0.006" valve clearance in new DOCH CAMs - Too tight?

[Nut124]

I see European makers offer new billet cams for my DOCH application (Fiat original twin cam). They suggest valve clearances as low as 0.006" and typically no more than 0.008".

This seems awful close to me. Factory clearance was 0.016/0.020".

What would be the acceptable clearance range? One cam maker says the decel ramp starts/ends at 0.20mm or 0.008".

The available shims come in 0.002" increments. How much would set clearance change due to thermal expansion during extreme operation?

Have you run clearances like this? What was your experience?

[ProPower engines]

I see European makers offer new billet cams for my DOCH application (Fiat original twin cam). They suggest valve clearances as low as 0.006" and typically no more than 0.008".

This seems awful close to me. Factory clearance was 0.016/0.020".

What would be the acceptable clearance range? One cam maker says the decel ramp starts/ends at 0.20mm or 0.008".

The available shims come in 0.002" increments. How much would set clearance change due to thermal expansion during extreme operation?

Have you run clearances like this? What was your experience?

While any performance engine has its challenges the import stuff makes more work in this area.
I would call the cam manufacture directly on the phone and discuss the issues you are faced with.
Other wise you will have to set the lash much looser of say at .008 to .010 and then run it to hot as in 210 degrees then shut off and pull the cover and check the lash on 1 cylinder.
Then let it cool over night to stone cold and recheck and adjust to suit what is needed.

This is of course an issue if all the valves are not the same height in the chamber relative to the deck surface of head.
And the valves are all the same over all length checked before assembly was done.

This is the reason you have an issue now and the difference in shim requirements is a PITA as you are stuck not able to tip a couple thou off the valves to correct the installed tip heights. I set all my heads up this way so that when I am finished the valve guide and seats work I don't need a selection of shims and have to accept a different lash amount.
I end up with 1 shim thickness and use them to correct the lash.

If you are using a shim that goes over the valve tip you can make a magnetic holder and dress the shims a few thou in the valve grinding machine to correct the thickness you need.
Or pull the head and tip the valves to correct the tip heights as needed .



Oh the fun we have doing HP import type engine/head work








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[modok]

Most OCH with bucket lifters I expect to see around
.006-.008 intake
.008-.012 exhaust
OE's probably have a generous margin of safety
And does depend on what kind of lash ramps are on the cam.....

I would set the exhaust .002" looser than the intake, because we know exhaust valves get a lot HOTTER

[NORSK]

It depends how long the valves are
0,006" on exhaust seems very tight unless the valves are really short and the stems are thin
Motorcycle stuff can run tight clearance due to short valves and really thin stems

[Walter R. Malik]

I see European makers offer new billet cams for my DOCH application (Fiat original twin cam). They suggest valve clearances as low as 0.006" and typically no more than 0.008".

This seems awful close to me. Factory clearance was 0.016/0.020".

What would be the acceptable clearance range? One cam maker says the decel ramp starts/ends at 0.20mm or 0.008".

The available shims come in 0.002" increments. How much would set clearance change due to thermal expansion during extreme operation?

Have you run clearances like this? What was your experience?

On some motorcycle stuff, I have seen .002" / .003" cold if that can be applicable to the aluminum block & head temp variance so, the valves are not held open when fully cold; the lash ramp ground into the cam being used has the most affect here.

[Nut124]

Thanks for the replies.

The valves are 4 3/16" long (4.2") overall. 5/16 (8mm) stem.

See cross section picture below. Aluminum head, iron block. Bronze guides.

The shims are right under the cam, in a recess at the bottom of the inverted cup. They are perfect cylinders in shape.

[CamKing]

With an aluminum head, the head will grow more then the valves, so you would set the lash about .003" tighter then what the cam company recommends for Hot Lash.
We normally use a .010" lash ramp, and set the cold lash at .007" on setups like yours.

[MadBill]

I see European makers offer new billet cams for my DOCH application (Fiat original twin cam). They suggest valve clearances as low as 0.006" and typically no more than 0.008".

This seems awful close to me. Factory clearance was 0.016/0.020".

What would be the acceptable clearance range? One cam maker says the decel ramp starts/ends at 0.20mm or 0.008".

The available shims come in 0.002" increments. How much would set clearance change due to thermal expansion during extreme operation?

Have you run clearances like this? What was your experience?

While any performance engine has its challenges the import stuff makes more work in this area.
I would call the cam manufacture directly on the phone and discuss the issues you are faced with.
Other wise you will have to set the lash much looser of say at .008 to .010 and then run it to hot as in 210 degrees then shut off and pull the cover and check the lash on 1 cylinder.
Then let it cool over night to stone cold and recheck and adjust to suit what is needed.

This is of course an issue if all the valves are not the same height in the chamber relative to the deck surface of head.
And the valves are all the same over all length checked before assembly was done.

This is the reason you have an issue now and the difference in shim requirements is a PITA as you are stuck not able to tip a couple thou off the valves to correct the installed tip heights. I set all my heads up this way so that when I am finished the valve guide and seats work I don't need a selection of shims and have to accept a different lash amount.
I end up with 1 shim thickness and use them to correct the lash.

If you are using a shim that goes over the valve tip you can make a magnetic holder and dress the shims a few thou in the valve grinding machine to correct the thickness you need.
Or pull the head and tip the valves to correct the tip heights as needed .



Oh the fun we have doing HP import type engine/head work \

Do we know that these are hot specs? As mentioned earlier, the Fiat 124 uses a shim on top of the bucket design, so fairly easy to change, but still a pain to measure hot and per the illustration, apparently a lot of components to remove for access. I'm sure any shop regularly servicing them would from experience have learned the hot/cold delta and so would set the them to an appropriate cold spec. with the assembled head on the bench and not run up the time clock by re-checking hot. Clearly Fiat specs. could be and should and be cold lash numbers..
.

[n2omike]

I have a Honda CRF 230, XR 250, XR 400 and a Subaru 2.5L boxer that I've set the lash on recently. All are SOHC with rockers. The Hondas are air cooled dirt bikes with rocker arms and the lash is 0.004"I and 0.006"E. The Subaru also has rocker arms, and is 0.008"I and 0.010"E. DOHC with bucket/shim is usually tighter that what I posted... as there is no monkey motion with rockers, ROCKER RATIOS TO MULTIPLY LIFT RATES, etc. Overhead cam stuff with aluminum heads has a lot tighter clearance than what we are use to with old OHV V8's.

[digger]

With an aluminum head, the head will grow more then the valves, so you would set the lash about .003" tighter then what the cam company recommends for Hot Lash.
We normally use a .010" lash ramp, and set the cold lash at .007" on setups like yours.

never understood how the head grew more than the ex valve while running under load.
the stainless valve coefficient of linear thermal expansion is only 20% lower than cast alloy but the ex valve is much much hotter than the head 400C vs 100C. Sure the head height to position of cams is larger than the length of the valve but i dont see the head growing more

[MadBill]

Since the exhaust valve is much hotter than the seat, it will grow in relative diameter and thus 'climb' the latter's 45° slope, offsetting some of the valves linear expansion.

[Nut124]

Since the exhaust valve is much hotter than the seat, it will grow in relative diameter and thus 'climb' the latter's 45° slope, offsetting some of the valves linear expansion.

MadBill, thanks for that! This makes perfect sense to me as a scientist. Never thought about that, always focused on stem expansion, ignoring the valve diameter expansion. This question had been bugging me for some time.

Now, maybe I will get into these low lash cams after all.

Thanks again!

[digger]

Since the exhaust valve is much hotter than the seat, it will grow in relative diameter and thus 'climb' the latter's 45° slope, offsetting some of the valves linear expansion.

is that effect large enough to offset? it would reduce the effective length of the valve.
surely there is a way measure the clearance on a running engine? i dont means for mere mortals but surely some lab has done it

i've measured engines hot but by the time the valve cover is off and you're taking measurements the valve would have cooled the most out of all the components and a fair bit to boot.

[MadBill]

Some historian in the ranks probably has the details, but back in the mid-fifties, Corvette road racers with the factory race cam (xx097?) were having trouble with burnt exhaust valves. Better materials didn't help much, but opening up the exhaust lash did; the hot clearance was going negative, hurting power and valve cooling...

[Sparksalot]

Valve stem expansion of the exhaust valve is primarily in the relatively short portion of stem exposed inside the port and within the part of the guide exposed in the port. The part of the stem within the cooled portion of the guide inside the head is much cooler and doesn't expand all that much. The cooled portion of the stem is longer than the hot portion so the total stem expansion is less than some people think it is.