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On looking at the JE ones the Hawk capacity is 650 cc and they quote an 11:1 compression ratio for that capacity.
Using these in an XRV 750 may take that C/R to well over 12:1 which would be unadvisedly high in a road engine that was not designed for it. Some machining of the piston crown would probably be necessary to reduce this, but without doing a dummy build this cant be confirmed.
The price also seems high and it is unclear if this is per piston or a set of two. They also quote the piston weight, but again without stating if this is just the piston or complete with rings, pin and circlips.
Interestingly, there is also a listing for the Hawk engine from Wiseco

Jeez! that's a big link!!
 
I found this thread from a much earlier thread which seems to answer a lot of questions;
 
Discussion starter · #23 ·
Ah, i hadn't seen those, but i was looking some time ago!. These are probably the pistons that Hordpower use as they are for a Honda Hornet (600cc?), so no guarantee that they are the same critical dimensions as a 750 XRV.
JE (USA) pistons are good quality forged items as they are for racing, but can be used in road applications. This company is not to be confused with JE pistons UK who also produce forged pistons but in much greater quantities ( min order in the region of 2000 sets!).
Motorsport is so huge in America that it is not difficult to get pistons custom made, as i have done from the likes of Arias, (min order 2 pistons!!) but other good makes are Wiseco etc.
From memory the Hordpower (JE) pistons would give a capacity increase to 780 cc (in an XRV application) which may not sound much, but any increase in capacity will give extra torque. I was interested mainly to increase the Compression Ratio to over 10:1 for more efficient combustion with E10 fuel, but as i stated before as these are designed for the Hornet 600, using them may not be a straightforward job. JE can supply the relative dimensions for their piston ( probably as specked by Hord) but would not necessarily know the AT 750 dimension to compare to. You would definitely have to alter the ignition profile to get the best out of them, but i think you already have that option.
The valve to piston clearance may be ok as the Hordpower engine would undoubtedly have race cams which would have the necessary clearance, but still cannot be taken for granted and would require a dummy build check.
Dammit you are getting me interested in doing this again !!
What mileage / KM has your engine done ?
My engine has near 300kkm’s :) i l know who used this pistons allready in AT build, he shared the link with me.
just txted for hi, to tell me more about project
 
Well if anyone was in doubt about the AT's engine longevity, yours is a testament to that ! That would explain the oil burning that is highly likely to be the ring groove problem mentioned earlier. If the rings had been a thinner width (less weight/ inertia) and the pistons forged ( superior grade aluminium alloy) rather than cast, then it would still likely to be in rude health!
I that were my engine i would do a complete strip down and check which would obviously be costly, but would be better than having some sort of failure later on. This of course depends on what trips you have in mind and how long you intend to keep the bike. With the amount of time and money you have spent on it, it would be a shame to get rid of it, but that is down to you of course.
Back to the engine build: with that amount of Km's it has done, as well as the pistons, i would also fit new valves. If you have ever seen the damage a valve head failure can cause, it would be high on my list!
You can crack test components (definitely crank and rods) but you cannot fatigue test anything without taking it to destruction to see where/when it happens. To give an example, anyone with an air cooled VW flat 4 engine that has done 100,000 miles (or Km equivalent) change the exhaust valves ( inlets also advised) as the heads will soon part company with the stem and cause a complete engine scrap pile! ( Usually a long way from home, so factor in recovery costs!). New valves will seem cheap then!
 
Discussion starter · #25 ·
I keen to do engine fully, dont like half of the job. I had years ago xr400 with valve failure :) .
I see wiseco pistons even can get better price, but its 82mm, if i remember right, JE is 83mm. But wiseco is better known for me.
 
Discussion starter · #26 ·
Well if anyone was in doubt about the AT's engine longevity, yours is a testament to that ! That would explain the oil burning that is highly likely to be the ring groove problem mentioned earlier. If the rings had been a thinner width (less weight/ inertia) and the pistons forged ( superior grade aluminium alloy) rather than cast, then it would still likely to be in rude health!
I that were my engine i would do a complete strip down and check which would obviously be costly, but would be better than having some sort of failure later on. This of course depends on what trips you have in mind and how long you intend to keep the bike. With the amount of time and money you have spent on it, it would be a shame to get rid of it, but that is down to you of course.
Back to the engine build: with that amount of Km's it has done, as well as the pistons, i would also fit new valves. If you have ever seen the damage a valve head failure can cause, it would be high on my list!
You can crack test components (definitely crank and rods) but you cannot fatigue test anything without taking it to destruction to see where/when it happens. To give an example, anyone with an air cooled VW flat 4 engine that has done 100,000 miles (or Km equivalent) change the exhaust valves ( inlets also advised) as the heads will soon part company with the stem and cause a complete engine scrap pile! ( Usually a long way from home, so factor in recovery costs!). New valves will seem cheap then!
By the way, i love this bike, and more likely i will get job done. I don’t what to trash it or sell. Its old only in others mind’s:)
 
Ha, you are certainly doing it justice!
According to the previous thread that i sent, there is a limit to how far you can go with the bore size without machining the crankcases, fitting new liners etc. Apparently the JE pistons are a direct fit, whereas the Wiseco ones are not. Also with regard to Piston weights the JE ones are closer to standard than Wiseco.
Standard piston 355 g, Wiseco 390 g, JE 366 g This info was supplied on the forum, as requested by myself, so i cant guarantee how accurate the figures are, but some emails to JE and Wiseco should confirm this.
You should try to get the piston total weight as close to standard regardless of whatever piston you use, or the dynamic balance that Honda have set will be altered and probably not for the best.
As you can see, the Wiseco is quite a bit heavier and it may be tricky to shed that amount of weight without compromising its strength.
If you do go for those, then i suggest that you have the crank, rods and pistons Dynamically balanced (+ clutch basket and cover as this fulfills the flywheel function) by someone that knows how to balance a 'V' engine as this is not such a straightforward operation as an in line engine. There is a balance factor that is a percentage of each piston /rod assembly, to make up weights that have to be bolted to the crank temporarily to balance it. The percentage value will alter with the RPM range the engine uses so this will require experience. I worked for a company that did this, their customers included a host of top drawer V8 drag racers, but this company has been taken over by new owners, due to retirement of the owner.
If you do a total rebuild, it would be wise to examine/ replace the gearbox sprocket shaft as this is an Achilles heel component that is highly likely to have very sloppy sprocket splines.
 
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