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[stinkwheel]

Do I need to use a paper gasket each side of a compression plate or just one between the plate(s) and the crankcase? Obviously the thickness of any gaskets will need to be taken into account when working out compression ratios.



Initial rough calculation suggests I may well need both plates (5mm of lift) as the piston deck height is -2.8mm with no compression plate, which very roughly works out at 14:1!

[Barry N]

I didn't put a gasket between the plates, but might have put a smear of Wellseal between them (I can't remember), although I doubt it would have been neccessary since their finish is so good - just one either side of the stack. Definitely both 3mm and 2mm plates though!

[Barry N]

Further to my previous post – I have in fact a total of 3 base gaskets, plus the 5mm plates, because when I first ran with the 3mm plate only, I used 2 gaskets underneath it, as well as one above. When I added the 2mm plate at a later stage, I left the two that were underneath.

[Alan R]

Hi guys----------Blimey Stinkwheel, how many more threads re}--- Compression Ratios do you need ??-------It's getting difficult to keep track and give helpful answers..LoL...Smiley logo etc........So, paper gaskets actually needed under the barrel ?....None !! ... As per Barry N and use a modern liquid sealant.......OR, if you feel more comfortable with the old ways then use the thinnest of paper ( NOT card !) and impregnate it with grease or Engine oil between thumb and forefinger ( Useful if doing emergency "Field repairs"..).....and in either case you really don't have to be so pedantic with the thickness as--- to all intents and practical purpose there isn't any.......After all we are not dealing with Formula One engines here, are we ??..........

With regards to Piston Deck Height}---- This is a linear measurement which for Compression Ratio ( CR ) volumetric calculations I feel is a Red Herring which appears to be distracting you from the main event ( not seeing the wood for the trees etc ??)............In another thread you demonstrated a very good method to actually measure the Clearance Volume ( CV ) by means of a coloured fluid....and it's that very CV value that you need to input in the CR formula.................For those that missed it in those other threads, here it is}-------------- ................

Finally a friendly request from me to you}---- Please, please, please will you use the time-proven, accepted Engineering nomenclature of Cubic Centimeters as we are actually dealing in VOLUMETRIC quantities here.....The use of ml is ok for wines etc. but like the Deck height values it can be a bit of a mis-leader for the brain......

Back in the day I designed, built and raced my own motorcycle Grass Track bikes using Tig Perry frames ( Antig )...in 250, 350 and 500cc classes and running on Methanol ( Methyl Alcohol ) and Castrol R using a total loss oiling system on the 4-strokes .......A CR of 14:1 would be very much welcomed !!......It was in the Southern Centre ACU with running number 176 and for the Tadley and District Sporting Motorcycle Club......These days I don't bounce as well as I used to so a slow, basic 6:1 Enfield is lovely for my auld bones !!

[stinkwheel]

ml/cc = potato/potato. Technically we ought to be using cubic metres to work on volumes but we get some very small numbers. My brian is used to ml.



Anyway here's my longhand working out of the clearance volume with no base gaskets at all:



Deck height is -3.3mm, so protruding volume of piston (not coumting the dome) = surface area x height = 59.45 x 0.3 = 19.62ml



Head gasket is 0.527mm so it adds 3.15ml



So clearance volume = Measured combustion chamber volume - dome - protrusion + head gasket = 110.37-50.93-19.62+3.15 = 42.97ml



Swept volume is 612.33.



So CR = 15.25:1. Well, we'd be mashing valves at that anyway.



So adding both compression plates and a base gasket will give us a lift of 5.5mm. So we add a volume of 0.55*59.45 = 32.7ml to the clearance volume.

New CR with 5.5mm lift = 9.1:1.



Could probably still do with a couple more base gaskets there?

[Jamesy]

I don't know why even back in the imperial past motorcycle manufacturers decided to define engine size by cc.Maybe 500 cc sounds better than 30.5ci.

[Alan R]

Hi Guys-------Hi Stinkwheel, as you say}--Potato or Potarto (?).....I believe our American cousins still use Cubic inches so there's another string to our bow......It would seem that the march of progress now dictates that the power of an engine be rated in Kw ( Kilo Watts ) thus making comparisons between Electric and Mechanical devices straightforward............. For me your figure of 9.1 is a gratifying result as I had earlier predicted an initial Ball-Park figure of 9.41..with a possible final figure of 8 or 8.5 even......Meanwhile, extra de-compression plates means the rear rocker cover becomes increasingly more difficult to remove, as does the top of the carb.....

Regarding the "mashing" of valves}---- If you use larger diameter headed valves you can then opt to reduce the amount of valve lift and still get the same volume of gas to pass through.........Max diameter of a poppet valve head for max gas flow is approx 1/4 to 1/3 diameter of head......thus by increasing the diameter we can reduce its required lift......But then valve overlap needs to be considered as well........OR}--- we can consider multiple, smaller valves for greater gas flow ....BUT, as you "Tweak" one part so another gives less as a result----It's all a matter of compromise and balance as to your expectations really.......I know I keep quoting my Honda XBR 500 ad nauseum but it's Tech. Specs do make for interesting reading for a production bike and a possible source for data ??....................................... http://www.motorcyclespecs.co.za/model/ ... 0%2086.htm .............and to give Beesa Bryan a thrill here's the 500 Goldstar specs as well....Remember this was in 1960 !!}------------ http://www.motorcyclespecs.co.za/model/ ... 4%20GS.htm

[Woodsman]

Stinkwheel, This is roughly the same area of rebuild I'm at on my Woodsman, with many of the same questions, so I'll defer my "Compression Ratio" thread over to this one. Lots of good info here. Thanks for your hard work. Looks like I'll also be using both compression plates. But I read with interest Alan R's reply about Cams & Valves. I'm still using my original Cams, but with the Stage One Head which is very similar in Valve size to my Woodsman's original Head. Please pardon my lack of technical knowledge, but are higher lift Cams and/or larger Valves related to an increase in Compression? I just wish I understood all I knew about this stuff............

[Alan R]

Hi Woodsman----- yes, to the bystander this can be a little bit confusing.........First things first then}----- Terminology...."Compression" is exactly as it says ie}--- gas being forced into a smaller volume.....Thus we get units of measurement such as the Imperial System of}--- "Pounds force per square inch" usually shown as}---PSI or psi.....In the SI ( Metric ) system we have the Pascal which is 1 Newton of force per square metre...........

"Compression Ratio" however is the comparison of volumes found above a piston when at Bottom Dead Centre ( BDC ) which is the Total Volume (TV) and Top Dead Centre ( TDC ) which is the Clearance Volume (CV).....Because it is a comparison it does NOT have any measurable values but is shown in the Mathematical way thus}---- 10:1 or 8:1 etc.....
So, if you have a 500cc engine ( like a Bullet ) whose TV is 560cc ( Cubic centimeters ) and CV is 60cc's then the CR is 560 divided by 60 = 9.33:1 ...........Now to answer your question about Compression}---- Basically to go faster we need to burn more fuel/air mixture at a greater rate to create more power.....If we compress more fuel/air mixture within the engine then that will help achieve this....These are typical ideas to make it happen}----Bigger inlet tract, bigger carb / uprated fuel injection system, Bigger inlet valves, multiple inlet valves, inlet valve held open for longer, Valve "overlap," Turbocharger, "Supercharger", using an additional oxidizer such as Hydrogen Peroxide ( Drag car racers etc) ....or using a fuel that can stand much higher Compression Ratios than petrol such as Methanol ( Methyl Alcohol )....
I would say that Stinkwheel's approach is that used by Engine designers, especially when no prototype actually exists...whereas Bullet Whisperer and I favour the more traditional "Hands-on" approach as when out in the Workshop or on site etc....Both are equally valid IMHO..

[Barry N]

I believe the base gasket is approx 0.5mm thick – therefore two or three of them (as opposed to none at all) would have an effect on the compression ratio, sufficient to be considered when building the 612 with Accralite piston. I know from painful experience, that just the 3mm compression plate (with 3 gaskets) – at least with my own engine – resulted in serious “pinking”, no matter how I tried to dial it out with timing and carburation settings, as well as different petrol concoctions. But after adding the 2mm plate to the stack, along with the existing 3 base gaskets (making a total of 6.5mm, give or take) – AND rounding off all the sharp edges from the piston crown – the pinking was eliminated and it’s a much sweeter engine altogether. So my advice, for what its worth, is not to strive for the highest compression you think you can get away with on these (612) engines – they don’t like, or need, sky high compression!