Machine Shop Hell

I had built the 400 SBC in Dec 2002 and went over it with a fine tooth comb. I finally was able to installed it in Jan 2004. That's when the problems started. The new engine has a major problem. When I started it up, there was a loud heavy slapping sound coming from the bottom end. So I reluctantly had to pull the engine and take a look. Piston #1's wristpin has started to work its way out. There's about 1/2" or more showing. These are press fit pistons and this shouldn't be happening. Thankfully, the cylinder wall isn't scored as the wristpin still had about 1/4" to go before touching it.

I've torn the engine apart and checked all 8 pistons/wristpins/rods. Only cylinder #1 was loose. Even after 2 years, my machine shop still stood by its work and had even offered to re-sleeve the block if the cylinder is bad. They replaced the rod and it is now nice and tight like it should be. They were surprised that they didn't catch it at the shop. They had this great attitude about let's fix it and get it right. Unfortunately, they didn't size the big end of the new rod right. They were off center by 0.0125" which means that the rod bearing stuck out 0.025" on one side. So they replaced the rod again.


Wristpin sliding out to the left - should be flush with piston skirt edge

Even though the wristpin could easily slide back and forth there was no damage to piston, cylinder or head - not even a scratch which makes me wonder as to what was making the loud slapping noise. After I put the piston back in its place, I rotated the engine and no interference noises. But when I put on the oil pan, there it was, the sound of something hitting the oil pan. Off came the oil pan with the crank in position where it made noise. The problem was that the rod nuts were making contact with center of oil pan

The piston's 3 & 4 rod nuts were making contact with the center of the oil pan. Two small knicks were reverberating all the noise. When I loosened one side of the pan, the noise went away. I had an old pan and it measured 3 9/16" clearance from the oil pan lip to the center. The bad pan measured 3 7/16" which was fixed with a couple of whacks with my trusty hammer so that it measured 3 3/4" clearance. No problemo now!

In a way, I'm grateful that there was the oilpan/rod nut interference because I was able to catch the bad wristpin before it did any damage.

One step forward, two steps back. Still having engine problems. Found out that the jumper that selects whether the tach is for a 4, 6 or 8 cylinder engine was not making good contact. The engine seemed to me to be idling about 600 rpm but showed 1200 rpm. Reseated the jumper and it indicates 600 rpm. Unfortunately, it means that I broke in the cam at 1000 rpm not 2000 rpm.

So I decided to break the cam in again and at 2000 rpm, there's a pretty good vibration and the front pulley appears to be wobbling. Hopefully its just the 24 year old harmonic balancer (damper) that's bad. I bought a new one and put it on - no difference. I may have to disconnect the torque convertor, check the flywheel to see if the external weights are all on and run the engine without the torque convertor connected (isolate problem - is it the engine or tranny). If there's still a problem, out comes the engine again :-( At least I'm getting a lot of experience on tearing down engines.

Looks like the newly machined crank is bent at the snout (where the harmonic balancer sits). I put in the new harmonic balancer ($100) and still have the vibration problem. Measured the crank snout and there's 0.010" out of round. It should be less than 0.001". Talked to another machine shop and they verified that there's pretty much nothing that I can do at this time. My machineshop has asked me to bring the crank in to verify and if it is out, they'll replace it free of charge.

The crank was bad, the machine shop dials in the crank using the area between the timing gear key and the harmonic balancer key. I measured it and it was deadnuts on but both the timing gear and harmonic balancer were out by 0.010". We looked at what we could do to salvage the crank and the only things that we could come up with were:

  • Welding up the snout and taking it back down on a lathe to size - the heat from welding would most likely warp the crank. Just needs 0.001"of warpage to screw up the crank.
  • Spray welding - this wouldn't work as there is key slots. The weld would peal. Spray welding is used for press fitted pieces. I had another machine shop verify this with two journeyman machinists.
  • Chrome plating - Best bet but it is expensive and the machineshop didn't have their chrome plating tank up and running.

They did have two extra 400 cranks in the back. The first one was junk so they loaded up the second and it was good. The result was that they supplied me with a new crankshaft turned to my dimensions and new rod bearings as they had to go 0.010" more on them to clean them up. So I have a new crankshaft that measures perfectly. It's in the engine and the engine finally after 4 months purrs better than I expected.

So what did the machineshop do wrong?

  1. They did not pressfit number 1 cylinder's wristpin properly
  2. They did not size the replacement rod's big end properly
  3. They gave me non-standard rod nuts which interfered with the oil pan. In all honesty, these would probably work perfectly fine with any other SBC. The 400 block has very tight crankshaft to block to cylinder sleeves to cam to oilpan clearances. Typically your looking at 0.050" with the crank and any other part. The 400 is unusual in that regard.
  4. They failed to measure the snout properly even though it was obvious to the eye and physical touch that the snout was not true.

What did the machine shop do right?

  1. There was absolutely no argument about correcting their work even if it was 2 years old
  2. The parts were replaced 100% free of charge
  3. The work was put on a priority and returned asap
  4. They were genuinely upset that I had any problems and very upset about all the problems that I had.
  5. Their attitude was to fix the problem period and they would do whatever it would take to fix it. Even replacing the block and crank, pistons and any part that was damaged.

I've talked to many other engine builders and they all have similar stories with machine shops. And the funny part is that even the best machine shops have bad days. I think that the moral of the story is to check and double-check everything that goes into the engine. There's just too much at stake that could go disastrously wrong.

If this page has helped you, please consider donating $1.00 to support the cost of hosting this site, thanks.

Return to

Copyright May 2011 Eugene Blanchard