hello hello! it's been a hot minute, yet again! Life is busy, getting married in 3 days, but the DT fire is still burning hot! My own projects have escalated to sourcing modern DT175 bottom ends from Australia, but more on that later!

My friend recently money shifted his DT125 into 1st gear right as it developed an air leak when he was riding home (high speed high rpm situation), and the previous owner never put the clutch pushrod ball back in, SO, some things may or may not have friction welded together. 

In any case, I've offered to help him as he's a fairly new rider and wrencher and I don't want to see him get discouraged with the vintage bike life.
We decided we'll toss a spare 175 top end that I have on there while we're at it.
The cases are getting split anyway to replace the main shaft, so might as well button her back up with some OEM+ upgrades

My main question is:

Is it necessary to swap over to the 6 plate clutch that came on the 175s, or is the 5 plate clutch that the 125s came with sufficient? He loves that bike and his riding reflects that...if you know what I mean   no judgement on my end, I love ripping my 175, but I've just never dealt with a 5 plate clutch on one. 

Appreciate any insight! 

Congratulation.
First thing to learn, is ask your wife before disapearing into the night to fix your mates bike.
Or better still, ask her nicely to bring down some nice baking & a cuppa to the Man Shed.
Happy wife happy life--i'm on wife two from about 35 years ago haha, so i'm a slow learner.
Anyway--long story going nowhere. Re-the clutch i suppose if the 175 6 plate clutch fits up then why not use it.
But if there's a problem then 175 clutch you'll have to try.
Ok reading again 175 on a 125!! , so just use it as it is now, 125 clutch.
It'll be ok unless the plates are glazed up from semi slipping previously.
If it wasn't gripping good before then it'll only be worse.
Have exactly that issue with my 175 on a 125 but was boarderline slipping as a 125 so really let go as a 175.
But funny thing is i just beat it up going hard around our track & the damn thing came right.
It's sat so long since that day it's probabaly forgotten that it came good & will slip again next time.

haha! She did give me a weird look when I told her I was heading out to rescue a DT. But more so because we both know I should be resting and not doing this right now haha! I'm not working on it...but I can gather knowledge!!

Sounds good, I think we will just rebuild his 5 plate clutch then. Will be cheaper for him than having to buy a 175 clutch basket on top of a rebuild. I just didn't want to rebuild the 5 plate just to have the 175 be a little too torquey for it.

Now as I'm typing this all out...I referenced some part numbers...and the clutch baskets are actually the same, it's just the difference in pressure plates, which I did notice visually when I took off the clutch cover for a quick preliminary inspection. So really to run 6 plates you just need the 175 pressure plate. Nice to know that if he does start finding the need for that, it's an easy job.

"But funny thing is i just beat it up going hard around our track & the damn thing came right."
It's truly funny when they do that haha. I've got a few machines that have good and bad days, definitely some kind of multiple personality disorder going on with those things hehe.

Thanks for the reply!!

If i pick a 125 then 175 off partzilla the both have different numbers for the hub or clutch boss as they call it. So wonder if they run a shorter hub for less plates but use the same basket. In saying that i now see the baskets are different numbers but might be number of teeth. Says 71 teeth on 175 but can't see teeth number on 125 on partzilla.

If your friend has an early monoshock (1978 or even 1977 outside the US), Yamaha experimented with using rounded pushrods with no ball starting around 1976...  saying this because not sure the adjustment would be enough to make up for a missing ball?   And yes, friction welding was a problem.  Modern parts lists only show the updated parts that used the ball.

Early 125/175's had the same clutch.  Monoshocks didn't.  You can always update/upgrade if there's an issue.  (I converted the clutch on the one I did for a friend, not sure if it was necessary but I had the parts)

Don't forget to use the wider small end rod bearing

Have fun! 

Ah! Was late when I was looking this up. I see now. The pressure plate and the clutch boss are different, the primary gear/basket is the same as the part number for that references both bikes. It's a 1981 btw!