running the rack

so today i finnally got around to setting the overhead on my mack, as much as i hate to admit it i have never had the overhead done on this truck, of course when i bought it had very few miles on a rebuild but now with about 300k on the rebuild and my fuel millage has seemed to be worse lately. i figured i better get it done. now i have also never done the rack on a semi back when i had my freightliner i had it done by a shop. i don't really use any shops for anything anymore and have done valve lash adjustments on auto's many times and have the literature that includes the procedure for my engine for setting the valve lash so not something i felt uncomfortable with in the least. took me probably 4 hours start to Finnish.

i wanted to share something i found very odd when i did the overhead on this engine that does not seem quite right to me, but having never done it myself i thought i would ask if its normal to find this when running the rack or was the rack was never properly set when the engine was inframed?

i checked the valve lash on each valve prior to adjusting them and here are some things i found odd

most intake valves were vary tight the tightest one i could feel a tinny bit of lash my pushing on the rocker arm but i could not get a .001 feeler gauge in there. spec is .016 with the exception of #3 witch was dead on .016 and #6 witch was .014 all the rest of the intakes had less than .003 lash.

exhaust valves all sloppy loose. with the acceptation of #3 again witch was only .025 (spec is .024) and #6 witch was .22 (too tight) all the rest were .028-..30

now for the jake breaks old spec is .015 new spec for updated jakes witch i have are .021? anyway i chose to set them at .015 but here is what i found for pre adjustment lash on the jakes #1 couldn't get a .001 feeler gauge in there it was so tight #2 .027 #3 .018 #4 .021 #5 .010 #6 .016

anyone else find it odd how far allover the place the lash was before adjustments?

i haven't drove the truck sense running the rack but it does seem to idle smoother now.

 

yeah the first set of valves i did and the exhaust being so loose first thing that went threw my mind is cam shaft. these mack engines are a bit known for eating cams and the cam was not done at the time of the rebuild. but the fact 4 out of 6 were about the same amount of looseness did ease my mind a little i doubt the engine is eating 4 exhaust lobes at the same time. i expected them to all be tight. and no idea why the jakes would be out of wack so much and not necessarily in sequence with the exhaust valves that were out of whack.

as far as being adjusted out of sequence that was triple checked when i first realized things seemed odd in the pre-adjustment valve lash. timing marked lined up on the flywheel opposite cyl in the firing order in valve overlap between exhaust and intake. adjusted the cyl tdc compression stroke as the procedure outlines. even moved the cyl 15* down the cyl on the power stroke after making the valve adjustment and double checked just to be sure i wasn't out of sequence or on the wrong stroke.

but just from doing valve adjustment on automotive i found this to be very odd, typically what i would find in automotive is the lash would tighten up and typically the exhaust would tighten up more so than the intake would. so it seemed quite odd to me that exaust valves as a majority would be loose and intakes would be very tight as a majority.

the only thing that really comes to mind is the procedure outlined that the valve adjustment is to be done cold, that the engine must be under 100*f. maybe they previously adjusted the valves hot and as exhaust valves run hotter than intake valves maybe that would explain the looseness on the exhaust valves?

of course that doesn't make sense that they would have set the lash hot during a infame cause the lash would be set when assembling the engine before it was run for the first time i wouldn't thing they would assemble it test run it and then re-adjust the lash before sending it out the door.