Cummins 6BT, 4BT among popular rebuilds at Colorado engine company

Updated Feb 3, 2020

When it comes to engine rebuilding, some powerplants are better candidates than others. Take the Cummins 6BT 5.9-liter 6BT diesel engine for example.

From humble, reliable workhorse to powerful performer on various auto shows, including the Diesel Brothers, the 6BT continues to enjoy plenty of popularity. And its little brother, the 4BT, has also been busy winning over plenty of fans.

We caught up recently with Big Bear Engine Company in Denver, Colo. which has plenty of experience rebuilding the 6BT, 4BT and several other favored workhorses.

Special thanks to Big Bear Engine Company managing partner John Clifford for taking the time to fill us in on the 6BT, 4BT and others.

HWT: What makes the 6BT such a trusted workhorse?

Clifford: The old mechanical 5.9 (6BT) is still widely used to this day nearly some 35-plus years from its introduction because of its simple design. It’s often regarded as the best diesel engine ever made from an engineering standpoint due to it being “dummy proof.” With old mechanical engines you have much less points of failure. The gear train is straight forward. There is no idler gear on the front gear train applications (12 valve). The engine is straight cam driven…the crankshaft gear drives the oil pump and cam. The cam gear then drives the fuel pump and accessory drive (air compressor, vacuum, hydraulic pump, if used). The cylinder block uses a full skirt design and a main cap between each connecting rod (cylinder) which makes the block very rigid and strong. Your water pump is belt driven. Keeping it simple means the engine can take a lot of wear and tear. Consequently, for the enthusiasts, the engines can be easily modified for performance uses with upgraded camshafts, governors springs in the old rotary or inline P-Pumps, ported heads, larger turbos, etc.

Of course no engine is perfect. The 5.9 L (6BT) had its issues with the infamous ‘killer dowel pin’ (KDP) and cracking with the 53 Style Blocks. The old 12 Valves had some early issues with the fuel filters from some of their truck packagers. They didn’t design them to filter enough microns and thus the fuel was full of containments. All of this is old news and has been resolved since.

HWT: Have you seen more interest grow in the 6BT in the past few years? If so, why?

Clifford: Performance builds with the 5.9 L have always been popular with the enthusiasts crowd but the past few years we have seen an bigger demand for customers wanting to do 4BT builds. The 4BT is essentially a 6BT with 2 cylinders lopped off. In fact, the 4BT (3.9 L) was engineered first in the early 80s as an industrial engine. It was in 1989 that Cummins partnered with Dodge to take their medium duty 6B5.9 found in smaller commercial vehicles and industrial equipment and convert it into the world famous 5.9L passenger truck engine; the rest is history. The advantage of the 4BT over the 6BT is the weight. A 4BT (albeit a heavy little engine) comes in at 780-800 lbs. with oil, which is just about what a gasoline big block weighs. A 6BT is around 1200 lbs., making it suitable for ¾ and 1-ton chassis. Many customers don’t want to have to upgrade the axles, suspension, u-joints to hold the weight of that bigger engine. The vast majority of customers want to go with an old mechanical 12 Valve vs. the 24 Valve simply because when you add electronics to the equation it is simply another point of failure—you see the ECM, wiring harness leads and then injector failures, not to mention adding complexity to conversion projects. With a mechanical engine it simply works when you need it.

HWT: What are the benefits of going with a rebuilt 6BT?

Clifford: Rebuilding an old 12 Valve 6BT is fairly straight forward. The engines can be bored and honed to oversized specs just like gasoline engines. Sometimes if the damage is too great the blocks will have to be bored and sleeved. Other than that, cam, crank, head and connecting rod remanufacturing is fairly standard back to OEM specs. A lot of times budget is a concern for enthusiast-customers. They desperately want to do a repower on their truck or yank out an electronic engine however the cost can steeply rise as they go forward with the project. They tend to seek out used engines. Not to say all used runners are bad but what we’ve seen is that it can be a gamble whether or not the engine will last 7 months or 7 years. Used engines are not our primary business. When we do offer a used runner it is from our own core vendors that we deeply trust to inspect and test run the engines. If you go with a reman or new component built engine you can rest a little easier knowing the quality of the parts and workmanship, and avoid the unknown history of a used engine. The vast majority of our 4BT and 6BT engines (about 200 per year) are considered component builds. A component build is considered a mostly new engine with about 90 percent OEM, 10 percent aftermarket parts. Few components are remanufactured. With component builds our quality control increases as many processes are refined and replicated across multiple build cycles. With repetition, lead time is also decreased dramatically. Customers should also be aware of oftentimes confusing industry verbiage with respect to “rebuild” and “refurbished” vs. “remanufactured”. ‘Remanufactured’ means the old core engine is machined back to the original OEM specs per the engine serial number. A ‘rebuilt’ or ‘refurbished’ engine could simply have new gaskets, cleaned up and repainted; nothing is done with the cylinder head, crankshaft, bearings, rods, fuel system, etc.

HWT: What are some other reliable light-duty oil burners that are good rebuild candidates?

Similar engines to the 5.9 (6BT) would be the CAT 3116, CAT 3126, CAT 3208, DT-466 (old top-kick van bread truck engines) or a Detroit 50 Series (A short lived design before the famous Detroit 60 Series). We see a lot of repower customers also looking at the Isuzu 4BD1 or 4BD2. It’s a good little gear driven overhead cam engine, popular in Jeep swaps; however it must be noted that those engines can’t be rebuilt. If you deck the block it changes the gear lash; both the block and head can’t be decked. The Isuzu 4BD1 and 4BD2 are throw away engines.