Subscribe to the Free

Automatic Four Fun

Swapping A TH700-R4 Into Your Weekend Cruiser

Step By Step

The early A-bodies came with a variety of transmissions, which is evident by the abundant crossmember mounting holes. Because of this, Moore was able to retain the use of his stock crossmember with minimal modifications. Moore went on to explain that it’s best to mount the crossmember onto the tranny first and then see where it lines up on the chassis. In Moore’s case, switching from a Powerglide to the TH700-R4 allowed him to use one factory hole, but he had to drill out the second one (arrow).

The Hooker Super Comps tuck closer to the chassis and necessitated the crossmember’s exhaust humps (arrow) to be heated and bent to get the additional clearance needed.

The TH700-R4 is slightly wider on the passenger side of the trans-tunnel, but a quick strike to the trans-tunnel with a hammer cured that.

With the tranny in place and the factory column shifter connected, Moore found he couldn’t engage First or Second gear. The fix came by cutting the engagement plate on the shifter assembly above the hole until the shorter throw would work. In the end he found that trimming 1/2 inch off the plate would correct it. Then he rewelded the arm back together.

The final step is to take measurements for either a custom driveshaft or to get your stock shaft shortened.

Not everyone's idea of fun is manually rowing the gears down the dragstrip, especially when you take into consideration the short amount of time it truly takes to cover the quarter-mile. So while some may consider that statement blasphemy, the fact of the matter is that there are many of us who would much rather cruise on the open interstates with an assertive right foot.

However, in order to take advantage of the upper speed limit flights, the handicapped automatic three-speed tranny should be replaced with a more cruiser-friendly three-speed automatic with an overdrive--namely GM's TH700-R4.

The TH700-R4 features a 0.70:1 final ratio over the 1.00:1 ratio TH350 and TH400 counterparts. And it's not surprising to find that those who have done the conversion have quickly grown to appreciate all the advantages of an overdrive. Overdrive not only delivers better gas mileage and turns lower-rpm highway speeds, but it also does away with the annoying upper-rpm exhaust drone. If that doesn't convince you, think of it this way--you could run a fairly steep 3.73:1 for that blip of the throttle-seat press and then the overdrive would take the gear down to an effective 2.61:1 on the top end.

Now rather than bore you with chart after chart of schematics, we convinced CHP buddy Tim Moore to place his ultra-clean '67 Chevelle onto the operating table and let us poke and probe into the much sought-after specifics. So if this sounds like something you've been pondering, then follow along as we explain how simple it is to shoehorn a TH700-R4 into your very own muscle machine.

Quick Tips

We know that many of you will attempt this conversion onto body styles other than a Chevelle. So if you're looking to add a TH700-R4 into an early Camaro, Moore's advice is to use a TH400 crossmember and employ many of the steps already given.

If you happen to be jumping on the '78-and-later A/G-body bandwagon, G-Force Performance in Remsenburg, New York, sells custom-built crossmembers to easily stuff the TH700-R4 with a true dual-exhaust system without any surprises. The cost is moderate at only $189. The company can also supply a crossmember for a TH200-4R with a special frame extension for the '78 through '83 platforms (since the tranny wasn't introduced until the '84 chassis) for $219. The options are there--it's just up to you to determine what you want.

TH200-4R vs. TH700-R4

Those of you familiar with the TH200-4R may be asking what's the big difference? The obvious is the steeper First gear in the TH700-R4's 3.06:1 ratio versus the TH200-4R's 2.74:1 and a slight difference on the final drive with the TH700-R4 using a 0.70:1 gear over the TH200-R4's 0.67:1 gear. Other than that, many will argue that the TH200-4R is stronger and more than capable of withstanding a decent dose of horsepower, citing as proof its use in the Buick Grand Nationals and T-Types. However, most neglect to mention that while TH200-4Rs are still plenty abundant, the more desirable and stronger TH200-4Rs were only in production from '86 through '88. Then there are those who praise the TH700-R4s and proclaim that the internals are larger and added strength goes along with its size.

Who's right or who's wrong is entirely debatable. Both transmissions are strong and only limited by the amount of resources that go into them. Is it better to use one over the other? That's ultimately up to you. What we can tell you is that there are several manufacturers who specialize in either one or both. What we do appreciate about the TH200-R4 is the fact that it uses the same standard flexplate, the same 27-spline slip-yoke, and virtually identical dimensions as the Powerglide and TH350. The difference lies in the location of the trans mount. The TH200-R4 sits about 6 inches farther back than a TH350 would, but there are options to fix this. You can either use a TH400 crossmember that's relatively close to fitting, or you can use the TH350 crossmember by cutting off the mount and welding it to the opposite side of the crossmember.

Driveshaft Science

Trying to figure out the correct driveshaft geometry can be somewhat intimidating, especially since it's usually a one-time deal to get the proper measurements. Because we wanted to get this correct the first time, we contacted Denny's Driveshafts in Kenmore, New York, to help us determine our needs. Denny's directed us to its Web site (www.dennysdrive where we found a series of questions regarding specific measurements. These included the dimensions from the end of the trans case to the flat surface area on the pinion yoke, the width of the U-joint, and how far the output shaft sticks out of the trans case.

Our particular example called for a 57-inch driveshaft, which is 3 inches shorter than the original one at 60 inches. But just don't go by our measurements as each combination is unique and will vary slightly. If you're still confused, be sure to call Denny's Driveshafts to help clear up any questions.


Rochester, NY 14625
G-Force Performance
Remsenburg, NY 11960
Denny’s Driveshafts

Connect With Us

Get Latest News and Articles. Newsletter Sign Up

sponsored links

subscribe to the magazine

get digital get print