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The Vintage Japanese Motorcycle Owners Group: Discussion Forums

Vintage Japanese Motorcycle Owners Group :: View topic - Cold compression values, T20?


Cold compression values, T20?
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dorT500
Full Throttle
Full Throttle


Joined: Jul 10, 2008
Posts: 3186
Location: Galveston County, Texas

PostPosted: Thu Nov 09, 2017 2:25 pm    Post subject: Reply with quote

oooh kay....stock set up....T20...


http://www.oldjapanesebikes.com/mraxl_GT_Resource/manuals/t20_service/index.php click on engine ....page 29



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fodder
Weekend Warrior
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Joined: Aug 28, 2017
Posts: 46
Location: Near Harlow, England

PostPosted: Tue Nov 14, 2017 1:26 pm    Post subject: Reply with quote

The saga continues:

I picked up the rebored barrels today. The machinist has done a very good job, working to the clearances specified in the workshop manaul I gave him.

I now have the pistons in the barrels and ready to go back on. The new pistons are a great fit, and fitting them with the rings on had to be carefully done. Whereas the old ones hardly needed any effort and did move around a fair bit in the barrels. The difference is pretty noticeable.

I had reassembled the bottom end at the weekend, so I was going to put the rest together this evening. However, I have just discovered that the new pistons are a slightly different design to the old and my gudgeon pins are too long by a couple of mm, so I can’t get the circlips in.
I could grind the pins down a little but don’t want to ruin them by getting the ends off-square, so I will probably pop back to the machine shop.

Its a good job I'm committed to this rebuild!!

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Nick
Suzuki T20 1965
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kbts250m
Weekend Warrior
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Joined: Jul 20, 2017
Posts: 86

PostPosted: Wed Nov 15, 2017 6:02 am    Post subject: Reply with quote

You would be OK grinding the ends of the wrist pins. There are no side loads on them. They just float and off-square slightly would not be a problem. They are probably too hard to lathe turn so your shop will grind them anyhow. Whatever you do with them, there should be around 0.5mm (.020") to 1.0mm (.040") clearance (back and forth float) between them and the circlips.

Can you post some pics of your pistons, rings, and re-bored cylinders? How is the chamfering around the ports?

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Currently - 75 Suz TS250, 02 Kaw ZX-6R

Previously - 70 Suz T350, 71 Suz TS250, 72 Suz GT380, 72 Suz TS125, 72 Suz TS250, 75 Yam YZ360B, 72 Suz GT380 diy cafe racer, 80 Suz GS750, 73 Suz TS185, 82 Suz GS1100
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fodder
Weekend Warrior
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Joined: Aug 28, 2017
Posts: 46
Location: Near Harlow, England

PostPosted: Wed Nov 15, 2017 12:42 pm    Post subject: Reply with quote

A few pics attached. A side by side comparison shows a fair bit of difference between old and new. New has thinner walls, smaller cutouts (same height but less wide), looks different inside and is not as well finished.

I bought them from CMSNL in Holland,CMSNL and they are listed as genuine Suzuki parts. The packaging looked genuine and the pistons are stamped inside with the Suzuki S.

The chamfering round the ports looks OK to my untrained eye - the guy did it according to the instruction in the manual.



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Nick
Suzuki T20 1965
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kbts250m
Weekend Warrior
Weekend Warrior


Joined: Jul 20, 2017
Posts: 86

PostPosted: Thu Nov 16, 2017 8:05 am    Post subject: Reply with quote

I think I read somewhere that GT380 pistons could be used in T20s. If that's true maybe that's what you have. The thinner plated rings on the new pistons look like the ones in the GT380 I had.

That's excellent the machine shop chamfered your ports. Before you put the pistons and cylinders back on don't forget to double check your piston ring end gap like we discussed before.

Have you got new gaskets?

_________________
Currently - 75 Suz TS250, 02 Kaw ZX-6R

Previously - 70 Suz T350, 71 Suz TS250, 72 Suz GT380, 72 Suz TS125, 72 Suz TS250, 75 Yam YZ360B, 72 Suz GT380 diy cafe racer, 80 Suz GS750, 73 Suz TS185, 82 Suz GS1100
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fodder
Weekend Warrior
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Joined: Aug 28, 2017
Posts: 46
Location: Near Harlow, England

PostPosted: Fri Nov 17, 2017 8:06 am    Post subject: Reply with quote

I agree, they do look like they could be 380 pistons.

I checked the end gaps and they are fine. Yes, new gaskets will be fitted.

I have had the gudgeon pins machined down, they needed about 4mm off. The guy did them in the lathe and it was amazing to watch - the carbide cutter took spirals off the hardened steel pins as if they were just aluminium hardness. The ends are nicely chamfered, as they were originally.

I'll get it all back together on Sunday, hopefully.

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Nick
Suzuki T20 1965
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kbts250m
Weekend Warrior
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Joined: Jul 20, 2017
Posts: 86

PostPosted: Fri Nov 17, 2017 10:57 am    Post subject: Reply with quote

That sounds like you've got a good machine shop there. We turn and mill hard metals too here where I work. We sometimes use carbide and coated carbide tooling but also use a lot of ceramics for deep high speed cuts in very tough aerospace allows. It is amazing to watch!

Good job on all new gaskets and checking the ring end gaps. It sounds like you're on your way to having a nicely rebuilt engine now. If I can give you a few last hints before re-assembly.
1. Be sure all your parts are immaculately cleaned.
2. I always like to put a light coat of Gasgacinch... http://www.oldbritts.com/image/78_401001a.jpg
...or spray Permatex...
https://www.carshopinc.com/images/PEX/PEX80065.jpg
...on the paper gaskets and let them dry.
3. Oil everything liberally with 2-stroke oil.
4. A little extra 2-stroke in your FIRST tank of gas is good insurance. 2oz/gallon = 72/1 premix ratio.

I bet you're pretty excited to be finally wrapping this up after all this time. Good for you! Can't wait to hear about how it runs!!!!

_________________
Currently - 75 Suz TS250, 02 Kaw ZX-6R

Previously - 70 Suz T350, 71 Suz TS250, 72 Suz GT380, 72 Suz TS125, 72 Suz TS250, 75 Yam YZ360B, 72 Suz GT380 diy cafe racer, 80 Suz GS750, 73 Suz TS185, 82 Suz GS1100
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