Yamaha 701 – Big Bore Kits

The introduction of the 1996 Yamaha 760 Raider and 760 Blaster II (both actually 753cc) has sparked much interest for additional displacement among existing 701cc Yamaha owners. While the ’96/’97 motors from Yamaha do have different crankcases, carbs, and cylinder… Many of the parts will be interchangeable with the older 701cc engines. Group K is offering 753cc – 780cc big bore upgrade kits for the existing 701 engines. However we will not be utilizing the new 753cc cylinder to do so. There are several mechanical, economical, and political reasons for this. The following information will explain these reasons, as well as the different cost breakdowns.

The Political Reasons For Boring and Sleeving? – In 1996, the IJSBA introduced the “Super Stock” racing classification in to all racing divisions. This new classification will basically be the old limited class with the addition of cylinder/case porting, decking, and pump blueprinting. While stroker cranks will not be permitted, big boring and sleeving the existing cylinder blocks up to 785cc will be allowed. Bolting the “oem” 760 cylinder on to an older 701 engine will not be legal because it is, technically, considered an “aftermarket” cylinder. (“only” oem cylinders for that crankcase and that model machine are considered legal)

This Super Stock class will be the premiere modified class for all endurance racing and all local level novice and expert closed course events (that means no modifieds). The current modifieds (with carbon hulls, stroker cranks, billet cylinders, total loss, etc.) will be raced only by pro racers at all events on the national tour. While the limited class will still exist for a while, it will eventually be phased out. In our opinion, this new Super Stock class is the best decision that the IJSBA has made in a long time. Local mod class racing had just gotten too damned expensive. It’s also important to understand that a machine built to conform with the Super Stock rules will usually be as competitive as any true “modified.”

Advantages of the new S/S class are as follows:

1. Inexpensive cost of building one. (especially when compared to the cost of current modifieds)
2. Inexpensive to maintain. (again, especially when compared to the cost of current modifieds)
3. Easier tech inspection for competitors and local promoters. (No cylinder removal required)
4. Wealthy local level racers will no longer be able to dominate with an “ex pro-tour” race boat.
5. Super Stock machines can also be reliable and manageable weekend recreational boats.
6. Most of the modifieds being raced at local events already conform to SS rules.
7. Current limiteds can easily be upgraded to Super Stock.
8. Super Stocks will be quiet (Stock waterboxes are required).
9. 701cc Super Jet, Blaster , and Raider owners can legally big bore their existing cylinders to 785cc.

Buying a 753 From Yamaha – Many 701 owners, not interested in IJSBA Super Stock rule compliance, will consider installing the new 753 cylinder until they take a close look at the cost breakdown. The 753 cylinder retails in the $500 – $600 range. The pistons, rings, gaskets and other misc. hardware will be another $400. The stock 701 heads can be machined to suit the bigger cylinders (about $75), and the cases will also need to be bored to accept the larger base sleeve diameter (about $60 machining & $250 teardown and reassembly). Raider owners will be able to maintain their existing exhaust manifold and bolts, however Super Jet/Blaster owners will need to buy a new exhaust manifold and the special Yamaha exhaust manifold bolts to accompany it (another $150). Some enterprising SJ/Blaster owners may attempt to drill out the holes in their existing manifolds, however they will learn that the exhaust port roof and water jacketing of the 753 cylinder badly mismatches their existing manifold. The port can be matched, but the water jacket mismatch will be an ongoing source of cooling system leaks (believe us…we tried it).

All this will put the cost of a 1996 753cc cylinder installation at about $1400 for a Raider, and $1550 for a Super Jet/Blaster. With stock cylinder porting, it’s not realistic to expect the 753 to run much better than a ported 701. If you want a high output 753 (who won’t?), another $350 must be added for porting. With all this done, you must understand that the 760 cylinder has only two bore oversizes…that is .010″ and .020″. The base sleeve of the stock 753 cylinder is not thick enough to safely sustain any larger size pistons. In the event that the cylinder wall was significantly damaged, the sleeves (or cylinder itself) would have to be replaced.

Group K 753/780 Big Bore Kits – Group K will be offering two different Big Bore modifications for the 701 engines, based on the cylinder that you have.

The first, and most affordable, is the Raider Cylinder Bore Only. The 701 Raider cylinder (which bolts onto SJ/Blaster cases with no case boring) can be bored to 753cc using the existing stock sleeves. While the base sleeve areas do become some what thinner, the are easily strong enough for high output long term use. Boring the 701’s leaves the ports badly undersized for a 753. That’s why cylinder porting is a mandatory part of all the Group K big bore kits.

The second type of big bore modification is the Sleeve and Bore style, which must be done on the SuperJet/Blaster 701 cylinders. These cylinders do not have enough base sleeve thickness to safely accommodate an 84mm piston (753cc). For these cylinders Group K installs a thicker wall sleeve, made to our specifications, that can safely accommodate pistons up to 85.5mm (780.44cc). Top case boring is required to accommodate these larger sleeves. The thick wall sleeve installation will also be available for customers who desire to bore their Raider cylinders to the 85.5mm diameter. (boring the Raider cases will be necessary when the thicker sleeves are installed)

Other Advantages of Group K Sleeving – Besides the obvious cost advantages, there are also reliability advantages. The Group K replacement sleeves are manufactured with a head gasket sealing surface around the bores that is much wider than the stock 701 or 753 cylinders. Fitting these sleeves requires additional machine work for us, however the improved head gasket sealing is well worth it.

Even bored to 780cc, the Group K exposed base sleeve is still thicker than the stock 753. This gives the 780cc sleeve much better integrity for engines that will be prepared for race gas applications.

Performance of the Big Bores vs. Stroker Engines – In a nutshell, the big bores are much cheaper to build, much more reliable, and every bit as fast as the “over the counter” strokers we tested against. Our big bore test boats were able to pull the same pitch prop as any stroker. It bears noting that when a big bore turns the same rpm as a stroker, the average piston speed of the big bore is about 7% less. That results in noticeably lower operating temperatures, not to mention greatly reduced loads on the lower end bearings.

The performance of the Group K Big Bores are as follows:

• Raider – FPP pipe, dual 38s, stock ignition,145psi compression, Solas J impeller 7000 rpm 60 mph
• Blaster – FPP pipe, single 46, stock ignition,145psi compression, Solas I impeller, 7050rpm, 55 mph

It bears noting that the pump gas big bores have only slightly better peak speed ability than their 701cc counterparts. Their greatest asset is the huge advantage in low range acceleration that is being accomplished with only 145 psi compression (instead of the 170 – 180 needed by the 701’s). Race gas versions of the big bore kits will push no more than 170 psi.

Our development work has convinced us that these big bores will completely obsolete strokers everywhere except perhaps at the “cost is no object” national championship level. We have not seen any “over the counter” pump gas compatible stroker that can equal the performance and reliability of these kits.

105+ Octane Racing Versions – We also have available race gas formats that run slightly more port timing and compression ratio. For owners who want the “maximum setup”, we have additional recommendations on race gas formats. The closest available piston diameter to the IJSBA 785cc limit is the 85.5mm piston set @ 780.44cc. Pistons are also available in 85.0mm (771cc) and 86 mm (789.6cc). All these larger diameters run noticeably stronger than the 84mm (753cc) formats. For longevity conscious IJSBA class racers, we recommend the 85mm which allows for one additional (displacement legal) overbore. For “grudge racers” and “confident” IJSBA racers, we recommend the 85.5 pistons. On these engines we have always found bigger to be better. If you choose any of these larger displacement variations, we recommend mandatory crankshaft truing and welding, along with optionally considering case porting.

These larger displacement variations maintain the ability to rev very willingly into higher rpm ranges. Given that, they will usually yield the best closed-course results with a modified cdi limiter, and a Solas “I” pitch impeller.

Limitations Of Existing Parts – Damaged 701 cylinders and heads that have been involved in “mechanical failures” can easily be used for 753 big bore conversions…with some limitations. Badly damaged stock sleeves are usually not a problem. However if that damage includes the aluminum cylinder block itself, we must examine the damage before performing the modification. We cannot perform the big bore modification on cylinders decked to an overall “head to base surface” height of less than 4.665 (118.5mm). That means no more than about .020″ decked off the stock height. Having an un-decked cylinder is defiantly preferable. Stock cylinder heads can be used, as long as they have not had more than .020″ or .030″ milled off of them. Completely stock heads are preferable. No aftermarket 701 “O” ring heads can be used because the new larger dome diameter dome cut usually breaks into the “O” ring groove. No changeable dome 701 heads can be used because the domes will nearly drop into the new larger bores.

What about Big Boring an Older 633cc Engine – The 633 cylinder has so little available material around the exhaust port that big boring to 80 – 81mm (685 – 701cc) is the safe limit. This 685/701cc modification is a kit modification that Group K offers for all the 633cc models. However the lack of available exhaust port width on the 633 cylinder casting makes 84mm big boring a very bad technical choice. For this reason, Group K will not be big boring 633 cylinders to 753cc. Having said this, the 633 cases can still be bored to accept the 753cc big bore Raider or Blaster cylinders. The slightly smaller reed cages of the 633 cases are easily adequate for high performance recreational machines, however they do not offer enough area to feed a pure race motor. Keep in mind that any 701 cylinder used on 633 cases would need to be accompanied by a compatible exhaust manifold. The exhaust port opening on 633 exhaust manifolds is much smaller than the Raider/Blaster openings.

Intake and Exhaust Systems – We have found that all the big bores work well with the Factory Pipe Products exhaust systems. The stock Raider pipe, and the stock SuperJet/Blaster pipes, restrict peak rpm abilities. The stock waterboxes on all the 701’s work fine.

On stock dual carb lower ends, we recommend the stock 38 Mikuni’s (modified by Group K) above all other arrangements. The combination of smooth operation, strong acceleration and good fuel economy are unmatched. Dual 44’s and 46’s offer slight improvement in acceleration, however they greatly increase fuel consumption.

For the SuperJet/Blaster lower ends, we recommend the single 46 Mikuni on a Group K modified inlet manifold or the dual 42 Kiehin kit offered by Aqua Sports. The peak rpm of these two arrangements is the same, however the dual 42s accelerate “a lot” harder. This added acceleration is usable in the Blaster hull, but very hard to control in the Super Jet.

Reeds – The stock Yamaha reeds and cages have consistently out performed any aftermarket reeds we have tested. Some aftermarket reeds can offer better power for 20 – 30 operating minutes. However after that amount of time the material “looses its memory” or begins to chip. As this happens, the performance offered by these weakened aftermarket reeds becomes worse than stock.

Ignition – The stock Yamaha ignitions have a built-in rev limiter. The 633cc rev limiters kick in at about 6500 rpm while the 701’s are set at about 7100 rpm. Raising the limiter on the 633 based engines is considered mandatory. However the 701cc limiters should not be modified for 92 octane formats.

Lightening the stock ignition flywheel offers slightly quicker throttle response at a very reasonable price. The aluminum “charging flywheels” available from various aftermarket shops are a little lighter and a whole lot more expensive than a lightened stocker. We only recommend aluminum flywheels to racers who have an “open” budget.

Cooling System – All of our Yamaha kits can operate reliably with the stock single input cooling system along with our cooling system upgrades. Our optional cooling system upgrade comes with two additional water bypass outlets. All fittings and hoses are included. (note: for customers who have aftermarket exhausts, our cooling upgrade kits require that we have your exhaust manifold along with your top end parts.

Case Porting – This is a mod that can make a noticeable improvement in overall power that comes without any loss of reliability at all. The case porting is an “all gain – no lose” modification that should be done on any 105 octane format. (note : the cylinder is required with the top crankcase half)

Pump Modifications – Pump blueprinting offers much better “hook up” in rough water conditions and is an important asset to machines that will be run hard in rough water conditions (as race boats are). If your machine will be operated primarily on smooth water, the blueprinting will offer little improvement for the money spent.

Handling – For recreational boats that will be operated primarily on smooth water, cutting out the center bars of the stock scoop grate will offer all the additional water access that’s needed. Aftermarket scoop grates are recommended for all machines that will be run at high speed in very rough water conditions. For riders who run their machine very aggressively in rough water conditions we recommend the “top loader type scoop grates. Be forewarned that while these top loader type scoop grates offer excellent rough water “hook up”, they also scrub 1 – 2 mph off peak water speed and often cause “eye opening” deceleration when you release the throttle.

Of all the ride plates and scoop grates we tested on all the Yamaha models, the “Jet Dynamics” components consistently out-performed every thing else. These plates and grates are more expensive than the rest, however in our judgment…well worth the price.

Assembly Information – All Group K kits are accompanied by a step by step instruction pamphlet that outlines the assembly, break-in, fine tuning, and maintenance of your kit. For any further questions you may have regarding your kit, you’re welcome to contact us directly for assistance.