Hi everyone. I'm contemplating an engine rebuild on a 3.5 X350 due to possible piston slap. Unfortunately it would appear that pistons are not available anywhere. Until I strip the engine down I obviously won't know the extent and cause of the 'slap' - I'm hoping that it may be a broken piston ring or a badly fitting small end on the connecting rod. However, should the offending cylinder require a piston I've been considering replacing the pistons, crankshaft and connecting rods with those from the 4.2 ???? I cannot find a part number for the V8 3.5 or 4.2 block so don't know if they are the same. Does anyone know the answer to that question - 4.2 parts seem quite easy to get hold of in the uk, unlike the 3.5 ones. I appreciate that the ECU may need re-mapping to be compatible with the 4.2 parts. Thanks in advance.

 

I can't answer your question but with what your facing could you swap in a later used 4.2L? They are the same family and should be the same size?
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I can't see how pistons, crankshaft and con rods out of 4.2 block can possibly be compatible with the 3.5 litre block. If the capacity reduction is by reducing the stroke the block has to be shorter, (like the old XK 2.4 litre engine) so the conrods are too long, and the crank throw too much, and if by a reduction of bore size, the pistons are obviously incompatible.
So I had a quick look on the internet and Wikipedia tells us the 3.5 V8 was based on the 4.2. OK. so if this is the case, then the obvious way to reduce the capacity of the engine is to reduce the bore width. A simple change that needs no other mods, so the crank and conrods can be used in either engine. When Jaguar were making the straight six XK in later years, they introduced a 3.5 littre version of the engine based on the 4.2 block and this was, indeed with smaller bores, so I suspect this is what Ford did for the 3.5 litre version of the V8.

 

Just to contribute to the hope that cylinder diameter is the change, When Oldsmobile brought out the Aurora, they used the Cadillac Northstar 4.6 engine sleeved to 4.0 liters. The engines are even completely interchangeable, no external dimensions changed, and no ECU change needed. (That's the Aurora hot-rod method, find a 300-hp Northstar 4.6 to replace your 250 4.0! )

 

why rebuild unless desperate..you live in the midlands

the 3.6 is a destroke for anti-displacement countries just like the old 3.2 tax evader. same deal with the 2.1 v6

 

Thank you everyone for your helpful comments. The car concerned is low mileage and, other than piston slap when cold, runs very smoothly. Compressions are good all round, including cylinder No 6 (which I identified by disconnecting the injector). I know some people can live with piston slap but I'm concerned that more damage could occur in due course. As I said in my opening message I'm hoping that, once stripped down, it may me a broken piston ring or some play in the gudgeon pin (small end). Used 4.2 engines (in the UK) go for around the £800 mark and tend to be much higher mileage than my car. A 4.2 crankshaft, pistons & con rods can be bought for just under £200, hence my original question regarding block. I agree with Fraser in that the obvious way to reduce capacity would've been a reduction in bore size. That's not the case here though, both are the same at 86mm - the stroke of the 3.5 being 76.5mm and the 4.2 being 90.3mm. On that basis it would appear that the two blocks DO differ. I'm either going to have to live with the 'slap' , strip engine and possibly replace piston ring/gudgeon pin or consider a complete 4.2 lump. Had this car been 'average' I probably wouldn't bother, but it's an exceptionally good one. Thanks again.