ive just finished a 98 blazer 4.3 rebuild because of a spun #6 rod bearing. my question has to do with the cause so it doesnt happen again. right before the engine started knocking it missfired 3 times in a row. each one was violent like it fired the wrong cylinder; the engine sounded like it stopped momentarily. it also done this at idle a few times.like i said, it sounded like it fired a cyl way too advanced . the dist was phased properly and i had no codes. any techs out there have an idea? i replaced the cam and crank sensor in the new engine as well as the injectors. ive started it but havent driven it yet

it just seems like something effecting the timing and causing an errant spark like the pcm or something

Gary,
Not sure if the 4.3 was affected, but some engines in those years had a problem with the crankshaft position sensors causing a "kickback" like you are describing. This bulletin shows a new crank sensor part number:

Backfire, Kickback, Hard Or No Start During Crank/Start
For 1995-2000 GM 4.3, 5.0, 5.7 & 7.4L Engines
The AERA Technical Committee offers the following information on backfire, kickback, hard or no start during crank/start for 1995-2000 GM 4.3, 5.0, 5.7 & 7.4L engines. These engines are used in pickup trucks and sports utility vehicles.
Customers with this condition may have any of the combinations listed below. If the condition exists and you have a stored DTC code P0338 and it does not illuminate the service engine soon light, you must replace the crankshaft position sensor with Part #10456607.
* Backfire during crank/start.
* Kickback during crank/start.
* No start.
* Slow or hard crank/start.
* Grinding or unusual noises during Crank/start.
* Cracked or broken engine block at the starter boss.
* Broken starter drive housing.
* Broken starter ring gear on flywheel.
A condition may exist that allows the crankshaft position sensor to command 50 degrees of extra initial spark advance during crank/start position. This exposes the engine to higher than normal cylinder pressures, which may result in an inoperative condition.
The AERA Technical Committee

thanks for the info bill. it never did it during start up but you never know. hopefully the new crank sensor fixed it