43.5mm valves on stock 41.8mm valve seats??

porting, development, valve and seat work, combustion chambers, cams, head construction, etc
Posts: 3
Joined: July 26th, 2018, 6:19 pm

43.5mm valves on stock 41.8mm valve seats??

Post by gtmedley »

I am rebuilding an US-spec 1756 head (casting 4277590), a mild street engine for a '74 Fiat 124 sedan. Rumor has it that it is possible (desirable?) to use the larger 43.5mm valves on the existing, stock seats for the 41.8mm valves.

Is this rumor true? Possible? Worth it? Apparently there is enough "meat" on the 41.8 seats to accommodate the extra 1.7mm width.

If possible and/or common, does anyone have any photos of how the new seat looks? Perhaps a diagram that shows this?

Thanks for any info. Gary in Oregon/
GC #166
Spider 1969
Posts: 159
Joined: September 22nd, 2013, 7:21 pm

Re: 43.5mm valves on stock 41.8mm valve seats??

Post by Spider 1969 »

Hi Gary,

Regardless of sufficient meat on the existing valve seat (of which I don't know if there is) valve seat configuration, valve throat size and combustion chamber shape dominate the effectiveness of a (larger) valve. It's no use installing a larger valve without improving these.
Plenty of info/pictures and even diagrams to be found on this forum. Even better: invest in the GC workshop manual 2013 version. Worth every penny if you want to endeavour serious and reliable engine improvement.

Best regards,
Guy Croft
Site Admin
Posts: 5039
Joined: June 18th, 2006, 9:31 am
Location: Bedford, UK

Re: 43.5mm valves on stock 41.8mm valve seats??

Post by Guy Croft »

Thanks to Charles and he is right about the technicalities, bigger valves work best only when the seat angles are optimised.

No you can't fit 43.5mm to the seat for a '42mm' valve - attempting to do this will leave the 43.5mm valve overlapping the seat insert even if you cut it at 45 deg to its extreme diameter - and then the cast-iron insert will fracture at the sharp outer edge. You must leave at least 0.5mm material outboard of the contact face with cast-iron seat inserts.

Moreover the throat diameter of the insert is way too small for a 43.5mm inlet valve and would have to be bored out.

I see this mistake often made.

Guy Croft, owner

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