1972 T350: Need help on rebore

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GT500-2stroke
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1972 T350: Need help on rebore

Post by GT500-2stroke »

Recently bought a T350, which needs rebore of cylinder heads.

I am new to the topic of reboring, even though I just know it in theory a little bit about it.

- How much rebore should cost in my area ? ( San Francisco Bay Area ).
- Need help in finding rebore service providers. I prefer local but I am open to reasonably priced providers which are located far away from me and cylinders need to be shipped etc.
Cylinder heads have been taken off the bike currently.
- Topic of rebore came up because previous owner was into middle of it, when he decided to sell it for reason unknown to me.
He said that he had cylinder measurements taken by somebody, and that bike was smoking as the reasons for reboring.
He also said that measurements showed that cylinder heads need reboring.
As per him crank seals are not a cause for smoking.
- I called few local repair shops, but nobody is able to do rebore on this bike.

Appreciate any help on help..

Thanks in advance.
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Re: 1972 T350: Need help on rebore

Post by karl pa »

GT500-2stroke wrote:He said that he had cylinder measurements taken by somebody, and that bike was smoking as the reasons for reboring.
GT500-2stroke wrote:As per him crank seals are not a cause for smoking.
I would not put much faith in what he told you. Crank seals are one of the biggest reasons these engines smoke !!!
Two stroke engines are not like 4 stroke engines where when the piston gap opens it pulls oil from below the piston.
The T350 uses transmission oil to lube the center crankshaft bearing,there is crank seals on both sides of this,also one behind the primary gear, when these seals go bad the cylinders suck transmission oil in and burns it,causing excess smoke and it lowers the transmission oil level.
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Re: 1972 T350: Need help on rebore

Post by GT500-2stroke »

karl pa wrote:
GT500-2stroke wrote:He said that he had cylinder measurements taken by somebody, and that bike was smoking as the reasons for reboring.
GT500-2stroke wrote:As per him crank seals are not a cause for smoking.
I would not put much faith in what he told you. Crank seals are one of the biggest reasons these engines smoke !!!
Two stroke engines are not like 4 stroke engines where when the piston gap opens it pulls oil from below the piston.
The T350 uses transmission oil to lube the center crankshaft bearing,there is crank seals on both sides of this,also one behind the primary gear, when these seals go bad the cylinders suck transmission oil in and burns it,causing excess smoke and it lowers the transmission oil level.
What are my options?
- Forget about reboring, get the crank seals replaced?
- Do both reboring and crank seals?
- Third option?

In either case, I will still need to find a shop near me who can do this for me.
So far, I have not been able to find local shop which will work on vintage 2-strokes.
( San Francisco Bay Area )
Thanks.
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Re: 1972 T350: Need help on rebore

Post by karl pa »

GT500-2stroke wrote:So far, I have not been able to find local shop which will work on vintage 2-strokes.
( San Francisco Bay Area )
I am in the northeast,so I can't help you much there. In this area most cycle shops don't even want to talk to you if your bike is more than 15 years old,I got very lucky that the Suzuki dealer is 2 miles down the street and is old school,and loves the old bikes.

If the cylinders are off,you can insert a ring and measure ring gap at different spots in the cylinder to check for wear,a few of my bikes are out of spec on ring gap,but still run fairly good.If it is to far out of spec you will get piston knock,also depends on what type of riding you intend on doing.
You can drain transmission oil,then fill the crankcases with diesel fuel,if the diesel fuel level drops and drains out transmission drain,you know there is a problem with seals.
Another reason for these engines to smoke is the oil pump,incorrect adjustment,arm mounted backward.
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Re: 1972 T350: Need help on rebore

Post by Alan H »

Rebore cylinder heads? Where's me chair and popcorn? This I would like to see.
There isn't anything to bore or rebore on a cylinder head. You rebore the cylinder if it goes oval or out of spec.
Does it smell like the back end of a cow in that shop? I wouldn't be his customer.
Think of how stupid the average person is, then realise that half of them are more stupid than that.
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Re: 1972 T350: Need help on rebore

Post by dollydog »

rebore cylinder heads? hmm :D
talking of rebores, what size pistons do you have in at the moment? if they are standard, you'll probably be ok for a rebore. if they are +0.5mm then you've had it, as far as i'm aware. don't quote me on it, but i'm pretty sure you can only rebore the t350 cylinders once. take the advice on seals first though, because if the bottom end is no good you'll never get the top end right :?
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Re: 1972 T350: Need help on rebore

Post by karl pa »

dollydog wrote: i'm pretty sure you can only rebore the t350 cylinders once.
According to alpha sports,you are correct the T350 pistons are std and 0.5 over. Is this due to wall thickness or some other reason?
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Re: 1972 T350: Need help on rebore

Post by GT500-2stroke »

Alan H wrote:Rebore cylinder heads? Where's me chair and popcorn? This I would like to see.
There isn't anything to bore or rebore on a cylinder head. You rebore the cylinder if it goes oval or out of spec.
Does it smell like the back end of a cow in that shop? I wouldn't be his customer.
Sorry, my mistake.

I meant to say cylinder ( but I am so in-experienced that I did not realize that I typed "cylinder Head". :oops:
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Re: 1972 T350: Need help on rebore

Post by GT500-2stroke »

karl pa wrote:
GT500-2stroke wrote:So far, I have not been able to find local shop which will work on vintage 2-strokes.
( San Francisco Bay Area )
I am in the northeast,so I can't help you much there. In this area most cycle shops don't even want to talk to you if your bike is more than 15 years old,I got very lucky that the Suzuki dealer is 2 miles down the street and is old school,and loves the old bikes.

If the cylinders are off,you can insert a ring and measure ring gap at different spots in the cylinder to check for wear,a few of my bikes are out of spec on ring gap,but still run fairly good.If it is to far out of spec you will get piston knock,also depends on what type of riding you intend on doing.
You can drain transmission oil,then fill the crankcases with diesel fuel,if the diesel fuel level drops and drains out transmission drain,you know there is a problem with seals.
Another reason for these engines to smoke is the oil pump,incorrect adjustment,arm mounted backward.
I am beginning to like your plan of action, and will most likely go along these lines mostly.
I do not intend to ride it hard.
Just occasional pleasure rides at low power levels mostly..
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Re: 1972 T350: Need help on rebore

Post by GT500-2stroke »

karl pa wrote:
GT500-2stroke wrote:So far, I have not been able to find local shop which will work on vintage 2-strokes.
( San Francisco Bay Area )
I am in the northeast,so I can't help you much there. In this area most cycle shops don't even want to talk to you if your bike is more than 15 years old,I got very lucky that the Suzuki dealer is 2 miles down the street and is old school,and loves the old bikes.

If the cylinders are off,you can insert a ring and measure ring gap at different spots in the cylinder to check for wear,a few of my bikes are out of spec on ring gap,but still run fairly good.If it is to far out of spec you will get piston knock,also depends on what type of riding you intend on doing.
You can drain transmission oil,then fill the crankcases with diesel fuel,if the diesel fuel level drops and drains out transmission drain,you know there is a problem with seals.
Another reason for these engines to smoke is the oil pump,incorrect adjustment,arm mounted backward.
- What method should I use to measure ring gap?
I have a digital caliper set, that is all. Will it help? or will I need to purchase some special measuring shims/tool?
- Bike came with a service manual, I hope it will have ring gap tolerances ( I will check on that soon )
- I like your idea of checking condition os seals using diesel fuel. I will work along these lines next..

thanks.
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Re: 1972 T350: Need help on rebore

Post by karl pa »

GT500-2stroke wrote:- What method should I use to measure ring gap?
The way I do it is I insert a ring in cylinder, then using a piston without rings, I use the piston to push the ring in the cylinder to where I want to measure, using a feeler gauge I measure the gap between the ring ends, I measure the gap at the bottom the center and the top, if the cylinder is wore you will see different gaps. Another thing to look at is the ridge at the top of cylinder at the top of ring travel.
I use this method to check cylinder wear, you must remember used rings are most likely worn so ring gap might be more than new rings.
GT500-2stroke wrote:- I like your idea of checking condition of seals using diesel fuel
If the diesel level drops fast it is not good!
Another benefit of diesel fuel is it softens hard seals, so if your seals are not torn or cracked you might be ok.
My GT550 is burning transmission oil because of bad crank seal, so this winter I will remove cylinders and fill crankcases with diesel fuel and blue devil seal repair.
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Re: 1972 T350: Need help on rebore

Post by karl pa »

Most old school cycle shops I talk to aren't to concerned about cylinder wear on these old bikes,as long as it isn't to bad.
I think if it were my bike,I would check that the cylinders aren't wore to bad,check ring gap,possibly replace rings.If diesel level don't drop fast,let seals soak in diesel and seal repair for a week.Then put together and get it running and see how it is,the only thing you will lose is a couple of gaskets if you need to strip apart again.
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Re: 1972 T350: Need help on rebore

Post by Alan H »

karl pa wrote: My GT550 is burning transmission oil because of bad crank seal, so this winter I will remove cylinders and fill crankcases with diesel fuel and blue devil seal repair.
The crank seal between the transmission end of the crank and the points cam takeoff can be changed without stripping the engine completely. I need to do that on my 550A this winter.
Not sure that seal repair would stand the heat, revs and pressure in a 2T engine.
I have been wrong though - witness two divorces....... :roll:
Last edited by Alan H on Sun Nov 25, 2018 10:37 am, edited 1 time in total.
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Re: 1972 T350: Need help on rebore

Post by karl pa »

Alan H wrote:The crank seal between the transmission end of the crank and the points cam takeoff
Just beware the seal behind primary gear is obsolete on T250/350,so may be harder to find.
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Re: 1972 T350: Need help on rebore

Post by GT500-2stroke »

karl pa wrote:Most old school cycle shops I talk to aren't to concerned about cylinder wear on these old bikes,as long as it isn't to bad.
I think if it were my bike,I would check that the cylinders aren't wore to bad,check ring gap,possibly replace rings.If diesel level don't drop fast,let seals soak in diesel and seal repair for a week.Then put together and get it running and see how it is,the only thing you will lose is a couple of gaskets if you need to strip apart again.
Which seal repair should I use and where do I buy it from?

Also, How do I remove diesel fuel after soaking?
Till what level should I fill in diesel fuel in crankcases?

Thanks.
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