TH350 to Dana 300 - 0.75" thick plus Rotation Plate

Benchmark Adapter

Kit Components

Kit #10-1009 -- $575.00 $460.00 ON SALE!

add 2 cart



Benchmark Adapter

Installed photo

Benchmark Adapter

Installed photo 2

Features which set our adapter apart from others are reflected in the quality of the machine work exhibited in the kit components. The rotation plate and adapter are machined from solid blocks of T6061 aluminum. Each component is 0.750" thick.

The adapter plate is milled to exact tolerances with large radius corners and a thick mounting foot. We do not suggest the use of the stock transmission support mounting face found on the bottom side of the transmission tailhousing. Using this stock transmission support mount creates substantial stress in the tailhousing of the TH350 by allowing the entire weight of the transfer case to be "cantilevered" from the four 3/8" bolts in the TH350 tailhousing. This is a common break point on TH350's and TH700R4's. Stripped threads and cracks in the tail section of the case are not uncommon. Our solution to this problem is to provide a strong mounting base machined into the overall part. This "foot" mount allows all of the vertical weight load of the transfer case to be transferred through the adapter, onto the rubber or urethane mount therefore removing the load from the rear of the transmission case.

Benchmark Adapter

Mounting Foot

The rotation plate supplied in the kit provides 4 optional mounting positions for the Dana 300. This feature allows for better ground clearance and the use of flat skid plates. Read more about the design benefits of this rotation plate here.

Benchmark Adapter

Photo of cutaway

Driveshaft clearance is excellent when using this kit. When using a Dana 300 with a TH350 or TH700R4, it is best to build the driveshaft with a smaller OD and a heavy wall thickness.

Benchmark Adapter

Driveshaft View 1

Benchmark Adapter

Driveshaft View 2

This kit also provides a new input gear for your Dana 300. Our new input gear has 27 splines cut to match perfectly with the GM TH350 output shaft. The new input gear is machined with the optimum length to properly engage the GM output shaft.

Benchmark Adapter

New input gear

Spline Engagement:

This adapter kit is designed to be used with the short style output shaft (27 spline) found in the 4x4 TH350. JB Conversions, Inc. can provide this GM shaft if you do not have one. Our adapters are designed to be short in length however we do not sacrifice spline engagement in any case. In fact, our main goal is strength over length and as you can see in the photo, our new mainshaft allows for a full 1-1/2" of engaged spline.

Benchmark Adapter

Close up of cut-a-way section

FRICTION WELDING is a common practice used in manufacturing processes for "hybrid" mainshafts found in transfer case adapter kits. Low production costs are among the incentives for using the friction welding process. Our company does NOT use this process. We only produce new, 1-piece mainshafts with our adapter kits. Notice the results printed in the destructive testing section of this site. Our shaft yields a torsional strength value more than twice the amount found in a welded shaft.

Installation Tips:

This adapter is intended for use with a TH350 equipped with a GM 4x4 short output shaft (27 spline). If you do not have this GM shaft, we can supply one for minimal cost.

Benchmark Adapter

Back of TH350 with short GM shaft

The stock Dana 300 input gear must be replaced with the new input gear supplied in the kit. This is an extremely easy task to perform. View the instructions for this task here.

A small portion of the gusset shown in the photo must be removed to allow for clearance of the nut or bolt head. This modification does not weaken the case.

Benchmark Adapter

Lower bolt interference
(either bolts or studs can be used at these two locations - both are provided in the kit)

When using studs, the two nuts (not shown) must be held in place as the transfer case is slid thru the adapter. As you can see there is no room to install the nuts after the rotation plate and adpater plate are pulled together.

Benchmark Adapter

Gusset area marked for removal.

The two studs in the front face of the Dana 300 should be removed. A tight bolt clearance can be noted near the governor cap on the side of the TH700R4 and TH350. We supply a 3/8"x3/4" 12-point bolt for use at this point however you may choose to use a short stud and nut. If you choose to use the bolt provided in the kit, it will be necessary to tighten this bolt as the transfer case is pulled toward the adapter during mounting.

Benchmark Adapter

Short bolt in place.