facebook1 youtube1 twitter1 instagram google plus linkedin1 pinterest1

logosmallNOTICE:  If you are not a free registered member of the site, you will not see the photos in the forum, and you won't be able to access our premium member content. Please consider joining our community!

Welcome, Guest
Username: Password: Remember me
  • Page:
  • 1

TOPIC: Piston to wall clearance on a 74 Dt360?

Piston to wall clearance on a 74 Dt360? 14 May 2019 11:20 #1

  • Zozr
  • Zozr's Avatar Topic Author
  • Offline
  • Novice
  • Novice
  • Posts: 47
  • Likes received: 10
What is the min/max clearance you engine experts out there would be comfortable running?

Please Log in or Create an account to join the conversation.

Piston to wall clearance on a 74 Dt360? 14 May 2019 14:51 #2

  • monel12
  • monel12's Avatar
  • Offline
  • Site Contributor
  • Site Contributor
  • Posts: 377
  • Likes received: 91
Contact Enduronut on this site, he will be able to give you all the info. You need and more.

Please Log in or Create an account to join the conversation.

1971 RT 1B
1971 RT 1B
1970 RT 1
1970 RT 1
1969 CT 1
1971 DT 1
1974 100MX
2016 BMW F700 GS
1982 Yamaha Towny
1971 CT 1
1975 DT400

Piston to wall clearance on a 74 Dt360? 14 May 2019 20:15 #3

  • RT325
  • RT325's Avatar
  • Offline
  • Collector
  • Collector
  • Posts: 3798
  • Likes received: 1214
My Yamy RT3 book says 0.045mm to 0.050mm clearance. 1.8 thou to 2 thou. Yamaha used to say 18mm up from the bottom of the skirt so probably about the top of the windows -- i'll go measure one,--ok top of windows is 30mm & 18 about halfway. Measuring as best i can with verniers, the skirt stays the same till just at top of windows then tapers of from there on to the top. Anyway there's enough skirt below the windows to measure, not like a YZ with the cutaway. She'll run ok at 5 though but rattle. Depends on what you're trying to achieve or just want to go riding. Ring land measure 14 thou undersize compared to bottom of skirt. That's measuring a 1mm oversize piston but i'd think difference between bottom & top would be the same on all sizes. This piston has the Dykes L shape top ring so i was measuring between top & second ring.
Now i've 'right confused' you & 'me' Deet or Eduronut , think it is, will be along to hopefully save the day & save me!!.

Please Log in or Create an account to join the conversation.

Piston to wall clearance on a 74 Dt360? 14 May 2019 23:49 #4

  • Zozr
  • Zozr's Avatar Topic Author
  • Offline
  • Novice
  • Novice
  • Posts: 47
  • Likes received: 10

RT325 wrote: My Yamy RT3 book says 0.045mm to 0.050mm clearance. 1.8 thou to 2 thou. Yamaha used to say 18mm up from the bottom of the skirt so probably about the top of the windows -- i'll go measure one,--ok top of windows is 30mm & 18 about halfway. Measuring as best i can with verniers, the skirt stays the same till just at top of windows then tapers of from there on to the top. Anyway there's enough skirt below the windows to measure, not like a YZ with the cutaway. She'll run ok at 5 though but rattle. Depends on what you're trying to achieve or just want to go riding. Ring land measure 14 thou undersize compared to bottom of skirt. That's measuring a 1mm oversize piston but i'd think difference between bottom & top would be the same on all sizes. This piston has the Dykes L shape top ring so i was measuring between top & second ring.
Now i've 'right confused' you & 'me' Deet or Eduronut , think it is, will be along to hopefully save the day & save me!!.


Thank you sir, that is great help, I was able to follow along just fine. Not concerned so much about getting every last hp out of it by running tight clearances. Just want it to run and not seize. I picked up a OS piston recently. That measurement point of 18mm up the skirt seems pretty standard compared to my 4 stroke "modern" bike experience, although pistons I've seen are usually tapered more aggressively, with a small "window" of a maximum measurement point for clearance spec readings. Interesting that the size is the same a good ways up.

I will aim for 2 thou at the minimum, if it ends up a little looser so be it!

Please Log in or Create an account to join the conversation.

Piston to wall clearance on a 74 Dt360? 15 May 2019 07:44 #5

  • MarkT
  • MarkT's Avatar
  • Offline
  • Site Contributor
  • Site Contributor
  • Posts: 5386
  • Likes received: 2650
The Yamaha factory service manual with the specs is in the Tech Library.

One of my pet peeves is shops that... way back when and even more common today... "add a little extra clearance so it won't seize because it's an air cooled two stroke"...

IMHO all you end up with is an engine that rattles prematurely, and depending how "loose" might even break a piston...

When the piston it can rock more and the rings won't seal as well. The rings do more than seal compression... they, especially the top ring, transfer the intense heat of combustion to the cylinder... so does the skirt... So with extra clearance you loose valuable piston cooling and on top of it get more "fire" blowing past the rings and heating the piston even more. Hotter piston = more expansion = more likely to have a piston seize or melt.

I used to desert race air cooled two strokes... Heavy loading in deep sand, high outside temperatures (often over 100 F), miles and miles of sustained full-throttle operation... the BEST, most reliable, engines always had the piston clearance set to what the engine (stock piston) or piston (aftermarket forged) manufacturer recommended.

If you want to avoid seizing, warm up the engine before running it hard and spend your time making sure the jetting and timing is good, run good oil and lots of it, and make sure that you don't have any air leaks by pressure and vacuum testing before running it.
The following user(s) Liked this Post: Devin, RT325, Snglsmkr

Please Log in or Create an account to join the conversation.

1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250

Piston to wall clearance on a 74 Dt360? 15 May 2019 12:25 #6

  • Zozr
  • Zozr's Avatar Topic Author
  • Offline
  • Novice
  • Novice
  • Posts: 47
  • Likes received: 10

MarkT wrote: The Yamaha factory service manual with the specs is in the Tech Library.

One of my pet peeves is shops that... way back when and even more common today... "add a little extra clearance so it won't seize because it's an air cooled two stroke"...

IMHO all you end up with is an engine that rattles prematurely, and depending how "loose" might even break a piston...

When the piston it can rock more and the rings won't seal as well. The rings do more than seal compression... they, especially the top ring, transfer the intense heat of combustion to the cylinder... so does the skirt... So with extra clearance you loose valuable piston cooling and on top of it get more "fire" blowing past the rings and heating the piston even more. Hotter piston = more expansion = more likely to have a piston seize or melt.

I used to desert race air cooled two strokes... Heavy loading in deep sand, high outside temperatures (often over 100 F), miles and miles of sustained full-throttle operation... the BEST, most reliable, engines always had the piston clearance set to what the engine (stock piston) or piston (aftermarket forged) manufacturer recommended.

If you want to avoid seizing, warm up the engine before running it hard and spend your time making sure the jetting and timing is good, run good oil and lots of it, and make sure that you don't have any air leaks by pressure and vacuum testing before running it.


Thank you for the advice, you make good points. I think it's a good call to trust what Yamaha says.

Please Log in or Create an account to join the conversation.

Last Edit: by Zozr.

Piston to wall clearance on a 74 Dt360? 15 May 2019 15:11 #7

  • RT325
  • RT325's Avatar
  • Offline
  • Collector
  • Collector
  • Posts: 3798
  • Likes received: 1214
Don't run it too lean on the needle, well, don't run it too lean anywhere but backing off to the needle from a full song run & they don't like 'lean'.

Please Log in or Create an account to join the conversation.

  • Page:
  • 1
Moderators: VinnieDEETyamadminDmanEnduronutMakotosunDevinJames Hart