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the only STOCK valve upgrade you can do is to use a 3400 intake valve, as its 1.76 inch diameter. stock 3.1L 3100 and 2.8's are all 1.72 inch.

as for aftermarket, you'll have to go custom with oversized valves.

unless you have some very wild engine specs going on your not going to need a larger valve. the valves were upgraded and are the larger size already compared to GEN1 heads.
not saying you might not gain much, you might be able to but it all depends on your engine specs as per what you will need for a larger valve.

the only company i know of that even makes 3.1L valves for us aftermarket wise is SI Valves.
these are stock size valves but are a bit better material than stock.
 

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ready to order valves from SI your only option is Stock sizing.

unless you know exactly what size of valve you need depending on the engines specifics the amount of power its putting out, the desired rpm. power adder. ect .. or youve gone through and did dyno testing to prove that yes this exact size is going to actually help your engine breath for X amount of CFM at this lift., not make less hp, well then, dont just guess a size.
if you dont know specifically that you need X.XX size then your best just going with stock.
 
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