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Quick and ?simple? compression plate question

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Quick and ?simple? compression plate question
Started at 07:59pm on the 13th April, 2018 by stinkwheel
stinkwheel Subject: Quick and ?simple? compression plate question
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!
Posted: 07:59pm 13th April, 2018
Barry N Subject: Quick and ?simple? compression plate question
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!
Posted: 08:43pm 13th April, 2018
Barry N Subject: Quick and ?simple? compression plate question
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.
Posted: 09:01pm 13th April, 2018
Alan R Subject: Quick and ?simple? compression plate question
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}-------------- Compute_Compression_Ratio_Piston_Shape_dished ................

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 !!
Posted: 11:39pm 13th April, 2018
stinkwheel Subject: Quick and ?simple? compression plate question
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?
Posted: 12:38pm 14th April, 2018
Jamesy Subject: Quick and ?simple? compression plate question
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.
Posted: 01:13pm 14th April, 2018
Alan R Subject: Quick and ?simple? compression plate question
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/Honda/honda_xbr500%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/BSA/BSA%20DBD34%20GS.htm
Posted: 02:21pm 14th April, 2018
Woodsman Subject: Quick and ?simple? compression plate question
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............
Posted: 04:22pm 14th April, 2018
Alan R Subject: Quick and ?simple? compression plate question
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..
Posted: 05:36pm 14th April, 2018
Barry N Subject: Quick and ?simple? compression plate question
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!
Posted: 09:31pm 14th April, 2018
Alan R Subject: Quick and ?simple? compression plate question
Hi Guys---- yes, spot-on there Barry......The two things not yet mentioned are a}--- These are long stroke engines and develop their power via an increase of Torque rather than higher engine speed, and b}--- The quality of commercially available fuel at the pumps is changing ---- and not necessarily for the better...
I think that Bullet Whisperer's idea of altering the inlet cam by 1 tooth is the best, no-cost idea yet !!!......has anybody got one of Performance Classics oversized Bullets ??.....Any idea what CR he runs them at ??..........Just for interest I had a look for long stroke engines and in particular Harley Davidson Tech. Specs and found this}----------http://hdhistory.com/most-popular-harley-davidson-engine-comparison-chart/ ---------- Notice how Comp. Ratios have gone up from 6 to 8.5 --or-- 9.0 and then settled there ??......
If you want to increase the Clearance Volume (CV) by another method then you can always have some metal removed from inside the head.............The length of the inlet tract can help torque development..For example}-----A long tract helps torque lower down the rev range whilst a short tract helps with top-end torque.............Have another look at that BSA Gold Star photo and notice that the carb is bolted direct onto the Cyl.head to help with the top-end torque as the engine reached it's max rpm........On my 250 Grass Tracker I made up several different tract lengths to try out...............Incidentally when we say a base gasket of 0.5mm then that's a metallic one, not a paper or card....We don't want it collapsing, do we ??

Using basic maths we can transpose the CR formula around such that the Clearance Volume (CV) can be calculated if we know what CR we'd like to be using..( Remember that point about poor quality fuels ??)...................So we now have this}---CV = SW + CR divided by CR.....If we want an 8:1 comp.ratio on an engine of 500cc swept vol. we get}------ CV = 500 + 8 divided by 8 which gives}--- 63.5cc.......and for an 8.5CR we get 59.8cc............and for 9:1CR we get 56.5cc..........and for the Bullet production CR of 6 we get 84.3cc..............Just for information 1mm = 0.0394" ( 40 thou if rounded up ) so half a mm = 20 thou. or just over 1/64"...
Posted: 12:52am 15th April, 2018
Bullet Whisperer Subject: Quick and ?simple? compression plate question
Hi Alan, to answer your questions about my 'Asbo' Bullets, there are probably no two exactly the same, as, with a couple of exceptions, each has been built to the owner's requirements and there are 350 [one is a Redditch unit engine], 500 and one or two 535's out there. The 612 was never included in the count, although with modified piston crown and valve timing, perhaps it should have been. I usually aim for 9 or 10:1 for 350 engines and 9:1 for 500's, but will go lower or higher, if requested. The 612 in question is a custom/bobber/lowrider, which I helped build and it runs very well, with power to spare - enough to rip a bolt on rear sprocket off while traveling at speed, as can be seen in the closing moments of this video 'nasty'! Regards, Paul. https://youtu.be/Fm-Nmpz18fI
Posted: 10:17am 15th April, 2018
Alan R Subject: Quick and ?simple? compression plate question
OO--err !!.......what a shame as it was going great guns indeed AND on a glorious biking day as well !!.....I have a desktop PC with an 18" screen and always go to "Full screen" for maximum effect so was riding with you all the way.....The sound reception is also very good...for example I could hear muttered Welsh expletives just before you park it on the pavement---LoL !! ...... Thanks for the info. and the vid.
Posted: 03:09am 16th April, 2018
Count Johnny Subject: Quick and ?simple? compression plate question
Yes. And then - having doubled up on the number of bolts, to spread the load - it did the same to me.

Luckily, this happened at the end of a high speed dash from Bury St Edmunds (when I was completing the final leg in Greenwich, a mile from home) not at 80mph on the M11....ping...weee!

Safe to say, that we now have a one piece rear sprocket.
Posted: 12:09pm 16th April, 2018
Woodsman Subject: Quick and ?simple? compression plate question
Hi guys..... Oh, wow! Lots of good info & advice here and I'm trying to absorb it all. Regarding our discussions on compression... I do not currently have a burette, nor do I plan on getting one. Therefore, with much of the knowledge already gained, I'm just going to wing it. I'll definitely be using both compression plates, 2 base gaskets, and also doing the inlet cam 1 tooth timing mod. I really don't mind a tad more compression as I can still get some decent premium fuel here, and I have the added benefit of being able to tweak the timing on my N1 magneto right from the handlebar. The last rebuild of this engine included a 9:1 piston and .5mm solid copper head gasket. Not sure what the compression ratio was, but it ran pretty good. And speaking of head gaskets, what are the thoughts on our host's new composite head gasket? I currently have one on hand along with a new .5mm & 1mm gasket, but not sure yet which one I'll use.
Posted: 05:00pm 16th April, 2018
Bullet Whisperer Subject: Quick and ?simple? compression plate question
Hi Woodsman, retarding the inlet cam by 1 tooth reduces the cylinder pressure, or physical compression, it wont give more compression, but less. It will reduce any tendencies towards pinking with a high compression ratio and increase power at higher rpm's by a larger margin than any losses at lower rpm's. Kickstarting will also be slightly easier, as a result. Regards, Paul.
Posted: 05:16pm 16th April, 2018
Felix Subject: Quick and ?simple? compression plate question
Be sure the small stud above the tappets is long enough before assembling. I noticed that adding a 2mm plate and two thin gaskets made the stud shrink. With no British Unfindable Fasteners in my town, the nut is half-engaged.
Posted: 05:25pm 17th April, 2018

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