After rebuilding the 400 in our '68 GS400, were back to square one. We took the motor out originally because of what we thought was a rod knock, which it turned out to be. We bored it .060 over, new crank, new pistons, new bearings, balanced etc. As far as the oiling system, we got TA Performances New oil gears and plate. But, we didn't get new connecting rods or con rod bolts. Ater breaking it in, it ran like a champ. We then put an oil pressure guage in it, and it normally idled at 40 psi or more in gear when still cold. But, after it had warmed up, the pressure dropped to like 20 or below (can't remember exact number). Well, like after 500 miles, it started the knock again, and we think it's the same thing all over again. What could have gone wrong this time? Could the rod bolts have stretched? Or is there something wrong with the oiling system? Thanks for any help.
you DID make the oil system mods, did you not? http://www.trishieldperf.com/455_block_mods.htm http://www.buickperformance.com/oilflow.htm http://mywebpages.comcast.net/jholthenrichs/hardway.htm#High Vol Oil Pump don't forget to upgrade your oil pickup (and the block passage) to the 5/8" dia. a 68 would have come with the 1/2". the general consensus is that Fram filters are no good for these cars, the filter material is too fine and causes low pressure at operating temp. one of the jim's (burek or weiss, i forget) likes Shell Rotella T 15-40 oil because it's for a diesel and has lots of extra lubricants and cleaners. TA has some new two groove cam bearings which are really trick. you can also have the bearings coated. their new cast aluminum timing cover is also good. it already has all of the oil mods necessary.
Should have had the rods checked and reconditioned at that time. Rod knock...The big end on some of the rods my be out of round. Factory rod bolts should always be changed out during a rebuild.
20 PSI hot idle is good. what presures do you get at higher rpms??? you want about 10 psi per 1000 rpm. 10 psi at idle is not good. i like to have around 20 psi at hot idle . did you have your rods resised????
Did you at least plastigauge the bearings? I know it's too late if you didn't, just wondering. What was the damage to the old crank that was so severe you had to replace it?
you want .001 for a street moter and .002-.003 for race. ifs its more you could be losseing pressure at the crank.
Is there metal in the oil?? Anything over 10 psi at idle is ok. Try running a little thicker oil. Have to resize rods if it had a rod knock. Unless it is a race engine the oil system is fine. If there is no metal in the oil, It could be a broken flex plate. This will sound just like a rod knock when it's hot. Rod
We didn't drill anyting, we weren't told we had to by TA. Well, the motor isn't back out yet, the car is in the backyard. I believe they did recondition and resize the rods at the shop. My dad thought (more like hoped) it was the flexplate, but the noise is bad on start up, then quiets down. All the bearings spec'd out at .002 as well. The machine shop we took it to said the original crank was too scored, from the spun bearing, and it had already been turned down once. This just really sucks, having to go through it all again. We'll be getting new con rods/bolts atleast, that's if the crank isn't too messed up. Thanks for the help guys.
FWIW, my 400 has exactly the same symptoms, bad clatter on startup - occasionally, that goes away within a few seconds. otherwise runs real nice. The local wiseguys tell me it's due to a bad lifter. Seems a rod knock would persist and get worse with more rpm. Maybe your problem is similar? BTW, what is your name? dewey
I was talking to my bro (who normally drove it) and said that it seems to do it when revving between like 1800 to 2500 rpm, so i think it's probably a rod again. Oh, my name is Brent, but sometimes my bro Brian writes on here
Pretty easy to isolate a rod knock vice some other noises. Rod knocks are typically quieter during acceleration and get louder as the engine returns to idle. You can find the guilty rod by pulling plug wires one at a time thus reducing the load on that rod the noise will change considerably when you find the right cylinder. Of course the above only works if it's a rod knock. With 20 psi of hot idle oil pressure I wouldn't be surprised to hear it's something else.
It's hard to say without hearing it, but before you tear it down take a real good look at the the flexplate for cracks as stated above....and also check for loose bolts there and on the balancer. Running it with the fan belts off is a good way to isolate noises......just don't run it too long or you'll overheat it. If the knock is not very loud, check your fuel pump.......mine was noisy and it sounded just like a pounding main bearing to me.......it went away when I replaced it. Maybe my ears are just too sensitive. :bglasses:
How well did you clean the block after machine work? A dirty block will wipe a crank out within a 1000 miles. It takes about 30-45 minutes to properly clean out every single oil gallery, and the lifter holes. Whats your oil pressure like now? If the rods were not resized, then that would be your problem. I would take the oil filter and cut it ide open like a can, then start looking at stuff. see if they are some metal chunks and stuff.
