Back cutting

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Back cutting

Post  quick 52 on December 29th 2013, 10:35 am

I'm removing my heads to fix a header flange bolt... While off will replace springs and a valve job. Is there any gain doing a back cut my heads they are A429...  I did some port work on the heads not knowning a lot about porting may of helped or not.. when doing a back cut what are the angles and margin used and which valves or both

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Re: Back cutting

Post  c.evans on December 29th 2013, 3:48 pm

quick 52 wrote:I'm removing my heads to fix a header flange bolt... While off will replace springs and a valve job. Is there any gain doing a back cut my heads they are A429...  I did some port work on the heads not knowning a lot about porting may of helped or not.. when doing a back cut what are the angles and margin used and which valves or both

Intake valves only. It always helps to backcut them. With a 45* seat angle, use approx. a 30* backcut. Carefully lap the valves first, in order to see where your 45* seat angle ends on the valve. Then backcut the intake valve using a 29*, 30*, or 31* backcut angle. It really doesn't make as much difference which angle you use, the more important thing is the width of the backcut. Come right up to the edge of that lap mark. Try to get somewhere between .080" wide to even .100" wide on the backcut. The backcut will help your low and mid-lift flow numbers.

Uncle Charlie

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Re: Back cutting

Post  quick 52 on December 29th 2013, 9:00 pm

c.evans wrote:
quick 52 wrote:I'm removing my heads to fix a header flange bolt... While off will replace springs and a valve job. Is there any gain doing a back cut my heads they are A429...  I did some port work on the heads not knowning a lot about porting may of helped or not.. when doing a back cut what are the angles and margin used and which valves or both

Intake valves only.  It always helps to backcut them.  With a 45* seat angle, use approx. a 30* backcut. Carefully lap the valves first, in order to see where your 45* seat angle ends on the valve.  Then backcut the intake valve using a 29*, 30*, or 31* backcut angle.  It really doesn't make as much difference which angle you use, the more important thing is the width of the backcut.  Come right up to the edge of that lap mark. Try to get somewhere between .080" wide to even .100" wide on the backcut.  The backcut will help your low and mid-lift flow numbers.

Uncle Charlie
Thanks Uncle Charlie for the info.

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Re: Back cutting

Post  rmcomprandy on December 29th 2013, 10:52 pm

c.evans wrote:
quick 52 wrote:I'm removing my heads to fix a header flange bolt... While off will replace springs and a valve job. Is there any gain doing a back cut my heads they are A429...  I did some port work on the heads not knowning a lot about porting may of helped or not.. when doing a back cut what are the angles and margin used and which valves or both

Intake valves only.  It always helps to backcut them.  With a 45* seat angle, use approx. a 30* backcut. Carefully lap the valves first, in order to see where your 45* seat angle ends on the valve.  Then backcut the intake valve using a 29*, 30*, or 31* backcut angle.  It really doesn't make as much difference which angle you use, the more important thing is the width of the backcut.  Come right up to the edge of that lap mark. Try to get somewhere between .080" wide to even .100" wide on the backcut.  The backcut will help your low and mid-lift flow numbers.

Uncle Charlie

Generally true.
Back-cutting the exhaust valves so that back-cut is about 3/8" wide most of the way up the valve backside in order to narrow the actual seating area does help in most instances. However, the improvement is usually miniscule when compared with the correct back-cut on the intake side.

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