undercut valves; worth it?

[cuzncletus]

Question: Are undercut valves worth the extra expense? The engine I'm working on is a .125 over 258 incher. Cam is an L100. Valves will be either 1.5 (most likely) or 1.6 stainless steel Federal Mogul Chev replacements, mild port/gasket match. The cost on the undercuts through my machine shop is $50 more. $50 is either chump change or a small fortune, depending on what time of the month it is. (Fixed income.) Is there $50 worth of horsepower in the additional money? 1.6" valves would cost me nominally more for machine work. There seems to be such a divergence of opinion on the 1.5 vs. 1.6 issue I really don't have a clue on a mild motor.

[51 MERC-CT]

Not to me. The first time you pull up beside someone with a 4 cyl. car with half the displacement and you decide
to have a little drag race and he blows your doors off, then you can ask yourself was it worth it?

[Ol' Ron]

The SNYA rules were 258ci and stock valves size. Some of then were so thin they'd get swallowed up in the port when hot. You can see them in both JWL's book and mine. However the flow testing offered little improvemet in flow and any benefit would be at high revs. I think a proper 3 angle valver job with the major diameter of the valve near the edge of the valve. This along with a .060" seat and 60 deg bottom angle. DO NOT relieve the block you need all the Cr you can get. We did elieve our blocks in the beginning but found that they ran better not relieved. We geared for 5K at the end of the straight a way but hit 52/5400 on the teltail sometimes. Re geared to being the revs down and inprved time a few tenths. Good luck the 258 is my favorite engine.

[cuzncletus]

51 Merc;

Street racing is both dangerous and illegal. I have never engaged in an illegal contest of speed on our public roads. At least that's what I told the judge the three times I was accused of it.

[51 MERC-CT]

Originally Posted by cuzncletus

51 Merc;

Street racing is both dangerous and illegal. I have never engaged in an illegal contest of speed on our public roads. At least that's what I told the judge the three times I was accused of it.

Ya' the good old days

[sanborn]

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The flathead standard intake port flow is very poor and if a cam is used with a valve lift of .375"-----that is about all the port will handle. If the intake port is enlarged then back cut valves would probably help---but not much. In my humble opinion back cut valves would be worth it---but that is my opinion only.

"This opinion is offered by a non-flathead experienced person but is going through some of the same questions as you". I am building my first flathead in 50 years.

[4dFord/SC]

Originally Posted by cuzncletus

51 Merc;

Street racing is both dangerous and illegal. I have never engaged in an illegal contest of speed on our public roads. At least that's what I told the judge the three times I was accused of it.

I heartily agree, and commend you for your mature and responsible citizenship. As I was telling Elvis the other day, for similar reasons arduous spirits never passed my lips until until I attained the age of 21.

[Fordestes]

an alternative I used rather than pay a machine shop to back cut the valves after purchasing them , I opted for a set of milodon, race flow and saved a little $. compare prices, only you will have to be the judge.
I am now fortunate to be able to cut my own chevy 1.5 vaves to suit my needs.

[cuzncletus]

The machine shop buys a lot of reboxed Fed Mogul, Melling and other parts under a discount brand name. I'm not sure of other parts (cams which I have used successfully) but I know the valves are Fed Mogul, made in US. I think Summit also uses these under their name. I'm guessing the other big brands to also. I can get them for considerably cheaper than what they're marketed for, plus no shipping. For $50 it's hardly worth chucking up 16 valves to undercut them.

As for street racing, I gave it up at a much younger age. I haven't done it since I was in my 50's.

[Ol' Ron]

If you want to undercut your own valves this is how I do it. After running several flow bench test on various angles I settled on 20 degrees. Getting a stock 1.5 and 1.6 valve from the auto discount store (2.95.3.95) I chucked them up in a modified 3 jaw chuck. I installed a special coller on the stem in a drill chuck in the tail stock. This was done to speed up the operation. I then took .750" round carbide cutter in a special holder and brought it up the valve stem about 1.5" from the top of the valve head. Running the lathe at a mediun speed I cut into the stem of the valve to reduce the size of the stem to 5/16. Then feed the tool to a predetermined stop under the head and bring the cross slide out at a 20 degree angle. Reduce the intake seat width to about .080" and the exhaust seat to .100" Valve is now ready fo its next operation. Turn it around and install the stem into the chuck and radius the majr diameter and brake the edge from the minor diameter. Install these in a knurled guide with .0005/.001 clearance. Use 50/55 lbs of spring pressure installed height. Now I did this on most of the engines I built, and I never used SS valves. they have poor ware qualities and the 2 piece OEM valves will last for ever. If you want to go racing use Titanium.

[Pete]

"As for street racing, I gave it up at a much younger age. I haven't done it since last week."

There are 2 kinds of hot rodders, those that street race and liars......

[Ronnie]

Spend your money on a good ignition system and you will get abetter engine in the long run. Just my 2¢

R

[lonewolf]

As much as it costs to build a flathead with heads,multiple carbs,cam,machine work etc.
Whats another 50 bucks....id do it they need all the help they can get