All, I am in the process of rebuilding a 400. I decided to rebuild the 400 as compression test determined low compression in cylinders 4 (120 PSI) and cylinder 5 (140 PSI). The leak down test (performed twice, to help pinpoint where the compression leak was going) indicated that cylinder 4 was leaking past the exhaust valve and could be heard coming out the tailpipe. There was a 42% loss in cylinder 4 (indicating poorly seating valve or other issue) and lesser loss of 20%+ in cylinders 5 and 6. Needless to say, the engine was tired.

After pulling the engine apart we identified a hairline crack in the exhaust port wall of cylinder 4 (picture attached) /passenger side cylinder. I assume that this crack may be contributing factor to the low compression in cylinder 4. It does not however explain the lower compression in cylinder 5 and 6 (which I think is due to poorly seated valves or other factors). Of note the crank also had a bit of rust on it indicating that water had seeped into the engine block at some point. And the main rear seal was leaking.

So, few questions that I welcome lessons-learned based suggestions on:

1. should I attempt to weld repair the passenger side head or just get replacement heads (I have the original 670 heads)?,
2. if replacement heads, given the costs of buying and refurbishing the heads, does it make sense to stay original or just get new aluminum?,
3. any other suggestions on what could have been causing the low compression (and leak down) and water entry into block?

This car is a weekend cruiser. I would like to boost the performance a bit - but do not want to complicate the build. I want to run 91 octane pump gas. I plan to regrind the crank (cast /nodular), replace the 066 cam with a 067 or 068 and am considering long brand exhaust manifold to increase airflow. Rochester carb has already been rebuilt with larger jets. Thanks in advance!

The crack in the exhaust port wouldn't affect the compression. But it looks like you do have issues with leaking coolant. Am I seeing rust in the combustion chambers? Could be just a bad head gasket and the coolant caused rust on the valve seats giving you the bad compression results. Or the crack could be a symptom of the head warping. I would not try repairing cast iron cracks in heads unless they were something special. My non expert opinion.

If the heads need a bunch of work anyway my advise would be to pick up a set that doesn't need that level of repair. For the cost of fixing the crack you can buy new cores. You have many excellent options including aluminum depending on the level of performance you are looking for. And iron heads are readily available although the refurbishing will cost a bit, even a fair percentage of the cost of new aluminum. But then you will have great heads that make all the power you want!

I did some testing on my engine (compression and leak down). I had one bad value. I freshen up the engine (gaskets, seals, new oil pump, new water pump, timing chain, basically everything easy to replace when engine out. I rebuild the heads.

You have found some puzzling issue swith your heads. I have built a few motors and I would suggest you take your engine into a engine builder and get some advice. You need the heads done but not sure about the welding. You can always find other heads so why risk fixing bad ones.

I have 2 engines that we did. One I freshen up and the other a rebuilt. I made my own engine stand. Have a look at my YouTube "reels".

Stick with stock...