The chassis on the walk-in van or stepvan you own, regardless of the body manufacturer, is not made by the same manufacturer. Body styles may include Grumman Olson, Morgan Olson, Utilimaster, and Union City Body. Chassis manufacturers may include GM, Workhorse, Ford, Freightliner, Oshkosh, John-Deere and International. All body manufacturers for walk-in van type vehicles use one of these chassis manufacturers and many times they may use more than one for an individual body style.
The chassis includes everything under the body including the engine and steering column. All frame components, brake parts, wheels, exhaust, and most of the time even the bumpers are part of the chassis.
Everyone has heard of the GM P30 chassis as it is very popular. These early chassis' had 16" wheels with front disc brakes and rear drum brakes or 19.5" wheels with 4 wheel disc brakes. Chassis styles came with either independent suspension or I-beam suspension. Independent suspension used front coil springs and could not support the GVW that an I-beam chassis could. I-beam chassis used leaf springs in the front and rear. Workhorse took over the manufacturing of the P-chassis when GM stopped making it in 1998. Navistar acquired Workhorse in 2005 and Sept of 2012 Navistar shut down Workhorse. In the 70's and 80's the 350 c.i. engine was the popular choice but diesel was available from 1982 for the 6.2L engines and from 1995 for the 6.5L engines.
Freightliner has three chassis' used for the walk-in vans; the MT35, MT45 and MT55. The MT35 was the first Chassis to replace the Oshkosh chassis when Oshkosh became Freightliner Custom Chassis in 1995. The MT35 had a GVWR between 9,000 and 14,000 lbs. Soon the MT45 came out which gave it an increased GVWR up to 14,000 from 19,000 lbs. The MT55 chassis can have a maximum GVWR of 30,000 lbs. Today the MT45 is the popular choice for a stepvan with a payload capacity of 10,000 lbs. The new units use the 200 hp Cummins 6.7L diesel engine with the reliable Allison transmission. 19.5" wheels are mounted on chassis that stretch from 138" to a whopping 208". Optional fuel tanks come in 30, 40 or 60 gallon.
The Ford E-Series Chassis was the popular choice for stepvans before the year 2000. Even the E150 was used by some of the bakery trucks in the 70's and early 80's when a single rear wheel was enough and weight wasn't an issue. Before Morgan Olson changed their name from Grumman Olson the E350 and E450 chassis' were popular picks for those vehicles. Today most of the walk-in van type vehicles that use a Ford chassis use the F53 (very popular on motorhomes but many were used on stepvans as well) and the F59. The F53 has a GVWR of 22,000 lbs. and the F59 has a GVWR of 26,000 lbs. The standard factory gasoline engine today is a 6.8L 3-valve V10 for the F53 and the F59.