CnC74 wrote: ↑Wed Dec 30, 2020 9:13 am
For whatever reason cutting these things back makes me super nervous considering we've never did it before. Since my son is only an apprentice and worried about the consequences of a broken engine, any chance of someone here on the forum willing to cut these down to the size we're looking for as well as lighten them slightly and maybe lighten the exhaust valves too if I sent them at a reasonable price? We're in Ontario Canada
Man up.
It's 101 and as simple an automotive machining job as you and he are ever likely to do.
Just make sure you have minimum chuck overhang and sneak up on the cuts; 005" depth or so will be reassuring.
Ok, we'll give it a shot. Since this will be a new one for me guys (well I did do valves on a grinding machine back in high school 40 + years ago but that doesn't count). Hera how we'll do it so chime in and stop me if I've got it wrong. First measure margin and then reduce the diameter by 2mm. Because the margin will be much thicker is our next step to face the combustion side of the valve to the original margin thickness or do we need to leave a little to make the 45° face where it will seal against the seat? Then a 30° backcut? If so how wide should that 30° cut be? Width I mean? Sorry I always Ned to visualize first. Probably some sort of disorder. Thanks again for the help guys
Nut124 wrote: ↑Fri Jan 01, 2021 9:24 pm
Dumb question: Why not cut the diameter, then the 45 for correct margin, then the 30 rather than cutting the combustion side?
That will make the valve spring installed height a tad higher. It's an option if needed if he's juggling different springs and retainers.
Nut124 wrote: ↑Fri Jan 01, 2021 9:24 pm
Dumb question: Why not cut the diameter, then the 45 for correct margin, then the 30 rather than cutting the combustion side?
That will make the valve spring installed height a tad higher. It's an option if needed if he's juggling different springs and retainers.
..a tad higher vs what? If the larger valve is designed to be fitted to an enlarged seat, then cutting the backside would reduce installed height and cutting the top side would keep it the same vs when correctly fitted to a larger seat - right?
I would try to assess the optimal installed ht before cutting the valves and determine the best method.
Have the seats been cut or will they? No use putting in bigger valves unless the seats are enlarged. What about the guides?
I do work like this for my own use and do not have the greatest tools but it is not rocket science. Cutting OD or the backside is no worries. Cutting the 45 requires a real valve grinder for precision.
You will need to measure the seat diameter for each seat. Blue and some lapping compound may be needed to determine where the seat is. The diameter you need is the outer edge of the 45 cut in the seat. The valve 45 cut must be quite a bit wider, in both directions. Seats may not be all exactly the same. I find a magnifying head set from Amazon valuable.
Cut the valve diam to seat diam plus say 0.030" or so, others will correct me here. Then determine spring and stem installed height: If too low, cut the 45, top side down. If too high or correct, cut the underside. Margin must be at least 0.050 or so intake, a bit more on the ex side. The 45 cut on the valve must be wider than the seat contact by a good margin. The 45 seat contact width should be typically no less than 0.050 IN, maybe 0.070EX but no more than 0.080.
Valves should be lapped into the seats to check and ensure proper seal after cutting.
Reduce the diameter first. I am not sure how to do it wrong, tho if you want to do the best job it helps to use a collet chuck rather than a 3 jaw chuck if available. or, at least put brass shimstock around the vale stem so you don't mar it up with a lathe chuck and that's probably good enough too.
The tolerance on the valve OD is not very tight. Get within 0.1mm of size that's good enough for the government, cheap valves are typically no better for size and for the most part nobody even notices much less complains about it.
Then grind the 45 face until the margin is the right width.
modok wrote: ↑Sat Jan 02, 2021 5:15 am
Reduce the diameter first. I am not sure how to do it wrong, tho if you want to do the best job it helps to use a collet chuck rather than a 3 jaw chuck if available. or, at least put brass shimstock around the vale stem so you don't mar it up with a lathe chuck and that's probably good enough too.
The tolerance on the valve OD is not very tight. Get within 0.1mm of size that's good enough for the government, cheap valves are typically no better for size and for the most part nobody even notices much less complains about it.
Then grind the 45 face until the margin is the right width.
If don’t have a collet, you can also take a cast iron valve guide and slit it lengthwise and slip that over the valve stem before you stick it into the Chuck. That will keep the jaws from getting after the stems. And if you have fat fingers like me, it keeps me from fiddling with the shim stock.
So what would happen if we used the new set of 39.5mm valves, installed the larger seats to fit them and did a little unshrouding with only mild porting job instead of hogging out the runners. Trying to keep some low end torque. I know we would be leaving some gains on the table but lessening the chances of velocity reduction wouldn't we? Or is this undoable?
Are the existing seat inserts cut to the max? In my application, the seat can be cut to about 0.5mm or so margin remaining outside the seat 45 cut. A 30 cut can be on this margin.
Installing new seats will cost you and needs to be done right. After installation, they need to be cut. If the existing seat has margin, you could cut them larger yourself, then reduce the valve OD to match.
If it was right in the first place then a larger valve will make it worse. If the valve was too small to begin with ,then the larger valve will work better.
It may also depend on the face angle and how high the valve lifts.
If the throat percentage is 90% and the valve only lifts .22 it's diameter then a bigger valve would be a sure thing.
Confession time I guess. This is for a Fiat SOHC 4 cylinder that factory stock is only 67HP. Way, way less than what you guys deal with in HP into the high hundreds. The seats are pretty much maxed out with the original valves at 36mm so no luck there. Larger valves and port work are said to be very beneficial yto upping the hp
Cnc, co confession necessary. Mine is a Fiat twincam 1800. I just finished building it up this last fall.
Bigger valves is usually one of the last things to do for power. Do you have a good header, cam yet? What is your CR? I think you should be able to make about 100HP without needing to replace the seats assuming this is the 1500cc.
I'm running 1mm oversize valves in my opened up factory seats and the engine is making about 140HP. But I have all the other pieces: 10.5CR, 4-2-1 header, dual webers, street cam properly adjusted.
Well cheers to you Nut124. I bought a 74X with a 1300 (same vintage as I had 30 plus years ago). I have a 1500 on an engine stand now. I also have the 4-2-1 exhaust (mines an ANSA), euro 1500 head without the US spec recess and have twin DCNF's ready for rebuild. Where did you find the 37mm intakes? Or did you cut them down?
