Differential Sun gear pins ARAC8008 - way out of spec.

[lee82gx]

Howdy, recently I've had the pleasure of rebuilding yet another diff, this time the aluminium upgrade diff also suffered a backed out screw, so much so that it just jammed the whole drivetrain. It only costed me a days running and another chewed up pinion as the spur / center diff remained jammed against the diff mount.

I took the opportunity to swap out the diff pin ARAC8008 as suggested by elders of the community. To my surprise the pins felt thinner to the touch and prompted me to do a quick check - here is the original pin as spec'ed 2.25mm x 9.8mm. Only ~2.22mm. I have swapped out a few by now and every single one measures 2.21 to 2.22mm

Here is a replacement from a HSS rod 2.25mm sourced from the nether regions of China.

Thankfully the diff input cups DO ACCEPT a 2.25mm pin, ie its holes are larger.

Assuming equal material properties this geometry netts a ~5% higher bending stress for the thin pin. I am also pretty sure in professional engineering these days you will be hard pressed get a 2.22 drill bit when you order a 2.25mm one, because the tolerances are usually much tighter than that. It takes a special kind of cost cutting to actually save some weight or cost to even custom tool a 2.22 extrusion hole. Just wondering.

FYA, I've started using Loctite 290 on the center diff and motor mounts. Previously the blue loctite 242 keep backing out on me for some reason. I'm praying for just a 10 pack continuous run with this. The last few rounds its been just 1 pack and back to the bench, 24hrs curing of loctite really sucks.

 

[RCbuzz]

Nice observation! I have also theorized the same thing you are describing.

With the pin being slightly undersized it puts additional stress on the outdrive, pin, and sun gear... Probably adds unnecessary play as well.

Seems to make the mod even more worthwhile. BTW, don't forget to chamfer the edges of your pins a little, it will help keep wear down.

I am still using blue and green loctite. So far so good.

 

[lee82gx]

Nice observation! I have also theorized the same thing you are describing.

With the pin being slightly undersized it puts additional stress on the outdrive, pin, and sun gear... Probably adds unnecessary play as well.

Seems to make the mod even more worthwhile. BTW, don't forget to chamfer the edges of your pins a little, it will help keep wear down.

I am still using blue and green loctite. So far so good.

I've recently switched to green loctite (actually genuine loctite is hard to come by in my region) so I'm using a different brand, at least not a knock off. Based on some enforced reading, seems like blue loctite is not recommended for small screws by Henkel. Not 100% sure the reason, could be that the gaps are too small for it fill? But if we apply excessively there is nothing uncovered anyway.

If you follow the test in the video above, perhaps we should all move to red for motor mounts, pinion grubs and pray for no more disassembly afterwards haha. I also can concur based on some very rough testing that the blue loctite does not hold up well for M3 screws when subjected to a lot of vibration.

 

[C Breeze]

Here's a sun gear pin out of a blown v4 center diff; seems like these pins have been out of spec for quite a while. Interesting info; I wonder if this explains why these pins are usually stuck in the sun gears? I assume the slop lets them get wedged in there. Now I'm motivated to try the pin mod.

 

[ARA8611V5]

I've recently switched to green loctite (actually genuine loctite is hard to come by in my region) so I'm using a different brand, at least not a knock off. Based on some enforced reading, seems like blue loctite is not recommended for small screws by Henkel. Not 100% sure the reason, could be that the gaps are too small for it fill? But if we apply excessively there is nothing uncovered anyway.

If you follow the test in the video above, perhaps we should all move to red for motor mounts, pinion grubs and pray for no more disassembly afterwards haha. I also can concur based on some very rough testing that the blue loctite does not hold up well for M3 screws when subjected to a lot of vibration.

I use red loctite on motor mount screws and pinion grub and it has worked well for me. I usually replace the motor mount screws every time I have to remove them. The shallow heads tend to strip.

 

[slick2500]

Welcome to the world of cheaply made Chinese ready to run rc cars. I am sure you will find that all of the pins throughout the entire car are going to be slightly undersized and/or vary in dimensions.

 

[lee82gx]

I use red loctite on motor mount screws and pinion grub and it has worked well for me. I usually replace the motor mount screws every time I have to remove them. The shallow heads tend to strip.

Is the red loctite actually removable? I’ve suffered an incredible difficulty to remove the factory screw, requiring torches and solvents and eventually screw extractor kit was even offered up as sacrifice to get it out. I’ve since replaced it with a socket head cap screw and a split spring washer, even though it is a bit too big headed.

as I said in the previous post i have also applied a generous amount of green loctite equivalent.

 

[Xjeepguy]

Is the red loctite actually removable? I’ve suffered an incredible difficulty to remove the factory screw, requiring torches and solvents and eventually screw extractor kit was even offered up as sacrifice to get it out. I’ve since replaced it with a socket head cap screw and a split spring washer, even though it is a bit too big headed.

as I said in the previous post i have also applied a generous amount of green loctite equivalent.

I heat the head of the screw with the tip of my soldering iron to melt the loctite. Then it comes right out.

 

[LibertyMKiii]

Is the red loctite actually removable? I’ve suffered an incredible difficulty to remove the factory screw, requiring torches and solvents and eventually screw extractor kit was even offered up as sacrifice to get it out. I’ve since replaced it with a socket head cap screw and a split spring washer, even though it is a bit too big headed.

as I said in the previous post i have also applied a generous amount of green loctite equivalent.

A quality hex wrench like MIP makes a world of difference. I was stripping hexes all the time untill I got a set of MIP hex tools.

Beyond that I just heat till I see it let out smoke. I exclusively use red loctite on everything.

 

[MrTitanium]

This post was most informative. However.

These “toys” may be designed in the US, but the components manufactured are certainly not. Overseas manufactures do NOT have the same manufacturing standards (if any) that we in North America have and follow. It’s a free for all. North American companies buy their crap by the ton and pass it onto us, the consumers. We all know this. One exception would be Mugen Seiki.

Those Arrma dowel pins are garbage. Buy Mugen pins or buy drill rod from reputable company, cut to length, chamfer, and install. But wait…. It doesn’t end there cause those gears are also made by an 8 year old Asian boy working for 3 grains of rice at the end of the day. It never ends.

 

[slick2500]

This post was most informative. However.

These “toys” may be designed in the US, but the components manufactured are certainly not. Overseas manufactures do NOT have the same manufacturing standards (if any) that we in North America have and follow. It’s a free for all. North American companies buy their crap by the ton and pass it onto us, the consumers. We all know this. One exception would be Mugen Seiki.

Those Arrma dowel pins are garbage. Buy Mugen pins or buy drill rod from reputable company, cut to length, chamfer, and install. But wait…. It doesn’t end there cause those gears are also made by an 8 year old Asian boy working for 3 grains of rice at the end of the day. It never ends.

IIRC Mugen stuff is made in Japan.

 

[MrTitanium]

IIRC Mugen stuff is made in Japan.

Precisely. Japan is way ahead of us all when it comes to quality. Their work ethic alone is most impressive.

Anyhow... so much for my rant.

 

[slick2500]

Xray also makes all their own stuff in house in Slovakia of all places but it is regarded as some of the highest quality on the market.

 

[lee82gx]

This post was most informative. However.

These “toys” may be designed in the US, but the components manufactured are certainly not. Overseas manufactures do NOT have the same manufacturing standards (if any) that we in North America have and follow. It’s a free for all. North American companies buy their crap by the ton and pass it onto us, the consumers. We all know this. One exception would be Mugen Seiki.

Those Arrma dowel pins are garbage. Buy Mugen pins or buy drill rod from reputable company, cut to length, chamfer, and install. But wait…. It doesn’t end there cause those gears are also made by an 8 year old Asian boy working for 3 grains of rice at the end of the day. It never ends.

An important part of design work is to check and test and repeat until the designer is satisfied there is not much to do anymore. A designer worthy of his salt checks and maintains the life of the product throughout the life cycle.

I too can “design” something in North America and send it for mass production in a 3rd world country. But to assume everything will come in spec is a criminal act of negligence or they are just simply within a very wide spec on purpose.