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 Post subject: Hi-Comp Pistons
PostPosted: Fri Aug 26, 2011 8:25 am 
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Joined: Sun Oct 17, 2010 2:24 pm
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Hi Guys and Gals, I have a set of Hi comp pistons for a 600 pantah and was considering dropping them into my TT2 race bike as its currently apart getting the heads shimmed up.

Is this as simple as putting them in and making sure the valves dont hit them, or will I have to take out the crank and have the lot re-balanced? If its the later I wont bother, as my aim is to build a larger capacity engine.

While I'm at it, Mick Walkers Ducati restoration book is quoting 0.02mm as the opening clearance on the valves. Seems a bit tight to me, given most recomend 0.1mm or 3-4 thou.

thanks

A


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 Post subject: Re: Hi-Comp Pistons
PostPosted: Tue Aug 30, 2011 2:05 pm 
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Joined: Mon Oct 04, 2010 7:47 am
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Hi TeeTee2,

You are on the right track. There are a couple of things to consider. Is your 600 motor stock? Has it had the crank and rotating assembly be balance before other then the factory. After market pistons generally have deeper valve pockets then stock and are usually the valve pockets are a little bit bigger in diameter. If the pistons are designed for your motor they may drop right in, but you do need to check a few things. The piston to valve clearance is important. I like to see 0.040" piston to valve minimum. The piston to head clearance in the squish area is also very important and should be in the 0.038" to 0.045" range. Another thing to check is the weight of the new pistons to what was in the motor. You should try to keep this weight pretty close but with hi-comp pistons are usually heavy. A lot of aftermarket pistons are thicker then needed to allow engine builders the ablity to custom tailor the setup. You may want to assemble the pistons in the motor with the top ring and check the volume of the cylinder to see what compression you will have. Generally the thickness of the top of the piston is way thicker then it needs to be and can either be cut down for less compression or material taken of the inside of the piston top to lighten it. Minimum piston thickness should be at least .220". I hope this helps and I'm sure other may have some imput.
Palmer


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 Post subject: Re: Hi-Comp Pistons
PostPosted: Wed Aug 31, 2011 12:34 pm 
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Joined: Sun Oct 17, 2010 2:24 pm
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Thanks Palmer

It is a stock 600, I should have made that clearer as the bike is a NZ built TT2 replica. The pistons do have quite large cut outs for the valves. At this point I will stick with the standard valves and cams. Its the first time I have had one of these apart and its been an eye opener, very neat,cool and logical, but the last bike I was adjusting valves on was my agricultural R80G/S.....

I hope to have the standard pistons off in the next few days so we can compare weight, fingers crossed its close. Thanks for the tips, very handy.

With some luck we can have it ready for a meeting in October...not Barber unfortunately.

cheers

Alec


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 Post subject: Re: Hi-Comp Pistons
PostPosted: Wed Aug 31, 2011 12:52 pm 
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Joined: Mon Oct 18, 2010 12:54 pm
Posts: 88
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Location: Philippines, Western Pacific
Palmer is a brave man to be the first to answer this request. My hat is off to him, as he obviously is commenting on what he clearly has experience.
My first comments are hopefully obvious; something which is often lacking in the field of engine-building. For the production of maximum engine power in a modern four-stroke engine the MAIN parameter to this end is Compression Ratio.
To simply install hi-comp pistons in any Ducati engine, with no further checks, and to expect a power increase, is pure folly. Indeed, it is quite likely that just "dropping" different pistons into your engine may simply prevent the engine from turning!
As Palmer states, you need to check (quite) a few things:
Is your cylinder height standard, or has a previous owner machined some metal from the cylinder base (to increase CR) and are you running with or without cylinder base gaskets?
Are you using "lumpy" cams? Using racing cams like NCR 7's can easily lead to piston-valve contact for the uninitiated. The same applies to larger-than-standard valves.

Its also best to check your engine one cylinder at a time. First, just fit one piston, install the head, including belt, and turning the engine by hand, ensure that your engine actually TURNS! This may sound a trifle comical to the casual onlooker, but MY experience with tuned-to-middle-C Pantah engines says that piston/valve/squishband contact is often the norm, rather than the exception!
Anyway, once these basic checks have been done, its time to steal your kids' plasticene, and get down to actually MEASURE the squish-band clearance,and piston-to-valve clearances.
Palmer's values for the various clearances are excellent guidelines, although I must admit that MY racing Panath engine enjoyed considerable smaller squish clearances, usually 22 to 27 thou.............!!!

:D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D :D


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 Post subject: Re: Hi-Comp Pistons
PostPosted: Wed Aug 31, 2011 1:19 pm 
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Cheers Poison...I will do what you suggest....after all that time adjusting valves it would be a shame to bend them...


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