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TOPIC: AT1 upgrade to 175cc

AT1 upgrade to 175cc 14 Apr 2019 02:03 #1

  • Peakbiker
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You helped me with a previous problem with my bike I am hoping you can guide me on my current question.

I have a 1969 AT1E which has suffered a partial seizure due to autolube starvation. The cylinder bore is marked and there is also piston slap so a rebore is needed.

I have the chance to buy a good condition used CT1 cylinder, piston and cylinder head and cannot decide whether to rebore or upgrade to 175cc.

I have three questions you may be able to answer.

a) Apart from the above used parts and gaskets do I need anything else to carry out the conversion ?

b) I notice from the parts list the crankshaft assembly is not the same on the AT1 and CT1, will the greater capacity over load the AT1 crank?

c) What performance improvements can I expect for the 40% increase in capacity?

What route would you take if it was your bike?

I would appreciate guidance from anyone that has carried out this upgrade.

Many thanks in advance.

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AT1 upgrade to 175cc 14 Apr 2019 04:58 #2

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I'd guess if you're seeing a difference in cranks it's probably looking at an electric start 125 compared with a magneto 175. Here [NZ] we mostly had magneto 125's [which i have one off]. I have a 175 top end on my "70 125. Top end i used [because i had it] was from the remains of a TY175 trials bike [which is a treed inlet 175 & absolutely flies!!. Back about 30+ years ago i had another ride to work AT1 & fitted a CT1 top end. We used to get the redundant stock list every few months at work from the Yamaha importer & was a scramble to be 'first in'!!. I got am MX175 exhaust which was nothing like a CT1 pipe but i modified it to fit & it had strong top end & revved crazy high if need be. Just another bike i wish i'd never sold haha. The one i'm talking about though has been in the family since mid "70's & was a trade-in at work . There's a video up here somewhere of my rough old Rat Bike Running, just not sure how to find it. Think i've told much of this story recently so i'm sorry if i'm boring you if it was you i told it all to lol. Love my old bikes. Cheers.
Not sure why but i put Worked for the local Yam Dealer for 20 years & it doesn't show. Maybe because i had it bracketed except other bracketed bits earlier are showing i think--doesn't matter.
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Last Edit: by RT325.

AT1 upgrade to 175cc 14 Apr 2019 08:49 #3

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Good question. There is no difference is strength between AT and CT cranks. If you look closely at the parts lists, you'll see only the ignition half of the crankshaft is a different part. That's because of what RT325 said... the electric start/generator on your AT vs the flywheel magneto on the CT (and some "kick start only" AT's in different countries).

You'll also need the piston pin and piston pin bearing from the CT. As long as you're staying piston port it's a very easy upgrade. Exhaust pipe and even the carb and carb jetting is the same. The cylinder swap is a very easy, cheap, and reliable way to gain a few HP (from 11.5 HP to 15.6 HP).

And easily reversible if you even decide to go back to 100% original. Just re-install the 125 top end parts.

One caveat is that sometimes the electric start will struggle to start the bigger 175, especially when cold. (They kick start very easy.)

This upgrade has been done many times... searching the forum should find a lot of information. One of the best threads I remember is here: yamaha-enduros.com/index.php/forums/rest...t175-sleeper-project

(Mako's thread is way overkill... a lot more was done than just the top end upgrade... a lot of good info on various mods and upgrades)

.

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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
Last Edit: by MarkT. Reason: added piston pin

AT1 upgrade to 175cc 15 Apr 2019 03:42 #4

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Looks like the CT1 barrel is the answer.

I am glad you mentioned the little end bearing change it could have been a very awkward job without knowing this!

Thanks for the information.

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AT1 upgrade to 175cc 20 Apr 2019 19:05 #5

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I also was wondering about this, as I have the CT1 cylinder, piston and head to do this conversion as well. I knew the pin and bearing (and probably clips) would need to be replaced, but was hoping to get an answer about specific part numbers to look for. Hopefully this is helpful for anyone else that would like to take this on as well.
-Pic 1, Taken from the AT1 Parts list HERE
Indicates that part 8, 9 and 10 (x2) need to be swapped for the conversion.

Pic 2, Taken from the CT1C parts list HERE
Are the ones that should swap in.

Follow up- There are two parts numbers for #8 and #9 that come from CT1, which are the correct ones to match AT1, or is there no difference?

Thanks all,
Chris
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AT1 upgrade to 175cc 20 Apr 2019 20:26 #6

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Just to be clear there are no "conversion parts". You are simply installing a complete CT1 top end on your AT1. You use exact parts you'd use on a CT1.

Always replace the clips. Never reuse.

CT1 prefix is 251. So the pin with the part number starting with 251 is the pin you need.

Bearing is a little trickier... original CT1 bearing was 933-10216-32-00 which a website I found superseded to 93310-216E1-00. That's the same bearing as a later DT175 so 93310-216E1-00 is correct for the CT1 top end.
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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
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