Need Part Number for BB 1.88 " Exhaust Valve

[Joe R.]

I posted this question in another thread but didn't get an answer. So I thought I would try it a different way. I have some 3873858 heads that I am massaging for maximum power. Duke recommended opening up the exhaust seat to accept a 1.88" valve. The exhaust valves are currently 1.72 ". I searched the archives, but was unable to find the GM part number for the 1.88 " exhaust valves. Please help. The big valve was used in the following GM part number heads: 3946072, 3964291, 14011076, 12363408.

Joe

[Duke W.]

Re: Need Part Number for BB 1.88 " Exhaust Valve

From the old 5th edition Chevrolet Power Manual 3946077 is listed as the "standard" 1.88" big block exhaust valve and a hollow stem version (340287) is also listed.

I expect both are discontinued, but you can cross it to a Sealed Power exact replacement at www.napaonline.com

The details will tell you that this valve is 21-4N stainless steel composition. If GMPD still lists this part I'll take virtually any bet that it is the exact same part from the same plant as the Sealed Power brand part.

The 1.88" exhaust valve (in conjuction with pocket porting) is an excellent upgrade for the early closed chamber heads because the exhaust port is restrictive, but I suggest you have the valve seat areas sonic checked for adequate material before the seats are ground out for the larger valve. In most cases they should be okay.

Duke

[Joe R.]

Re: Need Part Number for BB 1.88 " Exhaust Valve

From the old 5th edition Chevrolet Power Manual 3946077 is listed as the "standard" 1.88" big block exhaust valve and a hollow stem version (340287) is also listed.

The details will tell you that this valve is 21-4N stainless steel composition.

The 1.88" exhaust valve (in conjuction with pocket porting) is an excellent upgrade for the early closed chamber heads because the exhaust port is restrictive, but I suggest you have the valve seat areas sonic checked for adequate material before the seats are ground out for the larger valve. In most cases they should be okay.

Duke

Duke,

Thanks for the info. Do I want the standard or hollow stem version? Is the number for the hollow stem version right? It appears a digit is missing.

I will have the heads sonic checked before grinding.

Joe

[Duke W.]

Re: Need Part Number for BB 1.88 " Exhaust Valve

I doubt if the hollow stem version is available from any source, but you can check around. If it's available, it's probably very expensive.

GM offered this and a titanium inlet valve in order to increase valvetrain limiting speeds in racing engines. The six digit part number is correct. Back in that era GM was running out of seven digit numbers and they found some "old" six digit numbers that had never been used, so they were used in the mid-seventies while GM upgraded their system to 8 digits.

The "standard" 2.19/1.88" valves along with OE equivalent spec springs should allow at least 6500 revs, which is the OE redline, and the valve train limiting speed is probably something above that if everything in the valve train meets OE spec including correct shimming of the valvesprings to achieve the proper seat force.

Duke

[Duke W.]

Re: Need Part Number for BB 1.88 " Exhaust Valve

As long as I have the Chev. Power Manual out.... The recommended backcut on the BB valves is 20 degrees as I think Clem stated. The reference head is open chamber, and I don't know if it also applies to closed chamber heads, but this angle, or something close (in the range of 20-25 degrees) is recommended for most heads. The recommendation for the SB is also 20 degrees.

This cut creates a more gradual change in the annular flow area and should benefit flow on both sides at low lifts. It's very easy to do.

Once the seats are completed you die the seats, lap the valves to see the contact area, then backcut the valves to leave 0.10" overhang. On the downstream side the .010" overhang is achieved by grinding the 45 degree seat to within .010" of the OD of the seat surface on the valve.

My only concern with your proposed configuration is valve sealing. The second dual spring/integral umbrella seal is designed for the OE integral cast iron valve guides. If the guides were rebuilt with the K-line spiral bronze guides, then a "positive" type seal is required.

It might be possible to remove the integral umbrella from the second design spring and leave enough room for the "positive" seals at full lift.... I don't know, but guide-seal compatibility is something you need to evaluate.

Duke

[Joe R.]

Re: Need Part Number for BB 1.88 " Exhaust Valve

My only concern with your proposed configuration is valve sealing. The second dual spring/integral umbrella seal is designed for the OE integral cast iron valve guides. If the guides were rebuilt with the K-line spiral bronze guides, then a "positive" type seal is required.

It might be possible to remove the integral umbrella from the second design spring and leave enough room for the "positive" seals at full lift.... I don't know, but guide-seal compatibility is something you need to evaluate.

Duke

Duke,

Thanks for your help. I am definitely not smart enough to make a determination on the valve guides and seals. Is it too late to go back to the OE cast iron guides?

Also, I was comparing the specs on the valves. There is a difference on the length. The 1.72 " head valve is 5.355 " in overall length. The 1.88 " head valve is 5.3622 " in length. Will that cause any problems? If so, what is the remedy?

Joe

[Duke W.]

Re: Need Part Number for BB 1.88 " Exhaust Valve

I believe cast iron guide inserts are availalbe. Are the K-line spiral bronze guides installed? Maybe Clem can comment.

I don't think the approx. .010" difference in stem length is an issue as far as valvetrain geometry is concerned.

The new exhaust seat should only be cut enough to bring the head seat OD to .010" less than the valve seating surface maximum diameter on the upstream side. Any more tends to sink the valve.

An important parameter is the distance from the spring seat to the stem tip and the bottom of the retainer. The former is important to maintaining design rocker geometry, and the latter is what you use to determine shim thickness to achieve proper seat height/force.

Since the new exhaust valve is larger, proper attention to detail should not sink the seat. On the inlet side using the OE valve size on a freshly ground seat will probably sink the valve slightly, but unless the seats were very badly chewed up I doubt if the inlet valve will sink more than about ten thou.

Duke

[Dick W.]

Re: Need Part Number for BB 1.88 " Exhaust Valve

Joe, I have used the K Line knurled bronze guideliners for many years with no problems as long as you use a positive seal.

[Joe R.]

Re: Need Part Number for BB 1.88 " Exhaust Valve

Duke,

Yes, the K-line spiral bronze guides are already installed.

Joe

[Joe L.]

Re: Need Part Number for BB 1.88 " Exhaust Valve

Duke,

Yes, the K-line spiral bronze guides are already installed.

Joe

Joe-----


I would use the Sealed Power/Federal Mogul valves available in the aftermarket. There should be no problem, at all, finding these. GM does have some 1.88 exhaust valves available but they're not the original valves and I don't think they'll be any better for you than the Sealed Power. I think the GM valves currently available are Manley valves sold under a GM part number. Manley valves are excellent but they are expensive.

The GM #3946077 valves were discontinued several years ago. The hollow stem GM #340287 were discontinued in May, 1988.

[Duke W.]

Re: Need Part Number for BB 1.88 " Exhaust Valve

Duke,

Yes, the K-line spiral bronze guides are already installed.

Joe

Okay, so you need postive type seals, which probably won't work with second design dual spring with umbrella seal unless it can be removed. I don't know. I've never seen one of those assemblies close up or tried to disassemble one.

If the above spring won't work, you should be able to find an aftermarket dual spring that has similar seat force and rate. The trouble is, there are conflicting specs for this spring's rate. I've requested anyone who owns a set to go to a shop and have the force tested at nominal open and closed lengths, which will allow the rate to be determined, but no one ever responded.

Duke

[Joe R.]

Re: Need Part Number for BB 1.88 " Exhaust Valve

Duke,

You mean have a 627 spring tested? How would you like it tested? I should be able to manage that for you. You've been very helpful, I appreciate it.

Joe

[Duke W.]

Re: Need Part Number for BB 1.88 " Exhaust Valve

Any decent engine shop should have a spring tester.

The GMPP catalog says the seat force is 105 pounds at 1.88". This length is between the spring ends and does not include the retainer.

The listed rate in the GMPP is 267 lbs/in, which seems low (same as the production 3911068 SB spring), but I have also seen it quoted at 450 lbs/in, which is way too high to be beliveable. I would expect something near 325 lbs/in.

Dual springs can complicate things especially if the lower seat is at different levels, which I don't think is the case. (Dual spring setups are usually tested separately, then the rates are added).

The spring can probably be tested assembled with the seal and retainer. Test the force at about 1.88" between the ends of the outer spring (not including the retainer) then compress to about 1.38" and record the force. Test two of three springs and look for consistency of data.

AFAIK this is a linear spring, so the rate is:

(force at 1.38" - force @ 1.88")/0.5

If you do end up having to use an aftermarket spring due to the seal/guide issue, make sure the full open rate is about the same as the ....627 spring.

"Beehive" springs offer advantages, but by design they are variable rate. You want to make sure the open force is about the same as the ...627, but you can accept less closed force.

Duke