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Jake brake 25A vs. 25B & their use with a pulse manifold

US6x4

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I've been doing some light reading of the Jacob's install manual for the 425/425A model and I'm kinda leaning toward this model over the 25B since it's a bit more versatile with Big cam vs Small cam and single entry vs split entry applications:

20190901_181318.jpg

The downside is that they are a little more expensive.
 

US6x4

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20200227_210646.jpg


I'm missing 3 oil supply screws (p/n 199225)
and I cut off 3 of those special stud nuts that were rusted on. Does anyone have any extra of these they don't need anymore?
 

US6x4

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... If you use a divided entry turbo with a pulse manifold, the autolash screws might need to be installed or changed to a different type.
I think I found the Jacobs document mentioning the replacement of adjusting screws with Auto-Lash adjusters on page 3 (listed under Small Cam III) of the install manual for type 401 jake heads.

Do you think this change mentioned for a Small Cam 3 would work on our NHC-250?
 

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WillWagner

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That has a mess of wrong stuff. NH were non turbo charged....hence the engines in the 5T trucks, NHC 250. No such animal as a Small Cam III. BUT, If you are adding a turbo to your NHC, that would make it an NTC, small cam. Depending on your turbo selection, single or divided, they are both 25B, divided needs the auto lash screws. They still set at .018.

NHC.....N, New 4 valve head. H, nat. aspirated. C, Custom Rated.
NTC......N, New 4 valve head, T, turbocharged,. C, custom rated.
NH..... you get it.
NTCC..... Same, but the last "C" was for Ca. only, MVT...Mechanical Variable Timed. The ONLY engine sold in Ca, from Cummins in the 1984-85 time frame, fuel pressure and air pressure to control no to light load timing. Base timing set at .065 BTDC and advanced to .125. Even had a compound turbo 475!
STC......Step Timing Control. Used oil pressure to advance injection timing, base timing set at around .306 BTDC prior to fuel pressure coming up to around 85 PSI...still mechanical fuel pump.
N14......you get it, but the 14 was for displacement. Made in STC from 1991 to 1994 and fully drive by wire. 1994 got rid of mechanical and went to all electric.

Other engines, K series, inline 6 to V16....bigger now, V series, 504, 555, 903 and early, just rated by HP, like in the LARC series and the M123 series, V8 300. Not too fun to work on, V 8 470, lots of others. J series, a "C" series made in the 60's that is NOTHING like the C in the 900 series trucks. And there is more, but I am rambling.

Single entry turbo, no change to the screws, divided entry, change to auto lash screws. Part numbers are on top of the screws.
 

US6x4

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Will, thanks for chiming in with those corrections and other definitions- much appreciated. Looks like they want about $100 apiece for those Auto-Lash adjusters and at 6 per engine that's a premium upcharge just to run a divided entry turbo. Oh well - this is funner than collecting stamps 😁
 

US6x4

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Further info on the Auto-Lash part numbers for anyone pursueing them:

Jacobs Auto-Lash P/N 013369 superceded to 022576 which crosses over to Cummins P/N 3871407 for a 6-piece kit.
 
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