big bore vs overbore

[Waternut]

I pretty sure I just bought a cylinder advertised as a 62t 760 cylinder that is really just a 61x cylinder that's been bored out to 84mm. Wouldn't bother me too much but most people say that 84mm is the last bore on a 61x stock sleeve. So my question is, how can I be sure whether it's a big bore, 760 cylinder, or just something that's been hogged out?

 

[Matt_E]

The casting should say whether it's 61X or not.
62T cylinders have larger exhaust bolt holes and extra bosses at the top.

If it's a 61X and at 84mm, then the stock sleeve overhang at the bottom would be pretty thin.

 

[#ZERO]

You should ask questions before buying a big bore cylinder, including the sleeve outside diameter measurement along with many others. The stock 61X sleeve outside diameter is 88mm with an even thinner measurement at the bottom skirt area. Most big bore sleeves start at 90mm OD or larger and don't use the thinner skirts so they will sustain the cylinder bore contricity for use with larger piston sizes. If you need some 61X specific big bore sleeves for that cylinder casting, just let me know.

 

[Waternut]

You should ask questions before buying a big bore cylinder, including the sleeve outside diameter measurement along with many others. The stock 61X sleeve outside diameter is 88mm with an even thinner measurement at the bottom skirt area. Most big bore sleeves start at 90mm OD or larger and don't use the thinner skirts so they will sustain the cylinder bore contricity for use with larger piston sizes. If you need some 61X specific big bore sleeves for that cylinder casting, just let me know.

If it was ebay, I would've questioned the guy intensely but I figured people on here know what they're talking about and I can trust them a bit better. Unfortunately, that wasn't the case. I knew it wouldn't be a 61x resleeve but I was at least expecting a real 760 cylinder and not a 701 cylinder bored to 760. It was even sitting on a 64x case so without having a close up picture taken of the casting number, I'm just trusting the guy is telling the truth and it everything else seemed logical.

 

[Big Kahuna]

I have a 61x cylinder with 61x sleeves that have been punched out to 85mm and ran this way for years with no issues. I would not advise this, but it was ran like this for many years.

 

[Waternut]

Yeah I figured I might as well run it at this point but I guess it would be stupid to get it ported professionally which is what I was really hoping to do over the winter.

 

[McDog]

Maybe ported and resleeved. Or sold and buy something else. Or get hold of who sold it to you and see if they will make it right or take it back.

 

[Waternut]

Yeah I contacted him...no response yet.

 

[WFO Speedracer]

Personally I would rather have the 61x cylinder as for freestyle the porting would be better however either way you are dealing with very thin sleeves that are easily damaged, I have replace far more 760 cylinder liners due to damage in the transfer ports than any other cylinder, bottom line is you really need true big bore sleeves if you want it to last.

 

[SUPERJET-113]

So what exactly is marked on the cylinder casting? 62T?

There are 62T 701's that have the 5mm higher ex porting, but only are good to basically 84mm(like a 61X cyl).
Then there are the true 62T 760 cylinder that starts at 84mm and can go to 86mm.
Someone correct me if I'm wrong..

 

[McDog]

61x

 

[Waternut]

61x is actually what is marked on the cylinder.

 

[Matt_E]

Did you measure the bottom of the sleeves?

 

[SUPERJET-113]

I have a 61X sitting on the shelf that is to 84mm. I just measured the sleeve thickness at the skirt and its around .075" - .080"...

 

[Waternut]

Did you measure the bottom of the sleeves?

Yeah the bottom of the sleeve is at 88mm outer diameter. I'm working with the guy now. He seems like he wants to make it right.

 

[Freakrider]

So what exactly is marked on the cylinder casting? 62T?

There are 62T 701's that have the 5mm higher ex porting, but only are good to basically 84mm(like a 61X cyl).
Then there are the true 62T 760 cylinder that starts at 84mm and can go to 86mm.
Someone correct me if I'm wrong..

Max safe bore on a 760 is 85mm per several engine builders...some guys here have run up to 86mm and have gambled ok with it.

 

[JetManiac]

So what exactly is marked on the cylinder casting? 62T?

There are 62T 701's that have the 5mm higher ex porting, but only are good to basically 84mm(like a 61X cyl).
Then there are the true 62T 760 cylinder that starts at 84mm and can go to 86mm.
Someone correct me if I'm wrong..

I think you are wrong, the 62t casting is the same whether it is a 701 or 760, you can bore either to the same because the outer walls of the sleeves are the same diameter.

 

[rkm]

701 62t sleeves taper at the bottom of the sleeve. Same OD as 61x sleeves.

 

[#ZERO]

The 62T & 64X sleeves are around 1mm larger on the outside diameter than the 61X sleeves and the aluminum cylinder casting is 2mm thicker. You can quickly tell the 62T-701 sleeves from the groove around the bottom skirt area and the 64X-760 sleeves do not have these grooves which are around 1mm thicker at the bottom skirt area.