Engine/Transmission (1998.5 - 2002) How tight is to tight?

[StakeMan]

Minimum spec for valve adjustment is . 006 and . 016. Could we assume that Cummins left some safety factor there? If so how much?



I have been told that some set the valve lash to minimum spec and have not had a problem yet. Has anyone went under minimum spec?

 

[JFinke]

It all depends on how hot you run your motor. The hotter you run it the more parts will expand and use up the lash. It also depends on what is wearing on your motor at a faster rate. If you are getting wear at the cam, follower, pushrod, rocker, or valve tip the lash will increase. If its at the valve face or seat you will loose lash. Also the less lash you have the more carefull you need to be about RPM. I wouldnt try to figure the min. spec out unless I was willing to rebuild the motor from a failed experiment. And your truck may work with what someone elses hit's at. Or it may hit where theirs was ok. If you tow long grades you may find what has been ok is no longer acceptable.

 

[WeirCummin]

I have to assume that the specs your looking @ are from the Dodge service manual. I'd forget those numbers and go with the specs on the Cummins data plate. On my truck it's 10 & 20. Not min/max numbers from the DC manual. I work on Cummins motors daily from the 6BT to the N14 and all the specs given for valve lash in a dead on numbers

 

[Rob Thomas]

Little do you know Dan, but you just agreed with me. LOL





Rob

 

[Gary - K7GLD]

Don't wanna overlook the fact that valves USE time spent on the seats to transfer heat to the block - sure, you CAN run them tighter and still maintain cylinder seal - but sacrifice heat transfer and shorten valve life in the process...

 

[StakeMan]

Originally posted by Gary - KJ6Q

Don't wanna overlook the fact that valves USE time spent on the seats to transfer heat to the block - sure, you CAN run them tighter and still maintain cylinder seal - but sacrifice heat transfer and shorten valve life in the process...

I doubt we are affecting the time on seat that much. If that was the case then those running aftermarket cams would be burning valves.

 

[jelag]

An engine will tolorate loose... . wider gaps better than tight gaps...



An exhaust valve that is tight will allow excessive hot gasses past the valve when its open. . allowing damage to the valve and its seat... . Its like high pressure steam excaping from a steam vessel... . very hot gasses will cut and damage the sealing surface of the valve if the valve is off the seat... .



On the other hand... . a valve adjustment that is too large... . will allow for some clatter and lack of performance... . and down the road some damage... . but the tight valve can really cause damage... .



Ask the guys with the air cool VW's... the head of the tight exhaust valve would drop off from the heat and jam up the engine...



Just my thoughts...



Jim

 

[Gary - K7GLD]

Dan, I can't speak for the others, but much - or MOST of my past hands-on mechanical experience comes from working on gas engines, mostly SB Chevies - most carbed, a few injected, one B&M blown.



MUCH that applies to gassers does NOT apply to diesels!



I suspect that the apparently tamer profile of a Cummins in stock form will quite adequately provide proper heat transfer from valve to block unless lash is set WAY too tight. As fas as "Scavenging" of exhaust gases is concerned, I also suspect there's precious LITTLE to be done to assist that merely by a miserable couple of thousands less exhaust (or intake) valve clearance. In my book, there's LITTLE to be gained by tighter clearances - and MUCH to be potentially lost, if set too tight.



It's my understanding that "scavenging", or cylinder purging with camshaft overlap, is largely unneeded in a forced induction engine because air is FORCED into the cylinders under pressure - not SUCKED in by vacuum as in a "normal" engine - thus, I would assume, less - or no overlap at all. But I haven't seen the specs on the Cummins cam either...

 

[Diesel Gunner]

Aw heck, I just could not resist jumpin' in here. Hopefully this will attract one or two of those guys who really know these engines. I would not be suprised if a bit of overlap was used to scavenge the cylinders. After all, it would also cool at the same time. Any gurus know for sure?

 

[JFinke]

Originally posted by WeirCummin

I have to assume that the specs your looking @ are from the Dodge service manual. I'd forget those numbers and go with the specs on the Cummins data plate. On my truck it's 10 & 20. Not min/max numbers from the DC manual. I work on Cummins motors daily from the 6BT to the N14 and all the specs given for valve lash in a dead on numbers

I looked at my ISB5. 9 book today and it does give the same min/max #'s that Dodge uses. In my 12V B series book it gives dead on #'s. Jake

 

[Gary - K7GLD]

I suppose ya could pull the valve covers, and check to get a rough idea as to if and how long both valves are off their seats in a rotation...



Wonder how big a blast of air would pass thru a cylinder with BOTH valves open at 30+ pounds of boost...

 

[DF5152]

ok well like a few others im not a diesel mech but have been hot rodding cars for years. So here goes adjusting the valve to tight with do nothing for scavaging or over lap or anything else. over lap is controled by the cam and in a turbo charged engine you do not want it more over lap is for high RPM and thats what give a gas engine its lopey idle. If you make your valves to tight your valve will not seat and you will be sending ur compression right out the open vavle not to mention you will be giving up any oil clearance you have between you valve stem and rocker assembly. Without changing you cam timing i also dont think you will have a problem with piston to valve clearance but not having opened up one yet i dont really know how much piston to valve clearance there is. Basically what I think it come down to is oil clearance end temp if you are the tight side and run hot you are loosing oil clearance then the rocker/follower will be riding dry on the valve stem.

 

[StakeMan]

Does anyone know what the valve to piston clearance is? Wouldn't the valve lash change the timing?

 

[DF5152]

Originally posted by StakeMan

Does anyone know what the valve to piston clearance is? Wouldn't the valve lash change the timing?

it should only change lift proportianal the the rocker/folower ratio. in order to have any true affect on timing the cam would have to be rotated

 

[StakeMan]

Wouldn't you be changing when and how much the valves open? That sounds like cam timing to me.