I am in the process of building my 383 LT1...I have the bottom end put together and it rotates nicely....after torquing the heads on i can only get maybe a quarter turn on the crank before it brings up...same thing in the opposite direction...Please offer some ideas as i need this running before next weekend

 

Plugs installed? If so, you are working against compression.

 

plugs are not installed

 

sounds like posble piston to valve issue

 

Just the heads installed? Or heads, cam, lifters, rockers, pushrods....?? Check your valve train and make sure nothing is binding.

 

it was a complete rebuild. the shortblock rotated nicely, soon as the heads were installed and valvetrain installed i know something wasn't right..Valve train is good and propper pushrod length.

 

The most likely scenario is a valve-to-piston clearance issue. Back off the rockers until they are fully loose and try to rotate the crankshaft again. If it turns two full turns without hanging up, you'll need to track down the problem within the valve train. Did you use a timing chain set with multiple keyways? Did you set the intake centerline using a degree wheel and an indicator on the cam or the retainer? Did the marks on the timing chain line up correctly when you bolted the cam sprocket into place.

If it still locks up with the rockers backed off, the problem is within the shortblock. Did you check the block clearance to accommodate the full rotation of the rods with a stroker kit? Is there interference between the rods and the camshaft? What is the piston-to-deck clearance?

When you find the problem it will likely be merely an oversight of some sort during the assembly. Take your time and check all the details again.

Good luck.

 

I will try backing off the rockers fully tomorrow when the shop opens. The timing set that i used the cam gear just had the 3 holes for the bolts and one hole for the cam dowel. The cam was installed and degreed and it was dead on. Yes the cam/crank lines lined up properly. The shortblock was in the car previously, but with 5.7" rods...when i bought new pistons i replaced the rods for 6" rods...it rotated completly without the heads installed. What is the purpose of grinding the rod bolts on certain rods? Some of the ones in the shortblock with the 5.7 rods were ground down. There were no issuses with cleaance on the block so i don't know why they were gound down.

 

What type of pistons did you use. Flat tops with valve reliefs? If so, the pistons have to be installed so the large and small valve reliefs match up with the valves in the cylinder head. Just a shot in the dark.

Mec

 

They are probe 8cc dome pistons..they have valve relief pockets.

 

If the cam was accurately degreed, then one of the next potential issues could be a valve relief that is not deep enough or not aligned with the valve. Just because the piston has a relief doesn't necessarily mean that it is deep enough or in precisely the right spot. You didn't mention the specs on the camshaft. If it has a lot of duration and steep ramps, the interference could be traced to those elements. Did you use a dial indicator or a strip of clay to measure valve-to-piston clearance? While I usually set up an indicator, the clay method will give you more information in a short time in this kind of situation.

With only the #1 exhaust valve lash adjusted and all the rest of the rockers backed entirely off, align the degree wheel to indicate #1 TDC, rotate the motor through the exhaust cycle past full lift and into the valve closing event. When the pointer is aligned with 12 degrees BTDC, you will be very near the point at which the exhaust valve is closest to the piston. If you don't get contact at that point, loosen the exhaust valve and adjust the last on #1 intake. Rotate the motor until the #1 intake valve begins to open and check the clearance at 12 degrees ATDC. The 12 degree number may vary depending on the position of the intake centerline but it won't vary by more than a few degrees, usually between 8 degrees and 12 degrees. Even if you have clearance at those points, you should check to see how much clearance you really do have.

All of these suggestions are meant to help you through the situation and are not intended to sound ridiculous. Please don't take offense but here are some other possibilities based on things I have seen over 40 years of messing with motors:
1. Valve reliefs positioned at the lower side of the cylinder instead of the top (piston upside down).
2. Using 5.7" pistons with 6" rods (or the opposite although that's obviously not your problem in this case).
3. Rods that are contacting the cam or block (although that should show up before the heads went on).
4. A slightly misplaced valve relief (or a slightly relocated valve guide) that caused interference although a casual visual inspection showed that everything was fine. It was fine except on cylinder #1.
5. A piston dome that was not shaped correctly to fit the combustion chamber.

You will eventually work through this and find the problem but it will cost some sleepless nights until you do.

Again, good luck

 

No offence taken and i thank you for you suggestions. I am going to print this off and take to the shop with me tomorrow.

The cam specs are

LSA - 112
valve overlap - 19.4
centerline - 110
duration - 240.7/245.1
lift with 1.5 rockers - .558/.553