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Small Cam/Big cam Cummins

40656 Views 19 Replies 12 Participants Last post by  Ambrosepiu
Whats the difference between a small cam cummins and the big cam cummins engine?
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BCI's truck motors had non-top stop also. BCII and III Ag's also had non TS's. All BCIV's had OBC TS's.
I was going off of what I had experienced, my big cam one has top stops, and shows it was built with them in quickserve.
Done both. I use the old CPL book to know if it's built to spec or not. Just did two :)
We should look more at the camshaft itself to differ the 'small' verse 'Big'. This has nothing to the cam being big in lift or small in lift. It pertains to the physical SIZE of the camshaft as a whole. If you see an older 855 series Small cam you will notice the camshaft itself has a smaller diameter journal which entails smaller lobes, opposite is true for a big cam, large journals equals larger usable lobe size which do tune differently however the large journal cam can accept higher rates of acceleration at the lifter. The cam follower plates are also different in size where you adjust your valve timing.

I had a slight argument with some older co workers on this when I told them there was no such thing as a "big" cam or "small" cam at which they were thinking on a whole different perspective as I was.
Lobe size and injector plunger drive rates were a byproduct in afterthought of the original fix which was updating and repairing the ever-so-prevalent cam bearing spin in small cams. When big cams started having the same issues, tho later in engine hour life, they dropped the running oil pressure from 54/60PSI to 38/42PSI thru II and III series noting it was not tube load but wash out load that spun the bearing. Easy argument fix in cam size is
1) parts order ship weight
2) the Cummins stamp and small cams and the BIG CAM stamp on the latter

They do time and "tune" at same specs as early big cams tho fast cam timing was in truck and Ag use for SC, BCI, II, III and early IV... "Back 2" 7inLB click stop T. Either 14 and 27 or 11 and 23 could be used depending on valve set at OBC or 2 back plus hot and cold or dial indicator methods. .070"(+/-) was normal injection event in all early Cummins which was supposed equal 19* static in the "over 100 different" pistons and 40+ different cams and injector sets, tho the 176BK's turned up purdy often for injectors to match OEM rail... Good post tho
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tube load " pressure excerted on cam follower by lift of injector lobe thru injector tube" parasitic shock. injector plunger bottoming out. bearing washout " high oil pressure washing out bearing's, small cam including the 743 series ran 55 to [email protected] rated speed but held together, bc cummins 35 to 45lbs @ rated speed ran forever
Thanx Fest. :thumbsup bike didn't go so well... :bang
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