TH350 to NP231 - 0.75" thick

Benchmark Adapter

Kit Components

Kit #10-1006 -- $475.00

Select your NP231 transfer case year:



Features which set our adapter apart from others are reflected in the quality of the machine work exhibited in the kit components. The adapter plate is machined from a solid block of T6061 aluminum and measures a short 3/4" 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

Installed photo

This kit provides a new input gear for your NP231. 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. The adapter is equipped with an oil seal that contacts the new input gear in the OEM location on the ground surface. Notice where the seal makes contact in the photo below (arrow).

Benchmark Adapter

Close up of the 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.

SHIFT LINKAGE is another area where the Benchmark adapter design exceeds that of other adapters on the market. Machined mounting surfaces are machined into our billet adapter at four separate locations. These "bossed" areas provide excellent options for creating the optimum shifting system for your transfer case. The top mount found on our adapter works well with Lokar shifters.

Benchmark Adapter

Close up of the adapter mounting faces

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 it to you for minimal cost.

Benchmark Adapter

Back of TH350 with short GM shaft

The stock NP231 input gear must be interchanged with the new input gear we supply in the kit. This is an extremely easy task to perform. To get an understanding of how your NP231 is assembled, review the photos on our 231 SYE instructions found on this site.

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

Benchmark Adapter

Gusset area marked for removal

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

Lower nut interference

One stud on the stock NP231J must be removed or shortened to allow clearance for proper bolting. This stud location is found 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 shorten the stock stud and use the original 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 during mounting

Benchmark Adapter

Short bolt in place

The front bearing retainer on your NP231 will require modification as shown here. The three small aluminum bosses on the retainer will need to be clearanced slightly to engaged the adapter properly. We can supply a new, modified retainer for $15.00 if you desire.

Benchmark Adapter

Modified retainer