Ok I am doing a cam/heads install & even though I don't have a degree wheel I noticed that when piston 1/6 reaches tdc the timing mark on the crank gear isn't at 12'o clock, but like a few degree behind, when I get it to 12'oclock pistons 1/6 are still at tdc1 but a bit after 12'oclock & pistons start to move down which tells me that the mark isn't at 12'oclock when pistons just reach TDC, why? Is that why they put 4* advace into our cams, because our cranks are advanced making our cams retarded? LMK if anyone ever degreed a stock lt1 & found any of this out, LMK thanx.

 

There is a certain amount of dwell time that pistons will sit at TDC/BDC, this varies by rod/stroke ratio.

All you need to worry about is making sure the cam/crank gear are 6/12º (easiest to keep track of) when you R&R the cam. A board member did the swap and thought that 'close enough' was good enough, being off by one tooth made the car run like crap (HOTCAM).

 

that I know of, its called backlash, but once the piston reaches tdc the mark won't be 12'oclock, which tells me that the crank is advanced by factory?

 

The crank is what all engine timing relates to so by definition it can't be advanced. Is the timing mark placed inaccurately? Maybe, but you'll need a degree wheel and a dial indicator to see how much.

 

The wrist pin of most pistons is offset towards the thrust side of the bore by a small fraction of an inch (1/32"??). Reasons are numerous for this, but let's just say it's to help reduce noise and wear slighly. The net effect of this is that the piston will come up and start to dwell near TDC slightly before the crank throw reaches true TDC, and then will start to descend slighly quicker after TDC. That's probably part of what you're observing.

The other part is that the timing tab may be off, the mark on the balancer may be off and the keyway in the crank may be off. If they are all off slightly in the same direction you can easily find the indicated TDC off by 4* or more. I have run into this quite often.

 

so if it is 4* advanced(i should say) then the cam would be 4* retarded, & since every lt1 cam is 4* advanced it would make up for the factory 4* retarded & be 0*

 

What part of the above post (Bud M)is unclear. The crank is NOT adv. or ret., neither is the piston.

 

yeah crank isn't but the timing marks are but whatever maybe it is the piston pin I don't know.

 

There is a chance the the crank hub has slipped or was not properly aligned (if it was removed). Unfortunently the LT1 uses a press- fit hub, meaning that the hubdoes not have a grove machined in it to accept a key on the crank. In my case I have removed the hub once and reinstalled it allowing this press fit to be looser tha the factory intended. Also I use a supercharger which tugs on the hub, and yes, mine has slipped. the solution is to get a machined hub(with a slot) and a corresponding key for the crank.