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Rd07A compression

4K views 29 replies 4 participants last post by  een705 
#1 ·
Good day all.. well just not for me.. just did compression test for my rd07a
First cyl.
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Second
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#13 ·
Oil consumption with reasonable cylinder compression in high millage (or Km) can also be the top rings moving up and down in the ring grooves, which tend to pump a small amount of oil up to the combustion chamber with each stroke. If you remove the barrel, squeeze the ring all the way into the groove with your fingers and see if the ring can still move up and down noticeably in the groove. If it does, you need new pistons as the ring has hammered the groove wider. This is why high revving engines have thinner rings to reduce the effect of inertia at the top and bottom of each stroke.
If you remove the head and look at the piston top, if there is a clean area on the thrust and opposite side, with carbon in between, then oil is being passed up into the cylinder by the rings fluttering in the grooves, which removes the carbon.
This cylinder can still give good compression if the rings to bore clearance is still good.
The cylinder that is showing only just over 105 psi is probably leaking past one or more of the valves (seats). Although it is not bad yet, it will only cause a slight power loss at lower RPM only, but if you have the head off, it is worth re cutting the valve seats and valve faces etc. Also check the valve to guide clearance. A certain amount of wiggle is acceptable, but too much will allow oil down the inlet guides under high vacuum, but not so the exhaust. To check this before dismantling, with the engine hot, go down a hill with the throttle closed for some distance. Then blip open up throttle and looking in your mirror, if you see a puff of blue smoke (not black! too much fuel!) then the inlet guides/ valves/ seal are worn. This is best observed by someone following you and filming if possible.
If you replace the pistons and go oversize, depending how big you go, weigh the original piston complete with rings circlips and importantly the piston pin and try to make the new on the same weight as accurately as possible.
This is to prevent upsetting the engine dynamic balance, which can cause problems.
As the new piston is likely to be heavier, equalizing the weight is best done by removing material from the piston pin, as steel is heavier than aluminium. Do this not by boring out the inner bore as this will weaken the pin, but tapering from each inner bore end so as to create a small cone appearance. This as long as it is done in moderation will not affect the pin strength and is often how the manufacturers do it.
Do NOT remove metal from the pin ends as this will allow too much pin end float which will be catastrophic!
Lastly, if fitting new piston/rings, run the engine in for 250/500 miles on mineral oil, NOT synthetic or part sythetic, as the qualities of synthetic oil will prevent the rings from ever sealing with the bore and it will always burn oil!
After running in fully, then use Synthetic oil !
Hope this helps.
PS, If you do change the pistons i would be interested in knowing the standard weight, as i may do a big bore engine
in the future.
 
#15 ·
It depends on a few things, how good is the condition of the rest of your bike, how much money you are you prepared to put into engine rebuild. How much knowledge you have to rebuild. My first engine rebuild I needed help which I got from a friend so hopefully someone can help you. Its good fun when it goes right. Africa twins in the uk seem to keep a good price so factor that in. Another factor is time how much have you got to rebuild. Is your bike running. Last option is to drop it off at a good bike garage and let them rebuild it.
 
#16 ·
Its giod enought, last year spent a fortune on the bike
Tire Wheel Vehicle Automotive tire Motor vehicle

Full frame paint, all OEM bearings, even swingarm, full body paint, rear side panels kevlar composite, etc.

I just thinking, should I spend 1k eur on the top or just leave it in a corner. Nobody wants to pay me 4500 euros for the bike at the moment.

all work should only be done by me, no other workshop. I do not trust anybody..

or maybe there is simply a lack of inspiration at this time ..
 
#17 ·
I remember reading your thread now and you have a very nice Africa twin. I would definitely repair because you will get very little with a engine burning oil. My opinion you don't have much choice. Fix it up and start another thread showing rebuild. Use Honda gaskets for cylinder block and head. Good luck on your decision.
 
#18 ·
Going with the largest OEM pistons would be the easiest and probably the most practical solution. It would give a slightly higher compression ratio which would be a good thing if running on E10 fuel especially if you alter the ignition map as discussed elsewhere on this forum. The slight increase in capacity would also be no bad thing.
The only non OEM pistons i know of are made by Hordpower in America but these are Honda Hawk racing pistons which i was considering. However using these would be anything but straightforward as they have raised crowns, so things like valve to piston clearance would need to be checked and possibly adjusted. I have a history of building race engines so this work and other such pitfalls are second nature, but should not be undertaken by anyone without this experience or an expensive calamity is very likely!
I am not sure whether these pistons would be feasible to use as i would need detail information about piston pin diameter, compression height and overall weight in comparison with the XRV piston. I am not going to strip my engine now as it is running beautifully, but if you do decide to rebuild your engine, i would be interested in obtaining one of your old pistons complete, so i can do the various measurements and compare it to the Hordpower piston.
Of course if anyone else has an old piston preferably std size that they dont want or would lend me, i would like it to take the necessary dimensions etc.
 
#20 ·
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 ?
 
#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
 
#21 ·
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!!
 
#22 ·
I found this thread from a much earlier thread which seems to answer a lot of questions;
 
#24 ·
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!
 
#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:)
 
#28 ·
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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