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Of course a DOHC does not have lifters, but the Buell motorcycle does.
Push rod engines, SOHC, and DOHC engines can have hydraulic lash adjustment systems, which is maintenance free. It is one of the many systems available out there. I was under the impression that the shim system was usually used on higher revving engines like motorcycles and small cars.
The Mustang Cobra has a DOHC 32 valve engine with hydraulic lash adjustments. So one does not exclude the other. It is a matter of choice at the engineering level.
With regards to the allowable play, I've read somewhere that it can open or close over time, depends on the engine. When the gap closes, the valves eventually remain opened, and burn as mentioned by Tom. It happened to me on a old water cooled VW engine. Conversely, the logic is that if the gaps open, the valves do not open completely over time and only power is lost, but there is no mechanical damage to the valve. I wonder if it is the case with the Titan. Also when the gap opens, noise increases accordingly. This brings me to the statement in the Nissan manual mentioning valvetrain noise increase which should induce a valve lash check.
I agree with Tom - it won't happen overnight.
While I work on industrial gas engines (project engineer - ignition), I am no valvetrain engineer either, so I don't pretend to know that part. I am just curious why Nissan picked one system over the other. I think the X-terra uses the hydraulic system.
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