.060 oversize exhaust valves hit inlet valves

bsaboss · Dec 13, 2016

Just rebuilding a 750 engine and the exhaust valves were quite pocketed so I thought I would try the .060 oversize exhaust valves sold by Black Diamond. They solved the seat problem beautifully and looked a treat, but on assembly I found the exhaust and inlet valves just kissed each other lightly on overlap. Today's job is to pull the head again (I just love doing jobs twice!) and try and work out what to do. From a quick google search I'm not the first person to have had the problem and it seems my options are to set the exhaust valve further into the seat (i.e. grind the seat more so it starts it's travels further back) and perhaps slightly reduce the valve head diameter if that's possible without interfering with the seat area, or just pulling the valve out and having a new seat fitted so I can run a standard valve. I'm inclined to do the later but would appreciate any other thoughts and suggestions. In terms of a new seat, does anyone know who makes a suitable seat and what it's part number is?

lynxnsu · Dec 13, 2016

fitting an oversize exhaust is of little ,or none at all,gain regarding performance
fitting bigger inlets is something else

but to your question , putting the valve in a lathe and skimming it down in diameter is an easy thing to do
cutting the seat deeper is not an option for me as you are going to mask too much flow , especially with a std cam

if the bike is a road bike to be used at moderate speeds, reducing your oversize valve should be the cheapest option
if you like to use it in anger replace the seat and valve
if it is for track use ................

pesonally i would not want such a heavy and useless exhaust valve in my engine , but it all depends what you want to spend on it

any decent machine shop should be able to freeze a seat in your head , let them cut the contact area in 3 angles at the same time , as you probably know , the narrower the better

Torontonian · Dec 13, 2016

I'd go the latter route.

lcrken · Dec 13, 2016

According to my Norton mentor, the late C. R. Axtell, sinking the exhaust valve to get more valve-to-valve clearance does not have a significant effect on power. He sometimes had to do that for the heads he ported. They were fitted with 1/16" larger intake and stock exhaust valves, and with cams with the lobe centers close together (102° or so) it was sometimes necessary. There is probably some limit to how deep you can seat the valve before it affects performance, but I don't know what it would be. The factory did the same on some of their big valve race heads to get clearance for some really large valves.

Turning the valve down to stock size should be the simplest solution, unless you've done a nice 3 or 5-angle valve job, and opened up the throat enough that you don't have enough material left for a smaller seat. It sounds from your post like that might be the case. If so, you either sink the valve or put in a new seat and stock size valves.

I just went through this exercise by sinking the exhaust valves in a Maney Stage 3 head by .025" so they would have enough clearance for a large overlap cam. It was pretty straightforward. I started with .020" clearance between the valves, but you have the valves just touching, so you would need to sink them by a bit more to be safe. Still, it looks to me like a reasonable solution. Good luck with it, and let us know how it goes.

For the seats, I use Kibblewhite bronze seats, available in standard sizes, but there are other valve seat manufacturers who offer them in a variety of other materials.

Ken

jseng1 · Dec 14, 2016

Do it right. Get new tungsten carbide seats from Tucker valve seats and be done with it. They will last even with todays unleaded gas.

I think I had to special order them (can't remember). Here are my notes.

Stock Ex seat is .250” tall, seat recess is 1.5 (use 1.506 OD seat). ID is 1.180 to 1.9 x .159” wall, 1/32 wide seat mating as provided.

comnoz · Dec 14, 2016

I keep hard seats in stock.

I wouldn't use bronze seats for a streetbike on unleaded fuel. They wear pretty quickly. Jim

bsaboss · Dec 14, 2016

Many thanks for your replies. I'd put the oversize valve in as the standard valve was pretty badly pocketed and I'd naively thought that if Black Diamond made them they would be a straightforward fit. Anyway, I removed and stripped the head, and the valves had the smallest witness marks where they'd touched each other, so I turned down the exhaust valve heads by 18 thou and the inlets by 10 thou, and put a slight chamfer on the outside edge. That still left plenty of seating area, and happily after putting it all back together the valves now clear each other. For me this has been a frustrating way of learning a lesson - I think next time I'd just replace the valve seat. I wish I wasn't twelve thousand miles away from Jim Comstock shop - life would be so much easier.

Snotzo · Dec 15, 2016

bsaboss

your minimal static clearance will increase when the engine runs, so unless you miss gear changes, or over rev your engine, there should be no problem from lack of clash clearance.
Due to flex and lag in the valve train, intake opening in particular can often be delayed by several degrees or more from the static timings, so effectively clash clearance increases.
Flex on the exhaust side will also result in the valve closing slightly earlier than static, but here the closing lag is not as great as the opening lag of the intake.

The above will be instantly rendered meaningless if your valve springs are time expired!

lcrken · Dec 15, 2016

comnoz said:
I keep hard seats in stock.

I wouldn't use bronze seats for a streetbike on unleaded fuel. They wear pretty quickly. Jim

I'd take JIm's advice on this. All my experience with bronze seats so far is on race bikes, although I'm about to see how they work on my street MK3. I've had good results with them on both Norton and Rotax race engines. I discovered on the Rotax that they worked really well with titanium valves, showing almost no wear on valves or seats after a season of heavy racing.

Ken

Fast Eddie · Dec 15, 2016

lcrken said:
comnoz said:

I keep hard seats in stock.

I wouldn't use bronze seats for a streetbike on unleaded fuel. They wear pretty quickly. Jim

Click to expand...

I'd take JIm's advice on this. All my experience with bronze seats so far is on race bikes, although I'm about to see how they work on my street MK3. I've had good results with them on both Norton and Rotax race engines. I discovered on the Rotax that they worked really well with titanium valves, showing almost no wear on valves or seats after a season of heavy racing.

Ken

I've used bronze seats on race bikes a lot too and they've been great. I've used them less on road bikes, and even then they have not been high mileage bikes, so I don't claim any proof here.

But it was explained to me some time ago, by someone who's opinion I tend to value, that valve seat recession was due to the valve 'micro welding' to the seat in the absence of lead. But as a steel valve cannot weld to bronze, and as bronze work hardens, bronze is thus a good seat material.

Is this wrong?

comnoz · Dec 15, 2016

Fast Eddie said:
But it was explained to me some time ago, by someone who's opinion I tend to value, that valve seat recession was due to the valve 'micro welding' to the seat in the absence of lead. But as a steel valve cannot weld to bronze, and as bronze work hardens, bronze is thus a good seat material.

Is this wrong?

No, you are correct.
But from my experience I have still seen faster wear with the bronze seats. I have used them on racebikes with very good success but when I tried them on streetbikes I found I needed to readjust the valve clearance every 3 or 4000 miles or they would get too tight.

Also if I started with a nice .040 wide intake seat it would be the full width of the valve face in about 10,000 miles.
Of course this is also going to depend on the cam and springs used. Harsh closing ramps or valve bounce wears out any seat in a hurry.

I would have to say I have never seen full fledged micro-welding valve recession like used to happen with late 60's or early 70's Fords on any type of seat used in a Norton. Jim

bsaboss · Dec 15, 2016

Snotzo said:
bsaboss

your minimal static clearance will increase when the engine runs, so unless you miss gear changes, or over rev your engine, there should be no problem from lack of clash clearance.
Due to flex and lag in the valve train, intake opening in particular can often be delayed by several degrees or more from the static timings, so effectively clash clearance increases.
Flex on the exhaust side will also result in the valve closing slightly earlier than static, but here the closing lag is not as great as the opening lag of the intake.

The above will be instantly rendered meaningless if your valve springs are time expired!

That's good to hear. I've fitted new valve springs so hopefully all will be well.

lynxnsu · Dec 16, 2016

bsaboss said:
Snotzo said:

bsaboss

your minimal static clearance will increase when the engine runs, so unless you miss gear changes, or over rev your engine, there should be no problem from lack of clash clearance.
Due to flex and lag in the valve train, intake opening in particular can often be delayed by several degrees or more from the static timings, so effectively clash clearance increases.
Flex on the exhaust side will also result in the valve closing slightly earlier than static, but here the closing lag is not as great as the opening lag of the intake.

The above will be instantly rendered meaningless if your valve springs are time expired!

Click to expand...

That's good to hear. I've fitted new valve springs so hopefully all will be well.

do not rely on the fact that the springs are new , do check their pressure on full lift
i once did a valve job on a Lemans where the new springs looked the bussiness but were actually weaker thanthe (very) old ones
DO CHECQ , surely in your case