Need 701 bored.. but to what size?

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Ensue156

Location
Portland, Or
“Need advise today” 701 62t needs to be bored due to pitted cylinder. Was advised to just go a step up but since it’s already getting bored what about going bigger? 725,735, etc??

Also have the opportunity to buy a clean 760 motor and sell mine to a friend for a $100 loss. Would I need bigger carbs/electronics?

Surf/rec ski, needs it to be reliable.
FS2, bpipe, head, lightened flywheel, msd, 38’s, etc.
 
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Storbeck

Location
Minnesota
Most people will say that the 62t cylinder can be bored up to 84mm, which would put you around 753cc's. There will be some performance increase from the bigger displacement, and also because with the bigger displacement you will have a higher compression ratio unless your head is modified to match. There are potential issues with the shape/fit of the top of the piston and the head if you do not modify the head, but some people do it and get away with it. If you have an ADA head the "701" domes are okay up to 83mm dome but won't work (or at least not reccomended by ADA) for the 84mm dome.

So the two strategies here are basically to go the minimum overbore you can and leave yourself the most room for future repairs, or go the max bore and get the most displacement, or somewhere in between. Depends on where you put the priority between a slight increase in power or not having to replace the cylinder in the future.

The 760 cylinder has different port timing. A lot of people say they are more oriented to higher rpm, I don't know how much different it is in reality performance wise, but I have physically measured the port timing and it is different, and it is higher on the exhaust port which is usually better for higher rpms. It's not a huge difference. Most people are comparing to a 61x cylinder, your 62t cylinder has higher port timing than a 61x cylinder, and the 760 will be a bit higher yet.

Also the 760 has different dome sizes and different compression ratio between the two cylinders, and if you use the ignition from the 760 it will have slightly different ignition curves for the two cylinders. It had to do with detonation in the rear cylinder supposedly caused by twisting of the crank under heavy load pushing a big couch watercraft.

The stock 760 has bigger carbs, (44mm) where the 62t 701 has 38's, I would imagine it would run alright with the 38's.

If you're dealing with a more or less stock motor and you just want it to run, I would be going the minimum overbore to clean it up, but it's your motor and your priorities.
 
E

Ensue156

Location
Portland, Or
Storbeck said:
Most people will say that the 62t cylinder can be bored up to 84mm, which would put you around 753cc's. There will be some performance increase from the bigger displacement, and also because with the bigger displacement you will have a higher compression ratio unless your head is modified to match. There are potential issues with the shape/fit of the top of the piston and the head if you do not modify the head, but some people do it and get away with it. If you have an ADA head the "701" domes are okay up to 83mm dome but won't work (or at least not reccomended by ADA) for the 84mm dome.

So the two strategies here are basically to go the minimum overbore you can and leave yourself the most room for future repairs, or go the max bore and get the most displacement, or somewhere in between. Depends on where you put the priority between a slight increase in power or not having to replace the cylinder in the future.

The 760 cylinder has different port timing. A lot of people say they are more oriented to higher rpm, I don't know how much different it is in reality performance wise, but I have physically measured the port timing and it is different, and it is higher on the exhaust port which is usually better for higher rpms. It's not a huge difference. Most people are comparing to a 61x cylinder, your 62t cylinder has higher port timing than a 61x cylinder, and the 760 will be a bit higher yet.

Also the 760 has different dome sizes and different compression ratio between the two cylinders, and if you use the ignition from the 760 it will have slightly different ignition curves for the two cylinders. It had to do with detonation in the rear cylinder supposedly caused by twisting of the crank under heavy load pushing a big couch watercraft.

The stock 760 has bigger carbs, (44mm) where the 62t 701 has 38's, I would imagine it would run alright with the 38's.

If you're dealing with a more or less stock motor and you just want it to run, I would be going the minimum overbore to clean it up, but it's your motor and your priorities.
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Thanks for all the info, from what I’ve read very accurate. I’m not trying to crank every once out of if this thing. Just figured if we are doing the work why not go 735 or something? If it’s only a small difference I’ll keep it mellow. If it wakes it up like the guy below says I’d like to bore it farther. JetManiac head btw, domes aren’t stamped. Shop is going to measure, assuming 35’s.
 
zzylstra

zzylstra

Location
Valley Springs, CA
Ensue156 said:
Didn’t you have that Rok with a 735? That was the motor I had in mind with this post. If you think it wasn’t a lot better I’ll stick with mild bore.
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My first was a 718, 1mm Overbore. My second Rok was a 735 2mm overbore but only because the cylinder was in need of it anyways. That was awhile ago when I asked the same questions. I have a much different mind set with these things now days lol
 
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Ensue156

Location
Portland, Or
zzylstra said:
My first was a 718, 1mm Overbore. My second Rok was a 735 2mm overbore but only because the cylinder was in need of it anyways. That was awhile ago when I asked the same questions. I have a much different mind set with these things now days lol
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Yeah I currently have your original mindset to bore it out big.... You’ve been there, I’ll take your advice and keep it minimum, thank you
 
bored&stroked

bored&stroked

Urban redneck
Location
AZ
The way I see it, used cylinders are cheap. Even going 1mm over each time you have 3 re-bores. Seems most people get several years per top end. That's 12-15years of life per cylinder. Seems worth it to me.
 
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