bdk70monte Posted April 4, 2014 Share Posted April 4, 2014 I did a 350 to 402 engine swap in my 70 Monte Carlo over the winter. The 402 is a 71 block and was a fresh rebuild that I purchased from an individual. Now that I'm to the point that we can fire it up, it seems to run and sound good, but there is a knocking noise. The weird part about the knock is that it comes and goes. It'll knock for a minute or two and then go away for a minute or two before coming back again and so forth. I plan to try the usual things with it like adding a second valve cover gasket to see if it's just something hitting there. Also plan to try changing the fuel pump and rod. The fuel pump was already on the motor when I got it, so I'm not 100% sure if it's for a big block or possibly a small block one that got stuck on there. Also plan to get under it and check the flywheel that was changed with the engine swap as well as the torque converter bolts. Any other suggestions as to what might cause that kind of noise that comes and goes like that? Thanks Quote Link to comment Share on other sites More sharing options...
overdrive Posted April 4, 2014 Share Posted April 4, 2014 Sorry you have this problem. Use a piece of rubber hose or a piece of wooden dowel rod as a stethoscope, if needed. Can you narrow down the area the knock comes from? High, low, front, rear, left, right? Dan Quote Link to comment Share on other sites More sharing options...
bdk70monte Posted April 4, 2014 Author Share Posted April 4, 2014 It's hard to tell for sure, but it seems to be coming more from the top and passenger side. Quote Link to comment Share on other sites More sharing options...
Mike 57 Posted April 5, 2014 Share Posted April 5, 2014 Dull low pitch or sharp sounding? Does it have forged pistons? they knock until they get warm. Quote Link to comment Share on other sites More sharing options...
VETTEWITHLEGS Posted April 5, 2014 Share Posted April 5, 2014 LONG SCREWDRIVER IS WHAT MY UNCLE USED Quote Link to comment Share on other sites More sharing options...
bdk70monte Posted April 7, 2014 Author Share Posted April 7, 2014 Well, we eliminated the fuel pump and water pump from causing the knocking this past weekend. It's hard to tell even with a stethescope where it's coming from for sure. We did notice that it runs a little rougher when the knock starts and then smooths back out when it goes away. The strange part is that standing behind the car, you can hear the knock through the tailpipes. The local bodyshop let us borrow a camera, so we ran that through the spark plug holes and everything looked immaculate in the cylinders and pistons. When we ran the camera down the intake ports though we saw a lot of carbon build up around the vales. Although we were not able to find anything solid that would be holding the valves open at least on the intake side. I'm thinking I probably should pull the heads and have them reworked though since the rest of the motor is fresh. Apparently they just threw some used heads on it and called it good. I'm hoping that will fix the problem, although there are no guarantees that's what it is. Any other suggestions? Thanks Quote Link to comment Share on other sites More sharing options...
Patrick Sheean Posted April 7, 2014 Share Posted April 7, 2014 Hydraulic lifter failing? When it fails to pump up you get the noise and the valve does not open causing it to run rough? Quote Link to comment Share on other sites More sharing options...
overdrive Posted April 8, 2014 Share Posted April 8, 2014 My brother had a 72 Chevelle years ago and one day he was a couple hundred miles from home in it. The motor suddenly started knocking and he headed home. He said the knock got lighter the further he drove and completely went away before he got home. We think it was caused by a piece of carbon that came off a valve and when it got beaten small enough, it went out the exhaust. The carbon may be your problem, but it would have to be numerous pieces coming loose to give the symptoms you describe. I'd think that to be unlikely. Dan Quote Link to comment Share on other sites More sharing options...
badddoin Posted April 8, 2014 Share Posted April 8, 2014 A header gasket leak can come and go like that sometimes. Look at the gaskets real close when you pull the heads. Quote Link to comment Share on other sites More sharing options...
John S Posted April 8, 2014 Share Posted April 8, 2014 In 1982 had a 1970 402 that had a dull knock that would come and go. Put on different heads and had machine shop go thru them. Was told valve guide was worn. Installed heads knock was gone. John S Quote Link to comment Share on other sites More sharing options...
bdk70monte Posted April 11, 2014 Author Share Posted April 11, 2014 Well, the heads are off to the machine shop. Hoping that will take care of the issue. Fingers crossed. Quote Link to comment Share on other sites More sharing options...
bdk70monte Posted May 12, 2014 Author Share Posted May 12, 2014 Well, it didn't turn out to be the heads, but at least they are nice and fresh now. What it ended up being was the cam hitting the distributor. It wasn't doing it as much after we put it back together but what we ended up finding was that one of the lifters went bad and caused the cam to get wiped. So needless to say, a new cam, lifters, rockers, and push rods later and the knock is no more. Some of the rockers were looking pretty worn, so decided to replace them along with push rods as well. The cam is just a touch over stock with a 519 lift and 284 duration. It's got a great sound out the back exhaust though. Can definitely tell it's a big block. The next issue is that it seems to run a little too hot though, around 200, even with a new 160 thermostat in it. Planning to take the thermostat out and see what that does. Thanks for the input guys. Always great to be able to post on here and get advice and info when needed from others that have been through similar things. Ben Quote Link to comment Share on other sites More sharing options...
bdk70monte Posted May 15, 2014 Author Share Posted May 15, 2014 Well, it seems that it was just a bad thermostat that was causing the big block to run a little too hot. Tops out around 190 now, which is the range where the radiator always held the small block as well. Quote Link to comment Share on other sites More sharing options...
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