Flexplate to converter gap

[phantom]

I have a gap of ca 7-8mm which i found to be a tad over 1/4" inches. (we use mm's in Norway) between the two when the converter is pushed all the way in. I have read on the old interwebs that play up to 1/4" should be ok, but i am a tad over. Thoughts?

[68CoronetRT]

Proper gap would be 1/8" to 3/16". You can space the flexplate to converter bolts if needed, I believe.

[phantom]

Thanks. Just for the record, there are no spacers or washers going in between the flexplate and engine? I just bolted it straight on 

[68CoronetRT]

Right, you would add a washer in between the flexplate and converter.

[John_Kunkel]

Nonsense. A 1/4" pull-up is fairly normal; the pull-up distance depends a lot on the shape of the converter bowl, the bowl usually contacts the front pump housing before the drive hub bottoms in the pump.

Besides, adding a spacer between the flexplate and the converter drive lugs will place the starter ring gear too far aft.

[BSB67]

1/2" is not uncommon.

Don't use spacers.

[GreenMachine]

What if there's no gap? I have a new lock-up 727 and new converter and when I mated the engine/trans, there was no clearance between the flexplate and converter. When I installed the converter, the splines felt tight and I had to work it in. But, the converter spun freely after installation, so I went with it. It drives fine, but I only have a couple of miles on it so far. I'm just a little worried there may be a chance of some damage when I put a heavy load on it.

[Kern Dog]

Then you are fine.
Pulling the converter forward enough to bolt to the flexplate will not disengage the converter from the pump drive.

[phantom]

Well, i added 1,2mm shims last night, now the gap is just short of 1/4". Should i take them out again?

[GreenMachine]

Then you are fine.
Pulling the converter forward enough to bolt to the flexplate will not disengage the converter from the pump drive.

What I'm worried about is the no clearance issue. After the engine/trans was mated, I have absolutely no fore/aft movement of the converter. That's making me wonder if the snout is butting up against the pump. And I've read that converters will balloon out when under a heavy load.


Sorry for the hijack phantom, I thought you're issue was solved.

[68CoronetRT]

I'm just going off what PTC told me. Luckily mine was 1/8" and was fine.

For the people saying to not use spacers, my buddy who I personally helped install the motor/trans/converter had to run a washer to get the spacing correct. About 100 passes so far, 1300 wheel HP and zero issues with the trans/converter. So dont be scared of spacers!

[John_Kunkel]

Well, i added 1,2mm shims last night, now the gap is just short of 1/4". Should i take them out again?

Yes.

[phantom]

Ok, thanks.

[PRH]

What I'm worried about is the no clearance issue. After the engine/trans was mated, I have absolutely no fore/aft movement of the converter. That's making me wonder if the snout is butting up against the pump. And I've read that converters will balloon out when under a heavy load.

I'd b worried about a crank thrust bearing wear issue with no free play between the converter and flex plate.

I wouldn't have put it together like that.

[John_Kunkel]

Sigh. As I tried to explain in my first reply, the converter pull-up distance relies on several factors but, mostly, on the shape of the bowl. When fully installed, the converter drive lugs should be at least 1/2" in from the face of the bellhousing. This should result in at least 1/8" pull-up since the rear face of the flexplate is about 3/8" from the mating surface of the block.

A  factory 12" converter can be installed so far back that the pump drive hub bottoms in the pump before the bowl contacts the pump housing...in this case there will be way more than 1/4" pull-up. Smaller converters with smaller bowls will usually contact the pump housing before the drive hub bottoms out; this is easy to check by rotating the converter when fully installed and listen/look for the scraping as the bowl contacts the pump housing. In extreme cases, the pump housing must be machined for added clearance. (see pic below)

The point is, some pull up is necessary; I would be concerned if the pull up is less than 1/8". If the pull-up is less than 1/8" it's possible that the converter drive lugs are extra thick to compensate for an engine mid-plate or the converter is manufactured incorrectly.

[BSB67]

When fully installed, the converter drive lugs should be at least 1/2" in from the face of the bellhousing. This should result in at least 1/8" pull-up since the rear face of the flexplate is about 3/8" from the mating surface of the block.