i just finished rebuilding my f20c , started it up worked fine for the road test , drove for about 5 miles all was well got home turned car off and when tried to restart the motor would turn hard like if the battery was dead , i have installed darton sleeves cp pistons 12.5:1 c/r , bc connecting rods , stage 2 skunk cams springs & retainers , and also shaved off the head , when the car cools down it restarts fine , i drove the car for about an hour no problems just when restarting its like the battery went dead i checked battery voltage and altanator output all is perfect . what should i check ? i was thinking maybe the compression is to high and its having a hard time turning over !

 

Sounds like something fits a little too tight after it warms up. Different metals expand at different temperatures. You may have gotten your tolerances wrong on something. My only guess.

 

Battery heat soak probably. Do you have stock header with heat shields?

 

This would be my guess too, especially if you have a small racing battery. I have a Deka ETX16, and it barely starts the car after driving for 15 minutes.

 

check your valve lash clearance at cold and hot. it might be stuck a little at hot.

 

Next time when hot -- try manually turning the pulley and see if the engine is tight to turn.
I'm guessing Weasel could be right.

 

i don't think its a assembly prob cause when the car is hot and i turn the crank by hand it only gets stiff when its at compression the rest of the way it turns fine , i do have headers with no wrap but they r ceramic coated , im thinking maybe do to the high compression it wont crank as easy when hot , but y ? is it cause of the battery getting to hot ? i tried a new starter and it did the same thing , also i drove the car 4 like 40 miles straight today and the engine ran great , very smooth no noise or loss of power so there is no way its seizing up !

 

Our engines emit a good bit of heat. If it has no where to go, it sits in the engine bay. This excess heat can make it difficult for the battery to do its job.

 

My car used to stumble at start up only when hot. I switched to an Optima battery and I insulated the battery with some heat reflecting material, and I no longer get the stumbling that I used to get. I also have an aftermarket header.

 

Yeah you just told us that you have a non wrapped header and im assuming no heat shields either? if this is the case then you have found your problem. Heat coated or not they still emmit heat and it needs to be defelected (heat shield) or reduced further (wrap).