Silver Ghost
Member
- Joined
- Apr 17, 2013
Is a 1/16 to 1/8 gap between converter and flexplate too tight? The flexplate is a JW, the converter is a 9.5 PTC, the converter mounting pads are 1 inch from the trans face. Thanks.
Converter to Flexplate Gap
- Thread starter Silver Ghost
- Start date
Silver Ghost
Member
- Joined
- Apr 17, 2013
Thanks, perhaps I can stop pulling at what little grey hair I have left.
http://www.taperformance.com/PDF/Converter_Instructions.pdf 5/64-1/8 <<<Maybe THIS<<< Is for GM measurements only according to PATC So you maybe Exactly right guys.) Hughes converters shows 1/8-3/16 however.
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I have never seen a good explanation of what drives the requirement for the installed gap, and my guess is the numbers are a combination of calculations and experience. My thought is you have three things to consider, differential thermal expansion, centrifugal shorting of the converter, and fluid pressure. (Actually four, also need some room for thrust bearing movement.)
If the block, crank, transmission(even though the case is aluminum the small section that would impact this is likely negligible), and converter all changed temperature uniformally, then I would not expect thermal expansion to be a big concern, but consider the extreme case. If you took a cold engine and transmission (not saying anyone would do this), and loaded it up on a trans brake, you would put a lot of heat into the crankshaft and converter more quickly that the block and casing. The crank converter would expand more quickly than the block and the casings, taking up a certain amount of the installed clearance, the difference being the coefficient of expansion for the length from the thrust bearing to the end of the hub times the temperature difference between the crank/converter and the block/casing. This case would likely take up quite a bit of the gap. Every engine will see some transient case, just used the extreme for illustration.
The converter will likely shorten some amount as RPM increases due to centrifugal forces that want stretch the diameter, this would in turn shorten it in the axial direction. Have no idea how much.
The last thing I can think of is pressure causing the converter to to expand, not sure how much this will impact.
It would be interesting to hear the history on how the installation gap was/is established and how the use of the car also effects it.
If the block, crank, transmission(even though the case is aluminum the small section that would impact this is likely negligible), and converter all changed temperature uniformally, then I would not expect thermal expansion to be a big concern, but consider the extreme case. If you took a cold engine and transmission (not saying anyone would do this), and loaded it up on a trans brake, you would put a lot of heat into the crankshaft and converter more quickly that the block and casing. The crank converter would expand more quickly than the block and the casings, taking up a certain amount of the installed clearance, the difference being the coefficient of expansion for the length from the thrust bearing to the end of the hub times the temperature difference between the crank/converter and the block/casing. This case would likely take up quite a bit of the gap. Every engine will see some transient case, just used the extreme for illustration.
The converter will likely shorten some amount as RPM increases due to centrifugal forces that want stretch the diameter, this would in turn shorten it in the axial direction. Have no idea how much.
The last thing I can think of is pressure causing the converter to to expand, not sure how much this will impact.
It would be interesting to hear the history on how the installation gap was/is established and how the use of the car also effects it.
The only flex plate that I know of that is known to cause this problem is the JW flex plate. Even though this has been a problem,they continue to sell them and people continue to buy them. The most common damage will be that the converter snout will drive the transmission pump rotor into the back wall of the pump cavity. It will destroy the rotor and Back half of the pump body which will crack and loosen the stator support. The cause is the thickness of the flexplate. The solution is a different flex plate. The other problem that the thick JW flex plate has is that it is not flexible because of it's extra thickness. Many people in the turbo Buick community,including me,object to this. Another way to solve the problem would be to install washers between the bell housing and engine block. I would measure their thickness with calipers to make sure you have washers of equal thickness.
I've written this to make you aware of this problem,but if you have 1/8" clearance between the flex plate and converter when the converter is pushed all the way into the trans pump, you will be Okay. The problems I've seen were situations where there was no clearance and people put them together anyway.
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Silver Ghost
Member
- Joined
- Apr 17, 2013
I, too, wonder what governs the requirement for installed gap other than crankshaft endplay and thermal expansion. A stock flexplate bends away from the crankshaft flange by 3/16, a CAT plate moves outward 5/16, but a JW flexplate is perfectly flat and 3/16 thick. The JW ends up at the same position as the stock flexplate or 0.917 out from my stock block. The PTC converter I have is seated in the bellhousing by 1 inch, leaving a gap of 0.083 according to my math. If the converter mounting pads need to be trimmed for greater gap, this can be done, if necessary.
Wow TType I have a JW flexplate as well Over the years ive broken 2 rotors. I assumed i didn't have the correct washers in , the first time i used no washers. 2nd time i did ,,years ago. Then I put a billet rotor in,, haven't broken another but I just pulled a pump & had Mikestertwo check it out said it was scored bad & the rotor was no good...But like you said now I wonder if the flexplate could be the culprit.. I have mine pretty close to 3/16 gap now with very thin washers on the PTC converter pads. Maybe I'll be o.k tho it drives fine so far as I've gotten on it many times.. But I never considered the flexplate. I'll keep an eye out on that. THX for that.
If you use washers between the converter and flywheel,you're moving the converter closer to the back wall of the pump. I would remove the washers to maintain the greatest amount of clearance especially since,as Silver Ghost correctly stated, the JW plate is flat. With the stock and other plates,the plate is conical shaped and not flat so you want to use some washers so the body of the converter doesn't rub up against the plate and force the plate to move toward a flat position as you tighten the bolts. In that case,you have room for washers because of the thinner plate. Because of the pressure inside the converter,it is always trying to move forward away from the pump which keeps the crankshaft forward up against the thrust bearing as it is supposed to be. It's interesting that this problem never takes out the thrust bearing but ruins the pump.
What are the options and what is proven? I am currently running a JW, and it is very stiff. I don't have any problems changing it out if it is not a good piece, but do not want to choose something else blind.
As long as you have enough clearance,nothing will be harmed and you'll be fine.
