HELP: Piston-Valve Clearance Issues

[LONN17]

I was trying to get the cam swap finished today and ran into a problem. I was putting on the roller rockers and went to spin the engine over to do the last few and it wouldnt rotate the whole way around. Seems to me that the valves are hitting the pistons.

Here is what I have:
Stock 289 bottom end rebuilt ~20k ago. Flat-tops w/ small reliefs
Comp Cams XE274H (.520 int/.523 ext)
Comp Cams double roller timing set
'70 351 Heads w/ 1.94/1.60 valves (CC'd @60)
Felpro "performance" head gaskets
TFS pushrods (6.950")

When I first put the cam in and used my adjustable pushrods to determine the proper length I had clearance issues. To remedy this I used the -4* keyway on the timing set. Problems solved I thought. Can being off by 1 tooth on the cam sprocket cause this? When I tightened the rockers just to the point all slack was gone I was good. When I took it down the additional 3/4 turn (is this correct?) the issues began.

When I picked the cam I forgot to take into account that the heads have larger chevy valves in them. I would image this is the sources of my problem. With as much trouble as the headers caused me going on I have pretty much decided that I would rather swap cams than take the heads back off. If I find that my timing chain is correct I am thinking of stepping down to the XE268H or XE262H (.509/.512, .493/.500 respectively)

What is the best and/or easiest way to get past this? Any help would be greatly appreciated.

 

[69Rcode_Mach1]

First you need to check to make sure that it has atleast .1" if clearance. You can't just crank it and hope it doesn't hit that will end up with nasty results. Now you can either drop the cam a bit or get those pistons notched. It isn't just the chevy valves, the duration, notches in the pistons, and the lift all work together to determine how much clearance you have.

 

[rbohm]

rcode is right, remember that even with stock valves, small block fords are usually good to go with valve lift under .500". since yours is over that, you need to check v/p clearance, and like flycut the pistons.

 

[mustangdave]

You should always check p/v clearance by claying the pistons which involves installing the heads twice. No other proper way to install a cam. That is all.

 

[LONN17]

I knew thats what I needed to do but I wanted to hear something better. You know - "they'll self clearance, dont worry about it" . I do know that I have to go to a smaller cam because I am not going to touch the bottom end of the engine.

I want to see as close to 300 reliable/streetable HP as possible. Anyone have any comments on the XE262H or XE268H?

What I have:
.030 289 w/ flat tops
worked '70 351 heads w/ 1.94/1.60 valves & extensive porting (9.5:1 CR)
Weiand Stealth intake
Holley 600 (1850)
Tri-Y's and magnaflow 2.5"

Thanks for the help.

 

[69Rcode_Mach1]

I think the XE268H is a decent sized cam for your needs, my buddy has it in his 350 camaro and it pulls well. It will be different in a 289, but make sure to clay up and measure your clearance regardless. If you can't make it then it is time to flycut the pistons. Is the engine out of the car? If so then I don't see why you don't want to touch the bottom end, it is very easy to assemble and disassemble with the engine out.

 

[LONN17]

The engine is in the car. Thats why I didnt want to pull the head/header off to clay it. I am sure I will be happy with the XE268H over whatever was in there. The next engine I build will definately be out of the car. Thanks for the info.