65postgs
So... I have been trying to start my 401 that has been freshly rebuilt. I couldn't get it to fire a couple days ago. I checked a number of things to make sure I had everything right. When I took off the valve covers, I found that all of the exhaust valve pushrods were bent. The timing all lines up with TDC. The motor has been sitting in the garage for 3 years after a rebuild. I prelubed the engine along with relubing the cam a couple weeks ago. The cam is a 268 comp cam along with all the correct springs and valvetrain ( I believe)
Anybody know why only the exhaust pushrods are bent and not the intake pushrods? I really could use some help.
AMX69PHATTY
Piston to valve clearance ?
Rockers adjusted too tight, not allowing valve to close ?
Valve spring retainer hitting top of valve guide ?
Some cams have more lift and duration on the exhaust.
Doesn't sound like fun, sorry for the misfortune.
ironman_gq
how much lift can you run with stock springs before you need to worry about binding?
FuzzFace2
ironman_gq wrotehow much lift can you run with stock springs before you need to worry about binding?
I am running a Comp 280H cam .490 lift I am also running their springs and every thing clears on a stock 360.
BTW that was the largest cam Summit said I could run before I would need to do machine work to the heads. Also going any bigger in cam and you need to bump up the compression.
My .02
Dave ----
65postgs
This evening I checked the springs to make sure they weren't binding. I have about .060" clearance which matches the minimum Comp cams recommends. I pulled the heads and the pistons and valves look good. I am going to look at the rocker arm assembly tomorrow. When I crank it over by hand, the exhaust valve is not opening all the way before it binds up. The comp 268 cam has .456 lift on the intake and exhaust. I am soooo confused. Please help!!
tufcj
What compression pistons are you running? Double check the cam timing.
If you have to, pull a head and put some soft clay on top of the piston at the valve reliefs. Assemble with the old head gasket and torque to spec. Rotate thru several cycles and pull the head. cut across the clay and measure at it's thinnest. This will give you the valve/piston clearance. Minimum clearance is around .090", more if you're going to rev it past 6000. Better to err on the side of safety than drop a valve or break a piston.
I had problems on a 304 I built with a .468 lift and flat top pistons (10.5:1). I had to have the pistons fly-cut nearly .100" deep to get proper valve/piston clearance.
On old BBC engines, we had to retard the cam 4 degrees when we used cams with steep ramps that opened the valve quickly. The valve would open so fast that it would whack the piston after it passed TDC on it's way down.
Bob
tufcj
65postgs
these are stock 8.4 or 8.5 pistons. i have pulled one head so far and the #1 piston is at tdc and the timing marks lineup on the timing cover.
???
tufcj
You'll need to pull the timing cover and check that the timing marks on the cam and crank gear line up when the crank is at TDC. Just because the balancer aligns with the cover doesn't mean the cam is timed correctly with the crank. We had an incident with another member here where a shop used the oiling slot in the cam gear instead of the correct keyway when installing the cam gear.
Bob
tufcj
65postgs
thanks for the idea. i checked this out earlier today and the shop that put the motor did not have the timing chain anywhere close to where it should be. i appreciate all the help. thanks.
ol' school power
I'd be checking the valves too if they were all slammed shut by the pistons.
fifesjeep
Sounds like the garage who put the motor together needs to fork out some cash and repair/replace what damage they created... Definitely check the pistons for cracks... It would suck to get that motor back and you drive it down the road and the piston(s) blow apart causing even more carnage... You gotta watch these garages... just my 2 cents
65postgs
I have checked the pistons and the valves. Suprisingly no damage to either. The pushrods gave out thank goodness before anymore damage occured. And... I already have a call into the motor builder.
On a side note. I have ordered the matched set of dizzy/cam gear from bulltear. I am interested in doing the oiling mod to the front cover. Anyone have a link to some pictures or information. I would like to do this while the cover is off. I have already done the valley mod.
Also intake gaskets. Whats your opinion? I was going to not use the valley pan until I found some stuff about the pcv sucking oil on this website. Thanks for everyones help.
ol' school power
Not sure how you checked the valves, but damages don't have to visible to leak.
jeepsr4ever
It could have been some old assembly lube on the valves burned up and kept the valve from closing and then the pistons hit the valve bending the pushrod.
