Bigger valves, original seats

Bigger valves, original seats

Author
Discussion

227bhp

Original Poster:

10,203 posts

127 months

Tuesday 31st May 2016
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I am thinking about putting 2mm oversized valves in a head, what decides whether I need new valve seat inserts or not? How do you decide the limit?

Kokkolanpoika

161 posts

150 months

Tuesday 31st May 2016
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What engine? And what you are going to gain?

PeterBurgess

775 posts

145 months

Tuesday 31st May 2016
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You need to know if the larger valve will have a bigger OD than the valve seat, if it does you will need bigger OD insert. If you are on or near the OD of the insert it would be worth having a widish seat and bottom cut/s so you are generous with how much insert is left. If you go too large on the ID you may well risk the insert cow-lapsing.
Paul Ivey of REC fame and part owner of G&S valves has, historically, produced oversise valves which are on the limit of the standard inserts fitted on many alloy heads.
Peter

227bhp

Original Poster:

10,203 posts

127 months

Wednesday 1st June 2016
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PeterBurgess said:
You need to know if the larger valve will have a bigger OD than the valve seat, if it does you will need bigger OD insert. If you are on or near the OD of the insert it would be worth having a widish seat and bottom cut/s so you are generous with how much insert is left. If you go too large on the ID you may well risk the insert cow-lapsing.
Paul Ivey of REC fame and part owner of G&S valves has, historically, produced oversise valves which are on the limit of the standard inserts fitted on many alloy heads.
Peter
Thanks for the reply, I should have posted measurements up before, but the OD of the valve is equal to the OD of the seat and the head is aluminium.
Also the seat 'ring' material is 2.72 thick so when I enlarge the throat (by 2mm overall) it will reduce this to 1.72.
I wondered if it was a bit borderline, the engine is not known for seats coming loose (at all) with normal valve sizes, but am worried if I would be removing a bit too much of the tension which is holding it in.
My decision I guess, but it's maybe prudent to put some slightly bigger seats in.

PeterBurgess

775 posts

145 months

Wednesday 1st June 2016
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Just because you increase the valve diameter by 2 mm does not mean you have to increase the throat ID by 2mm. By cutting wider seat and bottom cut/cuts you will achieve more low to mid range lift air flow than with a wider throat. Overall you would end up with more bulk air flow than hogging out the throat and maybe not needing the very high lift flow anyway!
Peter

FordPrefect56

75 posts

95 months

Thursday 2nd June 2016
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http://www.pistonheads.com/gassing/topic.asp?h=0&a...

I went into some detail on valve and seat geometry here.

FordPrefect56

75 posts

95 months

Friday 3rd June 2016
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PS, if you follow my rules about throat diameter, 85/86% of the valve diameter on the inlet then the insert thickness will be fine for an insert that is the same size as the valve.