Losing Valve Lash

[TheJet-09]

Engine related, but I see even less activity under the Powertrain section than here, so...

What would cause valve lash to go away, or get tight? All aluminum engine (SBC), shaft rockers, valve lash set when cold (0.005"). I luckily saw a picture of the car under race conditions and noticed the #3 and #5 header pipes were glowing (but not #1 or #7, at least that I could see in the pic). Checked valve lash and sure enough, #3 and #5 exhaust rockers were at zero lash, cold (technically even less). Any idea what might cause this?

I believe I will start setting them with the engine warm, but it was used and I don't have the cam card. So what would a safe # be? Maybe 0.016" or even more? Any info is appreciated.

[MasterSbilt_Racer]

Engine related, but I see even less activity under the Powertrain section than here, so...

What would cause valve lash to go away, or get tight? All aluminum engine (SBC), shaft rockers, valve lash set when cold (0.005"). I luckily saw a picture of the car under race conditions and noticed the #3 and #5 header pipes were glowing (but not #1 or #7, at least that I could see in the pic). Checked valve lash and sure enough, #3 and #5 exhaust rockers were at zero lash, cold (technically even less). Any idea what might cause this?

I believe I will start setting them with the engine warm, but it was used and I don't have the cam card. So what would a safe # be? Maybe 0.016" or even more? Any info is appreciated.

Valve seats sinking into the head.

[TheJet-09]

Is that normal or something to be concerned about?

[MasterSbilt_Racer]

Is that normal or something to be concerned about?

Usually a concern. Are these really old heads? I had one fall out and that really junks everything.

[billetbirdcage]

First I want to explain a few things in case your not aware.

Lash: There is a ramp at the opening and closing of the cam lobe, that slowly accelerates (valve opening) or slows the valve down as it closes. This is to stop the valve from slamming into the seat and beat seats/valves up or break valves. How long this ramp is depends on cam design but that is basically what determines the max lash allowed. Lets say this ramp is .015 long so more then .015 lash (at the lifter NOT valve) and it will slam the valve on the seat. Why I said LIFTER LASH is because the valve lash is modified by the rocker ratio. Lash settings by the cam manufacturer are based on a recommended rocker ratio so running a different ratio actually changes the needed valve lash.

Example: recommended lash on cam card for cam with 1.5 rockers is say .018, so the lifter to cam lash is actually .018 / 1.5 = .012. So if you ran a 1.85 rocker the actual valve lash should be .012 x 1.85 = .022. As you can see you need to be way off on ratio to mess the lash up enough to cause a huge problem.

So the higher the ratio the lash will typically be larger in most cases. Also hot vs cold will be effected more with high ratio rockers, like all aluminum motor with 1.5 may grow .010 hot but a 1.85 rocker may grow .018 so keep this in mind. Also I had motors you can't set cold as it actually has no lash when really cold but it's really high ratio stuff like 1.85 to 2.3 stuff.

Now onto your losing lash problem: It could be the valves are starting to tulip, this is more common with large diameter valves and with titanium but does happen with steel valves and on the exhaust side if exhaust temp gets out of control. Tulip is when the head of the valve starts to deform and the stem of the valve basically moves up in the head towards rocker but the seat edge of the valve stays in the same place against the seat. This is hard to describe but basically the head of the valve tulips and effectively makes the valve longer and reduces lash.

Are there other things that could be the cause?, sure but most will cause lash to increase not decrease. If unsure reset lash as accurately as you can and run motor and recheck. lash will vary with temp so checking variance is more important then its exactly the same as last time. Again with high rocker ratios temp will effect this more and by the time you get to last cylinder on one side it can cool down enough to make some difference so keep that in mind.

[billetbirdcage]

See what rocker ratio the motor is and I can take and educated guess on lash, better to be too tight then too lose and not be on the opening and closing ramps on the cam. Typically most cams aren't under .012 and can be up to .028 so that can be a typical range but the smaller side is with low ratio stuff like 1.5 and the higher side with 1.85+. Usually .018 intake and .020 exhaust Hot usually works out and is safe for most cams but lean a little higher if when cold it closes up a ton with high ratio rockers, like .022 intake and .024 exhaust hot.

Setting cold is the preferred method as then temp isn't effecting the lash due to cool down while setting. However you need to set it hot and let cool down and check to get the adjust lash for cold. So set hot at say .018 and let cool down completely and check and its say .008 so use that from now on.

Typically aluminum grows .006 per item, so alum head steel block is usually .006 tighter cold and all alum is .012 tighter cold. Again will vary due to rocker ratio

[Jking24]

To add to what billet said i would relash everything and then check your adjuster depths on the suspect cylinders compared to the known good cylinders generally if the valve job was done correctly all adjusters will be in a similar position. If their different i would suspect a seat getting beat up or a valve tuliping

[TheJet-09]

Thank you for the responses, excellent information. In adding to the story: Brodix 16* heads, T&D rockers (1.70 Intake and 1.65 Exhaust). The engine has probably been around a while...was told it had 400 laps since freshened when I bought it (who knows to what extent though), and I've put just over 400 more on it.

I should also point out that I've always set valves (on any engine) with that cylinder at TDC. The last time I set the valves on this engine, prior to this issue, I used the method of setting INT as EXH begins to open, and EXH as INT begins to close. Again, a first for me, and it's quite possible I induced an error of sorts.

When I have some time I'll get some temp in it and set/check them, and check them again once cold. Thank you again for the responses.

[fastford]

Engine related, but I see even less activity under the Powertrain section than here, so...

What would cause valve lash to go away, or get tight? All aluminum engine (SBC), shaft rockers, valve lash set when cold (0.005"). I luckily saw a picture of the car under race conditions and noticed the #3 and #5 header pipes were glowing (but not #1 or #7, at least that I could see in the pic). Checked valve lash and sure enough, #3 and #5 exhaust rockers were at zero lash, cold (technically even less). Any idea what might cause this?

I believe I will start setting them with the engine warm, but it was used and I don't have the cam card. So what would a safe # be? Maybe 0.016" or even more? Any info is appreciated.

every thing billet said is correct , just curious though , are you really setting your cold lash at .005 or was that a typo error ?

[TheJet-09]

^^^ Yes, 0.005" as that was the spec provided from the previous owner. And yes, the feeler gage is so flimsy at that thickness it's obviously got room for error.

[billetbirdcage]

.005 is definitely in the range of what the lash should work out cold on an all aluminum motor with 1.7/1.65 rockers would be.

[fastford]

wow , I guess I have been screwing up for a lot of years because I have never set anything but a lawn mower that tight ...normally , I go 5 to 6 tighter than spec when cold ..with iron head/block , I go 2 to 3 looser cold ...

[over4T]

Did some research on this and found an older piece from Crane Cams that said a starting point cold was .002 looser iron block & head, .006 tighter aluminum head & iron block, .012 tighter aluminum head & block. We all know different types of metal shrink and/or expand at different rates. I've built many an iron/iron combo but my experience with the aluminum stuff is near zero. I've always set my iron stuff like FF, roughly .002-.004 loose and then checked again hot. Can't quite get my head around the extra tight numbers on the aluminum stuff. A little help?

[billetbirdcage]

An aluminum the block will grow in hieght (deck hieght) as the temp increases effectively making the block taller and the same for the heads, the whole head expands making the rocker studs or rocker stands effectively move up and loosen the lash. The alum just grows more then the steel in the valve train (lifter, push rod, etc) so the lash becomes looser.

That's basically why we have a .006 per aluminum component average for change in lash (Block and head)

[over4T]

Gotcha, I think. Thanks billet.