Ask Away! with Jeff Smith: Advice on Putting a 4L60E Behind a Chevy Inline 6 Engine

I’m installing a 1995 4L60E into my 1965 Chevelle that has an inline Chevy 230ci six cylinder engine. I want the 4L60E for the overdrive and versatility. The car has a Powerglide now and I just want to know if I can use the same flexplate to bolt up the converter? I think it should work, but if you could verify that I would appreciate it greatly.

W.D.

Before we get started into the technical side, this trans swap is a great idea. The Powerglide uses a pretty tall 1.82:1 First gear ratio with high gear at 1:1. The 4L60E uses the same First gear ratio as the older 700-R4 version which is a 3.06:1 ratio, and that’s dramatically better than the Powerglide. So acceleration from a dead stop will feel much stronger in First gear just because of this ratio improvement. It will make your little six feel almost like a V8! Plus you’ll have the overdrive feature at 0.70:1 in fourth gear. So the 4L60 is a great idea.

I’m not that conversant on inline Chevy sixes but I looked up a second generation Camaro application for an inline six flexplate and a 307 small block Chevy, and discovered they use the same flexplate part number—so that means the inline Chevy six crank bolt pattern is the same as a standard small block Chevy with the older two-piece rear main seal.

There appears to be a couple of other issues that cropped up, so I contacted Tom Lowe at 12bolt.com. Besides building 12-bolt rear ends, he also specializes in Chevy six cylinder engines with a catalog of custom parts. He reinforced that the small block flexplate will work and also dialed us in on converting to the larger diameter flexplate that will be necessary.

The issue will be the converter bolt pattern. The standard small block Chevy converter bolt pattern for a TH350 or Powerglide measures 10.750 inches across, while the converter pattern on the small block Chevy style 4L60E is larger at roughly 11.50 inches.

That means the 4L60E converter won’t line up to the holes in your flexplate.

This larger torque converter pattern will also require a larger 168 tooth flexplate to accommodate the pattern. As we mentioned above, the 230ci and 250ci Chevy six cylinders all use the same crankshaft bolt pattern as the original small block Chevy with the two-piece rear main seal. So this means you can bolt the larger diameter flexplate.

But this larger diameter flexplate will require a matching starter motor. But Chevrolet interchangeability is again in your favor. Your inline Chevy six uses the same small, 153 tooth flexplate starter as the small block Chevy. But when you change to the larger flexplate, this means you need to also change starter motors because the bolt pattern on the starter changes from the straight across pattern for the small flexplate to the offset bolt pattern used on starters for the large diameter flexplates.

Unfortunately, the Chevy six is not drilled for the larger starter motor pattern. But what you can do, is see if you can find a starter nosepiece that has both bolt patterns. We’ve seen this pattern on some aftermarket starter motors. If so, you can use the two straight across bolt to bolt it to the block and then use the third bolt hole as a drill guide to accurately drill the offset starter pattern hole.

If you can’t find one of these starter nose pieces, you will need to accurately locate the offset starter nose on the block and use it to establish the proper position for the offset bolt to drill and tap the new mounting hole in the block. This can be done by establishing the correct distance between the starter gear drive and the flexplate.

With that accomplished, you will need the proper flexplate. We’ve listed a couple of examples in the parts list. The application is for a two-piece rear main seal small block Chevy. What you are looking for is the torque converter mounting location dimension. Many aftermarket flexplates place two patterns on one flexplate. The smaller pattern measures 10-3/4 inches while the larger pattern for the 4L60E measures roughly 11-1/2 inches.

Next, you need to mount the flexplate on the engine and check to make sure it is flat and does not exhibit runout either from the radius or front to back. This is where you might consider spending a little more money to buy a good flexplate, as the budget versions will often suffer from poor quality control that can be exhibited as a wobble when bolted to the crankshaft.

That’s why it’s important to check this before the entire drivetrain is fully assembled.

The converter pad height dimension is another place to check for accuracy before final assembly. This pad height dimension is the distance from the bellhousing mounting flange on the engine to the torque converter mounting pad position. A Powerglide/TH350 transmission is the same converter pad height as the earlier 4L60E small block style transmission that you are using, which makes it easier.

But you should still check this. Measure the distance from the engine bellhousing flange to the converter mounting flange on the flexplate. This should be roughly 1.125 inches. Next fully seat the converter into the transmission and measure the distance from the transmission bellhousing flange to the torque converter mounting pad. This measurement should be roughly 1.00 inch or 1/8 inch (0.125 inch) less than the mounting pad measurement on the flexplate of 1.125 inch. This will produce a clearance between the converter and the flexplate of around 0.125 inch.

This is what you want. If there is no clearance between the converter and the flexplate mounting pad, this will damage either the transmission pump or the thrust bearing in the engine—or both!

So it’s critical that there be roughly 1/8 inch of clearance between the two. If there is more than 1/4 inch clearance, use three appropriate thickness washers between the pad and the converter. If the distance is greater than 0.250 inch, this will pull the converter out of the pump too far, which can also cause problems.

Once the transmission and converter are properly connected, you will also need some kind of controller since the transmission is fully electronically controlled. I’ve listed several aftermarket controllers in the Parts List below. The price and features vary wildly so it would be worthwhile to check into all of these controllers and investigate them fully before making your decision.

Hope this helps you with your upgrade.

GM Torque Converter to Bellhousing Spacing Chart

4L60E-to-Chevy Inline Six Parts List

• Summit Racing SFI-Approved Flexplate, 168 Tooth, Dual Pattern – SUM-G100SFI
• TCI Forged Flexplate, 168 Tooth, Dual Pattern – TCI-399273
• DIY AutoTune Microsquirt Controller – DIY-S0031
• Edelbrock TC Transmission Controller – EDL-36212
• Painless Perfect Torc Transmission Controller – PRF-66501         
• MSD Atomic Transmission Controller – MSD-2760