1985 YZ 125-N 58 mm Bore size

nicko-31
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nicko-31
Edited Date/Time 5/27/2016 6:06am
So I was putting my new top end back together after Bore to Max 58 mm and found that the piston hits cylinder head at TDC. I had a Athena gasket kit which is for that bike .I took head off and sure enough that's what's hitting. Does anyone know if because of 58mm bore size I need a specific 58mm head gasket ??http://www.ebay.com/itm/141979985762 . http://www.ebay.com/itm/141979988528
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MOgle80
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MOgle80 5/22/2016 10:42am
5/22/2016 10:42am
Yes I've had one or two YZ125 from that era, and when I bored the jug I used Cometic gasket specifically made with clearance for the larger piston.
nicko-31
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nicko-31 5/22/2016 11:04am
5/22/2016 11:04am
MOgle80 wrote:
Yes I've had one or two YZ125 from that era, and when I bored the jug I used Cometic gasket specifically made with clearance for the...
Yes I've had one or two YZ125 from that era, and when I bored the jug I used Cometic gasket specifically made with clearance for the larger piston.
Thank You ,I appreciate that.
Dick Trickle 5/23/2016 9:18am
5/23/2016 9:18am
I have two K's, almost identical motor to the N. One has a 56mm bore re-sleeved by Eric Gorr, and the other has a 58mm bore with Wiseco piston. I use the same gasket kit in both http://www.partzilla.com/parts/detail/aftermarket/XP-4142371.html and dont have any problems.
I had the cylinder, head and power valve on the 56mm cylinder all matched up by Eric Gorr. The 58mm cylinder came on one engine and to tell you the truth, i have never checked to see if the head was matched to it. However, i do know that when you bore these engines, you have to make sure you machine the power valve down to suit, otherwise it will catch the piston. I can tell you that i have been running both engines with the same kits and no problems. I fitted ProX rod kits to both engines.
See my engine rebuilds here.. http://yz125k.weebly.com/engine-and-transmission.html
nicko-31
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nicko-31 5/23/2016 1:40pm
5/23/2016 1:40pm
Thanks Dick, Yes Eric Ported this Motor and bored to 58mm.I was able to confirm that it doesn't come close to the power valve . I was able to get a hold of him "ONCE" to confirm that I need to run the Cometic 58mm bore head gasket." I hope " Apparently he forgot to tell me. He did say he would get them and ship to me but I hope to just get them myself and receive them sooner. I installed OEM rod and all lower end parts. I did however use a Wiesco piston kit though.

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The Sneak
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The Sneak 5/23/2016 7:41pm
5/23/2016 7:41pm
So...Ks rip but the L and N slugs right? Can you make an 84-5 run like a strong 82-83?
MOgle80
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MOgle80 5/23/2016 8:02pm
5/23/2016 8:02pm
Yes....take and put on a 83 cylinder and carb
nicko-31
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nicko-31 5/24/2016 8:52am
5/24/2016 8:52am
The Sneak wrote:
So...Ks rip but the L and N slugs right? Can you make an 84-5 run like a strong 82-83?
I don't know if a fully ported and decked cylinder head with proper jetting on a 34mm carberuator 84 or 85 would be any slower than an 83.There's a lot of porting done to the point of adding extea transfer ports.
nicko-31
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nicko-31 5/25/2016 5:00pm
5/25/2016 5:00pm Edited Date/Time 5/26/2016 6:28pm
In case anyone Is interested I received my Cometic gasket top end kit C7081 58mm and piston doesn't hit any more at TDC.
Dick Trickle 5/27/2016 4:46am
5/27/2016 4:46am
Thanks for that update nicko. I have added your link to my ebay watchlist. Its interesting, as i use the Athena kit and with my 58mm bore i have no trouble. Perhaps i will rip the head off and take a look, but i have swapped cylinders between both my K's and all good so far.
Cheers,
MOgle80
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MOgle80 5/27/2016 6:06am
5/27/2016 6:06am
nicko-31 wrote:
In case anyone Is interested I received my Cometic gasket top end kit C7081 58mm and piston doesn't hit any more at TDC.
Did your gasket come in a package that itemized the contents and describe the head gasket in a way that might be different that you notice ? Once I went online to get part numbers for Cometic and it was obvious that the oversized bore required a gasket that had been an option unlike a original yamaha oem head gasket. Maybe thats changed since then.
jhale588
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jhale588 12/18/2023 4:59pm
12/18/2023 4:59pm
MOgle80 wrote:
Yes....take and put on a 83 cylinder and carb

O cool, this will bolt up ok on the 85 model? I’m assuming cylinder and the head? Thanks

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