1974 308 gt4. Cylinder bores out of round

To clarify - do you really mean the bores are out of round, as in slightly oval-shaped or are you referring to taper within the bores where bore diameter is different between the top and bottom of the bores?

If it's taper, that's a product of normal wear over time however it is odd that the the bore diameter is larger at the bottom, below the rings. In fact, it's impossible for that to occur in an engine since there is no ring/bore interface there. If it exists, it has to be a product of the way the cylinder bores were originally made or the way they were rebored. Maybe Ferrari made them that way; never heard of boring being done in an aftermarket shop to produce that.

Now, if you are actually talking out-of-round, where the diameter varies depending on which direction the internal micrometer is aligned in the bore, that's actually not unusual. It is common to find bores that are perfectly round when cold are not when they are at operating temperature...and visa versa. IOW, out of round bores, measured cold may NOT be out of round when the engine is at operating temp. Bores that were perfectly round (either hot or cold) when the engine was produced may not be after all the metal has been heat-worked over time. Years ago, we used to specifically select engine blocks that had thicker castings on the thrust side of the block. This was totally a random occurrence as far as production was concerned during the block-casting phase. BUT blocks with the thicker casting on the thrust side maintained bore "roundness" much better than other blocks. Based on dyno tests, that fact formed my personal opinion that the power difference between identical factory engines occurred largely because the 'good' engines had better ring/bore sealing than the 'bad' engines. We saw as much as 50 HP difference between two identical big-block factory engines and the only difference between them was that the good engine had round bores when disassembled and the 'bad' engine did not.
If your bores are really out-of-round as you stated, then that's the reason - heat "adjustment" of the block over time. I ASSUME that back in the day Ferrari's casting process was similar to every one elses so bore wall thickness could vary from casting to casting, producing some engines that produced more power than other 'identical' engines.

There were (are?) shops that heat the blocks when boring to ensure they are round at operating temp but nowadays the entire process of block-making probably eliminates the issue.

 

They may have been bored with a torque plate pressing in (and deforming) the sleeves. This is the best way to bore them since they do deform once pressed in by the heads. Replace the viton o-rings at the bottom of the sleeves. If you need some I have some (£1 each.)

 

If you want to borrow my sleeve puller and/or torque plate send me a pm.

 

Derkw made a good point that I omitted - your shop should be using torque plates when doing the bore machining. If they don't, find a different shop - that's engine prep 101 stuff as far as quality engine building is concerned.

 

Ferrari didn't use torque plates and many very experienced experts don't use them-- they just slip in new replacement sleeves. I think it makes sense to use them and it could be the reason for your sleeve distortion.

Keep in mind also that the heads have to be torqued down for any measurement or work on the crank journals as the whole block flexes when the heads are torqued down.

 

Sounds like a factory race team or SCCA Showroom "Stock". We always heard that factory teams , that had to use "stock" parts went through the factory to hand pick which blocks, pistons, etc they used.

Lester