rmchrgr
Skate And Destroy
There always seem to be a lot of questions and general mystery surrounding the A999 variants of the 904 auto transmission. Most of the things people are concerned with are how do to identify one from a garden-variety 904, what vehicles did they come in, do they all have low gear sets, how do I know if I have a 5-clutch drum etc.
I have a genuine, factory A999, non-lockup unit that has all the parts that make it an A999 and differentiate it from a standard 904. I'm not an expert by any stretch but, I have at least confirmed the little amount of info that's out there in regards to these particular units. Just thought I'd share this info with some pics to help those trying to identify one out in the field.
If you're wondering what is unique or special about the A999, I can try to explain it a little here. They were/are considered (by the factory) as a 'heavy duty' 904. I don't know about them being heavier duty than anything else out there, but there is some truth to the legend. They have wider front drums to accommodate more clutch discs (5 as opposed to the standard 3) which equates to more holding power under load and hard shifting. Any 904 can be built to withstand power but it is difficult to piece them together specifically as an A999 based on finding and retrofitting factory parts into a regular 904. Not all parts interchange and there are a lot of variations over the years that make it really tough to cobble together. Of course there are many people out there that successfully run extra clutch discs in standard drums without issue but if you want to eliminate any questions or parts searching, the best place to start is with the '74-'78 non lockup units I am describing below.
One of the the main areas of mystery with these is that other than the "PK" # on the driver's side pan rail, there is no way to tell the difference between a standard unit and a 999 since all 904 cases are essentially the same. The only other external ID would be the input shaft but this would only determine whether the unit is lock up or not. Non lockup units are splined all the way to the end of the shaft while lockup units are not and have a machined step at the tip to fit a lockup converter.
There also tends to be some confusion in regards to low gear sets. Some builders favor a low gear set because it offers better torque multiplication off the line. Standard 904 low gear ratios are 2.45:1 while the '80-up low gear set is 2.74:1, an 11% increase. Stock stroke small blocks can benefit from a low gear set because of the tendency to have less torque in the lower RPM range. However, stroker small blocks provide greater torque so if you're putting a 904 behind a 408/416 etc., you probably don't need the low gear set. In any case, benefits are debatable after a certain point so choosing to use a low gear set should be based on application-specific needs. That being said, my main intention here is just to identify the parts, not to get into a discussion about drive line theory. You can start your own thread for that.
Low gear sets did not make an appearance in 904/999s until 1980 or so. My guess for this is that factory HP ratings for small blocks at that time were pretty low and the cars were getting heavier so they needed all the help they could get. The low gear sets only came in lockup units however so the '74-'78 non lockup units would not have had them unless someone swapped one in at a later time.
Low gear sets can be installed into older units but you need the sun shell that came with the low planetary gears to use them. The shells are specific and also require specific thrust washers so if you plan to put a low gear into an older unit, make sure to get the sun shell as well.
So there are three specific PK#s that I know of which designate factory, non lockup A999s. They are as follows; (taken from the Mopar Chassis Book)
'74-'75 3681844
'76-'77 4028465
'78 4028878
The one I have is the 4028465 from '75-'77. I have another in my Duster, don't remember which one off hand right now. Per the P.O., it has a low gear set in it but I have not had it apart so I can't say for sure how it was built. It's held up great to plenty of abuse so I can attest to how stout these things really are
Regardless of what I have, any one of the three particular units listed above would have been found behind 360-equipped passenger cars. Just about anything built after '78 would most likely be a lockup unit (which I am not concerned with here). The '78-up 999 is actually more plentiful but due to the lockup design is less desirable in a street/strip performance build. There may be other non lockup units out there with different numbers from different years/applications but I do not know what they are. Finding a newer 999 with the low gear set and lockup design would be harder to build into a non lockup unit because of the lockup parts not interchanging with the non LU.
That being said, original non lockup units are in fact pretty hard to find, especially with all the original guts. A little side story - a few years back I put a want ad in a couple of places asking for the proverbial haystack needle. Took a while to locate one, like over a year. Most guys who replied didn't know what they had and would offer me a regular 904 or a lockup unit. I had to reply to a few messages and make sure that the seller was knowledgeable about what they had but most were not. I'm not saying that everyone should have this minutiae at their fingertips so please don't take that to mean I thought guys were morons. There's just limited info about these particular units which most don't concern themselves with.
In any event, I waited it out and luckily member jamesdart sold me the one he had. I bought it on a spec based on the PK number he provided but I knew I would only be able to verify it was a 999 by taking it apart and looking at the hard parts. This was around 4-5 years ago. I finally disassembled it the other day and was glad to find all the correct pieces. Checks' in the mail, James. :salut:
Again, besides the PK # on the pan rail, there is no other way to identify one unless you take it apart and inspect the internals. You also need to do that with a certain amount of knowledge of what to look for. (hopefully I can try to provide some of that here).
So once you get it to where you can start to inspect stuff, here is what to look for;
Non lockup input shafts are splined all the way to the end of the shaft. A lockup input shaft is not and has a machined step at the tip to fit a lockup converter
For reference, here's a pic I found off the web comparing the different shaft types. Again, lockup is the one with the machined tip, non lockup is splined all the way to the end.
The A999 non lockup front pump is specific. It has a bevel on the back of the support to fit in with the wider 5 clutch drum. Here's the back of the pump. You can see the bevel at the base of the support where the tip of the pointer is. This fits with the wider 5 clutch front drum and does not interchange with other drums. You may be able to machine one down but I'm not sure exactly how to accomplish that.
Here's the 5 clutches and steels from the front drum. The rear drum is to the left and has 4 of each.
The 'wide' 5 clutch drum mates to a specific rear drum with a 1 1/8" splined hub. The regular 3 clutch drum hub is only 1" tall.
Here is a pic comparing the two different sun shells. The low gear sun shell is the one with the step in the bottom while standard gear set shells are smooth. So low gear on the left, standard gear set on the right. The standard one came out of my '75-'77 non lockup unit.
In addition to what I have here, there are some archived threads on Moparts about this subject but this one here is one I refer back to regularly. I have gleaned a lot of specific information from that thread. This was where I learned about the PK #s, the beveled front pump and the 1 1/8" splined hub.
There's also the [ame="http://www.mymopar.com/downloads/transguide.pdf"]My Mopar Torqueflite reference page[/ame]. It's probably easier to use it to know what not to look for instead of trying to find one of the three specific PK #s. Next time you're out hunting for one of these, bring a copy of the list so at least you'll know whether to even bother with potential finds.
Again, the 904 can be built all sorts of ways and can handle power but what I describe here is based on using the original, factory, non lockup recipe. I have no idea how many of these original units are still out there in unmolested form. I also don't know exactly which model cars they came in but based on the info provided, you should be able to narrow your potential donor search down to a fairly small group. Good luck finding one, or even finding someone who knows what it is!
Hope this helps anyone trying to either find or ID one they might have. Please take it for what it is though, mostly just my own observations and confirmation of info I have learned about through my own reading.
Greg
I have a genuine, factory A999, non-lockup unit that has all the parts that make it an A999 and differentiate it from a standard 904. I'm not an expert by any stretch but, I have at least confirmed the little amount of info that's out there in regards to these particular units. Just thought I'd share this info with some pics to help those trying to identify one out in the field.
If you're wondering what is unique or special about the A999, I can try to explain it a little here. They were/are considered (by the factory) as a 'heavy duty' 904. I don't know about them being heavier duty than anything else out there, but there is some truth to the legend. They have wider front drums to accommodate more clutch discs (5 as opposed to the standard 3) which equates to more holding power under load and hard shifting. Any 904 can be built to withstand power but it is difficult to piece them together specifically as an A999 based on finding and retrofitting factory parts into a regular 904. Not all parts interchange and there are a lot of variations over the years that make it really tough to cobble together. Of course there are many people out there that successfully run extra clutch discs in standard drums without issue but if you want to eliminate any questions or parts searching, the best place to start is with the '74-'78 non lockup units I am describing below.
One of the the main areas of mystery with these is that other than the "PK" # on the driver's side pan rail, there is no way to tell the difference between a standard unit and a 999 since all 904 cases are essentially the same. The only other external ID would be the input shaft but this would only determine whether the unit is lock up or not. Non lockup units are splined all the way to the end of the shaft while lockup units are not and have a machined step at the tip to fit a lockup converter.
There also tends to be some confusion in regards to low gear sets. Some builders favor a low gear set because it offers better torque multiplication off the line. Standard 904 low gear ratios are 2.45:1 while the '80-up low gear set is 2.74:1, an 11% increase. Stock stroke small blocks can benefit from a low gear set because of the tendency to have less torque in the lower RPM range. However, stroker small blocks provide greater torque so if you're putting a 904 behind a 408/416 etc., you probably don't need the low gear set. In any case, benefits are debatable after a certain point so choosing to use a low gear set should be based on application-specific needs. That being said, my main intention here is just to identify the parts, not to get into a discussion about drive line theory. You can start your own thread for that.
Low gear sets did not make an appearance in 904/999s until 1980 or so. My guess for this is that factory HP ratings for small blocks at that time were pretty low and the cars were getting heavier so they needed all the help they could get. The low gear sets only came in lockup units however so the '74-'78 non lockup units would not have had them unless someone swapped one in at a later time.
Low gear sets can be installed into older units but you need the sun shell that came with the low planetary gears to use them. The shells are specific and also require specific thrust washers so if you plan to put a low gear into an older unit, make sure to get the sun shell as well.
So there are three specific PK#s that I know of which designate factory, non lockup A999s. They are as follows; (taken from the Mopar Chassis Book)
'74-'75 3681844
'76-'77 4028465
'78 4028878
The one I have is the 4028465 from '75-'77. I have another in my Duster, don't remember which one off hand right now. Per the P.O., it has a low gear set in it but I have not had it apart so I can't say for sure how it was built. It's held up great to plenty of abuse so I can attest to how stout these things really are
Regardless of what I have, any one of the three particular units listed above would have been found behind 360-equipped passenger cars. Just about anything built after '78 would most likely be a lockup unit (which I am not concerned with here). The '78-up 999 is actually more plentiful but due to the lockup design is less desirable in a street/strip performance build. There may be other non lockup units out there with different numbers from different years/applications but I do not know what they are. Finding a newer 999 with the low gear set and lockup design would be harder to build into a non lockup unit because of the lockup parts not interchanging with the non LU.
That being said, original non lockup units are in fact pretty hard to find, especially with all the original guts. A little side story - a few years back I put a want ad in a couple of places asking for the proverbial haystack needle. Took a while to locate one, like over a year. Most guys who replied didn't know what they had and would offer me a regular 904 or a lockup unit. I had to reply to a few messages and make sure that the seller was knowledgeable about what they had but most were not. I'm not saying that everyone should have this minutiae at their fingertips so please don't take that to mean I thought guys were morons. There's just limited info about these particular units which most don't concern themselves with.
In any event, I waited it out and luckily member jamesdart sold me the one he had. I bought it on a spec based on the PK number he provided but I knew I would only be able to verify it was a 999 by taking it apart and looking at the hard parts. This was around 4-5 years ago. I finally disassembled it the other day and was glad to find all the correct pieces. Checks' in the mail, James. :salut:
Again, besides the PK # on the pan rail, there is no other way to identify one unless you take it apart and inspect the internals. You also need to do that with a certain amount of knowledge of what to look for. (hopefully I can try to provide some of that here).
So once you get it to where you can start to inspect stuff, here is what to look for;
Non lockup input shafts are splined all the way to the end of the shaft. A lockup input shaft is not and has a machined step at the tip to fit a lockup converter
For reference, here's a pic I found off the web comparing the different shaft types. Again, lockup is the one with the machined tip, non lockup is splined all the way to the end.
The A999 non lockup front pump is specific. It has a bevel on the back of the support to fit in with the wider 5 clutch drum. Here's the back of the pump. You can see the bevel at the base of the support where the tip of the pointer is. This fits with the wider 5 clutch front drum and does not interchange with other drums. You may be able to machine one down but I'm not sure exactly how to accomplish that.
Here's the 5 clutches and steels from the front drum. The rear drum is to the left and has 4 of each.
The 'wide' 5 clutch drum mates to a specific rear drum with a 1 1/8" splined hub. The regular 3 clutch drum hub is only 1" tall.
Here is a pic comparing the two different sun shells. The low gear sun shell is the one with the step in the bottom while standard gear set shells are smooth. So low gear on the left, standard gear set on the right. The standard one came out of my '75-'77 non lockup unit.
In addition to what I have here, there are some archived threads on Moparts about this subject but this one here is one I refer back to regularly. I have gleaned a lot of specific information from that thread. This was where I learned about the PK #s, the beveled front pump and the 1 1/8" splined hub.
There's also the [ame="http://www.mymopar.com/downloads/transguide.pdf"]My Mopar Torqueflite reference page[/ame]. It's probably easier to use it to know what not to look for instead of trying to find one of the three specific PK #s. Next time you're out hunting for one of these, bring a copy of the list so at least you'll know whether to even bother with potential finds.
Again, the 904 can be built all sorts of ways and can handle power but what I describe here is based on using the original, factory, non lockup recipe. I have no idea how many of these original units are still out there in unmolested form. I also don't know exactly which model cars they came in but based on the info provided, you should be able to narrow your potential donor search down to a fairly small group. Good luck finding one, or even finding someone who knows what it is!
Hope this helps anyone trying to either find or ID one they might have. Please take it for what it is though, mostly just my own observations and confirmation of info I have learned about through my own reading.
Greg
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