Exhaust valve to piston clearance?

[Juha Vane]

Took the heads off and had them flycut so a new little problem pop up.

The exhaust valve to piston has quite little clearance. When the piston
is coming up and "chasing" the exhaust valve a bit before TDC the clearance
is 0,83mm 0,033". These measured with 2mm solder rod.

There is very little mention of this clearance, tried to search and read a lot of
posts and found two figures:
0,040" - 0,045"
0,070" - 0,100"
What exhaust to piston clearance is recommended, cam is Elgin 7008?
P&C is AA 22deg big bore kit with cast pistons.

I would not like to add shims under the cylinders, none now and the quench
is 0,75mm 0,030", I know a bit tight? but here was mentioned that even
0,025" is OK.

[C J Murray]

.025" is NOT good for a street engine. Those are racing numbers and then only if you have confidence that not any part of the chamber is any tighter. .035-.050" is good for the street quench.

Valve to piston clearance is another dimension that should not go too small. Clearance there is what saves valves from being bent during over-revs such as a missed gear. There is very little loss of compression from taking a deeper cut to the piston to make valve clearance so its a good trade-off. .070-.100" is good but you may have limited piston crown thickness. Most likely you should shim the barrels a small amount and cut the valve pockets if you will still have a piston crown no thinner than .200".

OR, get a more reasonable cam which will make the car run better anyway.

[Wes Bender]

Cliff's advice is good. If it were me and the engine was running well before, I would shim the cylinders by the same amount as the heads were flycut.

[Don Gale]

I had a set of C/912 1720 pistons machined for my 1958 1600 Normal engine, the shop recommended .050". I did not use spacer shims. The shop machined the clearance by deepening the already existing relief for the exhaust valve in the piston top. I verified the clearance with clay, similar to your solder method. I forget what cam I used. I'm thinking it was a S90, SC, or 912. I do not recall if the seal surface of heads were flycut but if so, only a token few thousands for cleanup. He did have to machine a chamfer on the outer edge of the piston tops and a corresponding slight taper on the squish region on the heads. I believe his calculations predicted a 9.3 to 9.5 compression ratio.

[Ron LaDow]

Cliff and I disagree on several issues and this is one.
Assuming forged (light weight and close skirt clearance) pistons, a max RPM of 5.5K, no plug interference, and that you are willing to do the considerable effort to make sure the numbers are consistent, I can see no reason for more than .025"-.03" quench clearance on a street engine.
And several reasons to spend the effort.

[C J Murray]

Oh Ron! My main concern is that when an inexperienced builder goes for .025" they often screw it up because the slightest imperfection in the chamber surface or the slightest non-concentric alignment between the piston and the head will result in big problems. Experience tells me to clay the entire surface of the chamber, not use solder, and I always find tight spots. I set the clearance to the tightest dimension. If you don't know there are tight spots contact is possible.

In a street engine the difference in power between .025" and .035-.045" is not worth the risk. Advising a novice to shoot for very tight clearances is doing them a disservice.

In a race engine with the very best parts .025" is fine even at 9000rpm.

[Matthew Devereux]

I'm running that cam in my 1720 and I like it. I think I aimed for 0.040" quench which is the tightest recommended clearance for street use. At least that was my understanding at the time.

[Ron LaDow]

Cliff, you drew a distinction between "street" and "race" engines; that was the point of my objection. Any disagreement, if it exists concerns that dichotomy.
I certainly agree that an inexperienced builder shouldn't try to achieve the accuracy required for those sorts of tight quench clearances in a street *or* a race engine. Or should, and learn from the experience, as most all of us did.
But given the required skills, I see no reason a street engine should not benefit from the effort. If Alan's dyno were still available (howdy, if you're lurking Alan), I had hopes of investigating quench well under that. Both Jenkins and Yunick claimed to be successful at .008" using pistons with very wide flat surfaces and pretty loose skirts (forgive me...) in SBC engines; I'd bet the numbers now are in the low-single digit range. Real tempting...
Added by edit: Harry focused strongly on the rotating and reciprocating assembly, and given (possible) frictional losses, there was reason to do so. But any IC engine makes power strongly dependent on the chamber; where, when, how, how fast, how efficiently the mixture is burned. Burned, not exploded. Harry pretty much ignored that, other than to make some incorrect assumptions
In the 616 engine, we are given a chamber which RR rejected in the development of the Merlin as too prone to detonation; the "ramp" chamber (if anyone asks, I'll get the book and cite page number too; it's in the hallway "engine" book shelves).
I have images of John W's chambers (simply by finding them; no "spook" work on my part) and they certainly extend the quench area as much as possible by some clever efforts, and I'm guessing pull it as tight as can be. For street engines.
I've yet to see any images of Cliff's chambers and he owes them to none of us, they are the result of his efforts; I've yet to be generous regarding the matter either.
tl;dr: The chamber matters and the quench clearance goes a long way to defining it...

[Juha Vane]

Thank you for the replies, highly appreciated.

Been working some with engines, but this little air-cooled thing seem to throw curve balls
at me constantly. Also as I'm getting older, I like to increase my standards. Hate to remember
the things I put together when I was young...

The quench is now 0,030, so it's between the street minimum 0,035 and the race 0,025.
I will verify that I have this 0,030 quench all the way around in the quench area.

But the exhaust valve to piston clearance? Here is again quite a variance in numbers.
0,070 - 0,100 and 0,050.
I understand that this is depending on the cam profile. With my Elgin 7008 the clearance is
smallest just before TDC. It is only few degrees where it is the smallest, now 0,83mm (0,033).
The piston cut is very precisely angled, clearance was even.

Matthew you wrote:

I'm running that cam in my 1720 and I like it. I think I aimed for 0.040" quench which is the
tightest recommended clearance for street use. At least that was my understanding at the time.

What is your piston to exhaust clearance as you are using same cam?

[C J Murray]

Cliff, you drew a distinction between "street" and "race" engines; that was the point of my objection. Any disagreement, if it exists concerns that dichotomy.
I certainly agree that an inexperienced builder shouldn't try to achieve the accuracy required for those sorts of tight quench clearances in a street *or* a race engine. Or should, and learn from the experience, as most all of us did.
But given the required skills, I see no reason a street engine should not benefit from the effort. If Alan's dyno were still available (howdy, if you're lurking Alan), I had hopes of investigating quench well under that. Both Jenkins and Yunick claimed to be successful at .008" using pistons with very wide flat surfaces and pretty loose skirts (forgive me...) in SBC engines; I'd bet the numbers now are in the low-single digit range. Real tempting...
Added by edit: Harry focused strongly on the rotating and reciprocating assembly, and given (possible) frictional losses, there was reason to do so. But any IC engine makes power strongly dependent on the chamber; where, when, how, how fast, how efficiently the mixture is burned. Burned, not exploded. Harry pretty much ignored that, other than to make some incorrect assumptions
In the 616 engine, we are given a chamber which RR rejected in the development of the Merlin as too prone to detonation; the "ramp" chamber (if anyone asks, I'll get the book and cite page number too; it's in the hallway "engine" book shelves).
I have images of John W's chambers (simply by finding them; no "spook" work on my part) and they certainly extend the quench area as much as possible by some clever efforts, and I'm guessing pull it as tight as can be. For street engines.
I've yet to see any images of Cliff's chambers and he owes them to none of us, they are the result of his efforts; I've yet to be generous regarding the matter either.
tl;dr: The chamber matters and the quench clearance goes a long way to defining it...

It's called risk/reward. A few years ago the geniuses at BMW ran a very tight quench on their inline 6cyl M3 engines. They had the distinct advantage of a pent-roof chamber and modern, precise machining. Oops, they had to recall them all and install thicker head gaskets because the pistons were whacking the head. When you leave no margin for error, poop happens. In a race engine you either test on the dyno or if it acts up at the track you load the car back into the trailer, back to the shop, make adjustment.

As for chamber shape for racing.... SCCA allowed no addition of material to the chamber or ports so SCCA racers were subject to tear down and loss of wins so those rules tend to be followed. Vintage racers have more latitude but I don't think anybody is doing anything radical.

I have seen some unusual approaches to modifying the 356 chamber for street but they look compromised to me. Any mod that requires an odd shaped lump on the piston is counterproductive by requiring a heavier piston and a convoluted flame path. The last Porsche factory engineer(https://en.wikipedia.org/wiki/Hans_Mezger) to test and modify the 616 chamber knew more about engine design than the people playing with them now. Beware of marketing winning over function.

When I was growing up I lived 3 blocks from Bill "Grumpy" Jenkins shop in Berwyn PA and when I was a bit older we drank at the same bar. He taught me everything he knew about engines. LOL, just kidding, he didn't like an audience, not even at the bar!

[Matthew Devereux]

Matthew you wrote:

I'm running that cam in my 1720 and I like it. I think I aimed for 0.040" quench which is the
tightest recommended clearance for street use. At least that was my understanding at the time.

What is your piston to exhaust clearance as you are using same cam?

I only checked the piston to head clearance

[C J Murray]

I only checked the piston to head clearance

If it is still running it is adequate and you are lucky. It still may be tight enough that you do not want to miss any shifts.

Here are pictures from a street engine that a "professional" shop built and that car ran for years this way. It didn't run well but it ran. The cam was the once popular 105* lobe separation aftermarket cam. When you change from a stock cam you had better check all the clearances.

[Matthew Devereux]

Its a regrind of a stock 912 cam. I'm sure I did do one test with clay and there was lots of clearance if I remember correctly so I fibbed. That was about 3 years ago now. The piston to head clearance was the one that was running tighter. I know I had to add thicker cylinder base shims to get adequate piston to head clearance.

[Juha Vane]

Cliff,
I understand very well your concern, it's very valid.
I assembled both heads with exhaust valves using "weaker" springs.
Bolted the heads and valve train on and measured the clearances with
solder rod and clay.

The quench is on all cylinders ∼0,030 all around the quench area.
The piston "deck", the narrow circular flat ring outside piston to cylinder top is ∼0,039.
The exhaust valve to piston clearance is ∼0,035

These are measured with digital calliper so there is some tolerance, but I'm convinced
that the piston to cylinder head is OK. What concern me is the exhaust valve to piston
clearance. Probably to be sure, I'll cut the valve pockets to achieve 0,070 clearance.

Find it a bit strange that there is very little discussion about this? Is my cam too radical?
The car run OK (well, the carbs was not OK)

[Craig Richter]

Cliff, It's amazing that your "box of parts" photo above didn't actually bend any of the exhaust valve stems. Even the slightest bend will cause the motor to run really crappy. Back in the day, we almost always had to deepen the exhaust pockets in the pistons then available to use the popular Isky 1010A cam.