MK 10 tuning suggestions

[Candiru]

I am the original owner of a decent mk10 from 1992. It has been 2 years since I've had it apart. Prior to removal the IP locked up pretty quickly (couple seconds) to right between 125 and 130 on my gauge then slowly creeps to a max of 130 over a couple of minutes. I had in the -1 seat, no shims, and a 90 duro viton hp oring.

I am soliciting advice about which way to go with the rebuild. Ive got 4 + seats, 4 -II seats, 3 -1 seats, shims, aftermarket trident hp seat, and a NOS piston.

Im leaning towards reinstalling a -I seat with no shim and the new piston. My other choice would to put in the -II seat with no shim and hope the IP creep didnt get too high. I am somewhat hesitant to use the new piston since I don't think I would ever be able to find another and the current piston appears to have a good edge.

Thanks for your opinion.

 

[ams511]

What makes you think the piston is bad? What does the hard seat look like? Also is the creep to 130 or did you mean to say it creeps to something higher?

 

[halocline]

I am the original owner of a decent mk10 from 1992. It has been 2 years since I've had it apart. Prior to removal the IP locked up pretty quickly (couple seconds) to right between 125 and 130 on my gauge then slowly creeps to a max of 130 over a couple of minutes. I had in the -1 seat, no shims, and a 90 duro viton hp oring.

I am soliciting advice about which way to go with the rebuild. Ive got 4 + seats, 4 -II seats, 3 -1 seats, shims, aftermarket trident hp seat, and a NOS piston.

Im leaning towards reinstalling a -I seat with no shim and the new piston. My other choice would to put in the -II seat with no shim and hope the IP creep didnt get too high. I am somewhat hesitant to use the new piston since I don't think I would ever be able to find another and the current piston appears to have a good edge.

Thanks for your opinion.

I don't think the seat choice will affect the creep. To be honest a couple of PSI creep on a MK10 is nothing unusual and nothing to worry about. I'd try a + seat, no shims, and that should bring the IP up to 130-132 to start, maybe it'll creep a bit, maybe not. My experience is that with a new seat, they don't creep at all (once it's been cycled a few dozen times) and immediately after a dive they don't creep, but when I check one that's been sitting around, it creeps a bit just you described. It's annoying but not a problem in actual use. So maybe the first thing for you to is to try it on a dive, then check the IP shortly after.

You could try the new piston, but it would be better to carefully inspect the edge on yours with a jeweler's loupe. You'll see any imperfection in the edge. Save the NOS piston unless you find a nick or gouge, I bet you won't.

 

[Candiru]

Finished rebuild. Close inspection of piston showed no issues. After rebuild with original piston and plus seat the ip lock to 130 at 2000 psi and creeps to 137 after a few minutes.

 

[halocline]

Try tapping the purge lightly a few dozen times and then leaving it pressurized for several hours, like overnight. Then see if it still creeps. I'd be curious to know if this works for you, it has for me.

 

[Candiru]

I've cycled the 2nd stage over 100 times and left the system pressurized for two days. The Ip dropped a little and the creep is significantly slower. I'll use it as is until the next rebuild then switch out the piston and redress the edge.

 

[Fishpie]

Having to roughen up the piston shaft where the small o ring moves is a more common problem than dings in the knife edge.

 

[Candiru]

Please elaborate what you are fixing by roughing up the stem and the process you use. I have never heard of this before today.

 

[Fishpie]

The shaft can get worn smooth, and, rather like a combustion engine cylinder surface, can be re-honed.
Fine emery cloth works for me.
.......there are just too many double-entendres here for me to go into too much detail!

 

[Candiru]

I'm unclear how this will decrease ip creep.