post by  more coffee 
#1 ·
I have recently rebuilt a bbc, 454, 76 motor with new just about everything but have a slight problem I have never experianced before. When rotaitng the motor by hand (huge bar on crank or in blower drive) the engine rotates evenly and freely. If you leave the engine stood then rotate it, it gets very tight - very tight, to the point where it stops rotating and I would normally asume there is a major mechanical defect. If you continue never the less (not my normal practice) it will free off and if you keep it rotating be free and even.

A few notes, on the problem -if there is one.

so tight it seems to be locked up and bar requires good heave but only on the initial rotation -1st valve spring compression.
had the lifters and rockers backed off and rotates freely
checked valves and chambers for seal after tight spot to see if valves bent - all ok and no issues there, clearances where all ok anyway.
rotates freely when moving
every time stood initial tight spot seems to be on valve spring compression but not in the same place. Dependes where stops and tightens up ridiculously on first spring compression phase of rotation.
also creaking heard on the tight spot -like a spring creak, but only on this first initial rotation.
valve springs are new and comp cams units matched to cam not particulaly high pressure - just above stock
never had an engine this tight, had some realy tight ones but this seems very unusual as the problem goes away on further rotation.

I have had the sugestion that this is newness, especialy of the valve springs, as all the clearances are correct and the problem is resolved on rotation I would be inclined to agree but realy don,t know and that,s the bit that bothers me, the not knowing bit.

any body any experiance to pass on, I have pulled this bloody thing apart on the top end twice now but will do it again if needed - thanks in advance and best wishes from the UK.

Simon
 

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#5 ·
Hpefully u did lube the pistons real gd and the rings,because it sounds like dry rings much like when u shut off an alcohol or nitro motor because of the non lubricating qualitys of the fuel there is always both squeak and initial tightness when u go to run the valves even with the plugs out. Try and squirt some oil in the cylinders and then try it .
 
#6 ·
Thanks Guys for the info, I have checked all the clearances and the motor was the same before the blower was on and as stated turns freely once rotating, the cam is gear driven and I have watched the cam movement on rotation and it appears to be stationary, no walk forward or backward that is noticeable.
The crank end play on the thrust also seemed fine when the rotating assembly was installed and this rotated perfectly at the time and does so if you remove the rockers and push rods, the problem disappears completely.
I have oiled the bores though these where oiled up before and the whole motor is well lubed up anyhow.
The bores are fresh honed however with moly rings, 25 tho gap for street blower application.
The problem however still persists, it is absolutely fine on rotation but stop rotating and leave a while and its always there on that initial rotation then goes and is never in the same spot.

Will keep looking

Thanks again Simon
 
#8 ·
I had a 454 do me that way once and I thought it would be ok since I could move it once you got it going, but I was wrong, we started it up to break in the cam and it locked up tight after about 5 minutes of running at over 3500 rpms. The problem on mine was the crank wasn't polished. I had it turned .010 under but the machine shop that turned it didn't polish it. Since then every motor I've built I've had the crank polished, and I have never had anymore problems. We also have our brand new cranks for our Pro Mod motors polished before we ever put it together. Our Pro Mod blown alcohol motors turn freely by hand via 3 foot long bar on the blower pulley. Hope this helps.
 
#9 ·
I am no professional mechanic but a hobbyist. I would check lifter bore maybe. I once put a 4 cylinder together w/ the timing chain wrong. my cam lobe was coordinated with crankshaft stroke in such a way that my piston was at TDC at the same time the valves were opening up. They basically were crashing into each other. I had the same effect though. It would turn over but got real tight. I used a big bar to see just how tight it would go and it eventually would turn over ( after it bent the valve, that was what gave) anyways, just a thought. Good luck
 
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