Help: 400 sbc counterweight clearance problems...

Hi

I built up a mild 400 sbc about 2 years ago and finally installed it in my 1954 Pontiac custom. It has had a hone alignment on the crankshaft, stock pistons 0.040" over, edlebrock intake and performer cam (350 size cam not the 400 one), etc.. The crankshaft had #1 bearing originally toasted and the crank main journals were reduced to the 350 small size and oversize bearings used (the same ones used to destroke a 400 to make it 377 cid).

When I started it up, there was a loud heavy slapping noise from the bottom end. Not piston slap sound which is a tinny sound. This was loud and heavy. The engine ran quite smooth regardless.

I've pulled the engine and removed the oil pan and quickly took a look inside. Everything is where its supposed to be thankfully as I expected to see broken parts. I did finnd what seems to be interference between the front counterweight and the top of the engine block as there are score marks on the outside edge of the counterweight.

  1. Is there a size difference between the 350 sbc and 400 sbc edlebrock cam? Is the 400 sbc a smaller radius for clearance?

  1. Why would I have clearance problems now when the crank came out of the block originally. Can there be a machining problem when the crank/block was done?

  2. Can excessive crankshaft end play cause clearance problems? I'm wondering if the crank was incorrectly ground down to the smaller journal size.

I won't get a chance to take a good look until Thursday afternoon so any ideas that you have, I'll be glad to hear them and investigate.

Thanks in advance

Reply to
Eugene Blanchard
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Reply to
buggs

Good for you. Read the post again.

~atleast KJ knows what a crankshaft is~

Reply to
KJ

The 400 is the odd one in the SBC lineup and that is because of different clearances. You did a hone on the main bearings which put them a little closer to the top of the engine by a bit and this may be where you are in trouble. I'll also note that 350 blocks don't have the clearance naturally for the 400 crank. Drop the crank and see where it was hitting and hope that it was the cast iron of the block rather than the cam and that can be clearanced the few thousandths of an inch needed to make it clear with a die grinder in a few minutes.

-- Bob May Losing weight is easy! If you ever want to lose weight, eat and drink less. Works every time it is tried!

Reply to
Bob May

Reply to
atec77

Reply to
buggs

Reply to
Eugene Blanchard

Reply to
Eugene Blanchard

Funny how the consensus is that it is hitting the engine block. I don't think a normal person would go at the cam shaft with a die grinder. Would you?

~KJ~

Reply to
KJ

crankshaft,

originally

Nope a SBC cam is an SBC cam, they all run the same sized cam journals. Now lobe size is a different matter, however if it fit in without having to fight each lobe (under .500 lift) it isn't a problem and even if it was more the crank couldn't hit it.

You stated the block was align honed, that could have shifted the crank up enough to hit, especially on a 400 crank. Not a lot of clearance on those throws inside the block in the first place.

It could but it would have to be VERY excessive, What does yours measure out at?

Reply to
Steve W.

Better check for a bent rod now.

-- Regards Gordie

Reply to
The Nolalu Barn Owl

Either the Pin diameter is below spec size or the piston pin Bore in the ROD is above spec size. I would still check the crank counterweight to block clearance. Brent

Reply to
Brent Popp

The interference fit is supposed to be about 0.0008 to 0.0015 inch. Stock they are sometimes a bit tighter. It takes tons of force to make them move, like when intentionally taking them apart.

I suspect one of two possibilities/

1 - that rod was set up for oversize pins in the past ( 0.0015 and 0.003 inch O/S were popular sizes once-upon-a-time 2 - or it was honed to provide clearance for a poor man's "full float" using pistons with lock rings

either way, all the other rods need to be checked for proper interference fit, which is part of the procedure for properly assembling press fit rods and pistons. Are the rod ends discolored straw or light blue, as if they were heated to assemble?

At the very least the other 7 assemblies need to be tested in a press with special support fixturing to withstand 2000 pounds without the pin moving.

Reply to
djtcz

The rod ends are discolored light blue and the machine shop says its cause they heat them up for the interference fit. The other rods are solid. The machine shop was surprised that they didn't catch it when they assembled the pistons/wristpins/rods. They said that they should of "felt" something different when it was assembled. They basically said the same things that you did about oversize pins or poor man's full float.

The cylinder, its piston, etc. are all fine - no damage. The other 7 pistons are fine. The shop had no hesitation about fixing the problem including putting in a new sleeve if the cylinder was damaged. It was quite refreshing to deal with someone who stands 100% behind their work. My cost - $0.00.

Of course there was the frustrati>

Reply to
Eugene Blanchard

The machine shop replaced the rod and triple checked that the wristpin was snug. I installed the piston and rotated the crank - no noise. It bothered me that there was very little play on the loose wristpin and no marks or scuffing on the cylinder walls to indicate the noise that I heard cause it was LOUD!

When I put >

Reply to
Eugene Blanchard

Had that problem once when a guy used a crank splash sheild and its main studs for a 327 on a 350. The little extra stroke was enough to interfere the rod nuts with the baffle. Nice to see you used a shop that stands behind their work.

When I put on the oilpan and rotated the engine by hand, the noise was back! Anyways, rods 3 & 4 nuts were just touching the oil pan enough to make a loud noise. A quick bang with a hammer provided 1/4" more clearance and the noise was gone. I'm lucky that there was the oilpan interference otherwise I would never of noticed the loose wristpin until it was too late.

I've got some pictures on my website at

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Reply to
Rufus T. Firefly

Does this mean the shop checked the other 7 pins by applying pressure with a press, as if they were dis-assembling the rods and pistons? It really needs to be done.

Reply to
djtcz
6.49 a.p capeche?

~KJ~

Reply to
KJ

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