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Piston help needed!

3K views 15 replies 8 participants last post by  JAWS 
#1 ·
Hi all
Am in the process of building a "new" engine from scratch for my BSA M21.
It should be possible to bore out the barrel from stock 82 to 89 mm`s to take it from stock 600 to very close to 700 cc`s
A stock EVO piston at 3,5" is almost perfect for this, except for the pinheight (center pin to top piston) is about 3 mm`s or 1/8" to tall.
To avoid using a stroker plate under the cylinder I´ve got the following quetions:
Is there a stock bore EVO piston for a 4,5" stroker crank? Imagine that would be a perfect fit
What is the approx pinheight on a 1200 EVO Sportster? That one is also 3,5" bore not?
Any other piston suggestion, 3,5" bore approx 32 mm or 1 1/4" pinheight?
Anyone got a set? (a friend is doing the same, so need 2), or know a good source for it?
Thanks!

Stock piston to the left, EVO piston to the right.
 
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#2 ·
S&S Cycle has such a piston 4 1/2 stroke eve piston. So this motor can be bored 7 mm [which is .275 inches which sounds crazy. ] You said you are not using a stroker plate . Thats good because a stroker plate without more stroke will lose lots of compression.
 
#3 ·
Dave brings up a good point about compression. The combustion chamber and piston deck height to wrist pin is gona play in here. Also when using a different piston, keep an eye on the skirt. I like to dry fit everything together (except one side of case) and do a few rotations to see if there is any fitment probs gona rear their ugly heads.
 
#4 ·
you could condsider the a dome piston and walk the dome back for the correct compression forged weisco piston have solid tops so milling is cool But watch the bottom of the valve pocket area as it is thin to start with just stay away from it

.791 is the rist pin harlet sporty size and the BSA piston pin in the picture looks 1/2 inch but its a picture
 
#5 ·
Dave B: thanks! No loss of compression with a plate. The Evo piston will stick1/8 out of the cylinder, so a 1/8" plate will just keep the piston from banging into the head. Maybe a stroker plate is the wrong term, let's call it a cylinder height correction plate then:)
Have been going through an online S&S catalog without luck. It's either 4,25 or 4-5/8 stroke pistons. I'd imagine they wouldn't sell a lot, as most people probably wouldn't spend a lot of money on a stroker crank to only gain 1/4 stroke and keep the stock bore. If you by any chance have a link, it'll be greatly appreciated:)

Little D: It's a sidevalve, so a flattop piston that just goes flush with the cylinder is what is needed. Stock combustion chamber will be fine, and the compression will be raised simply by the added cc's. Rest assured that everything will be thouroughly checked:) there's larger (Chevy) valves going in, and the cams are out being welded and reground. An odd thing about this engine, is the very long (8"?) conrod, making the piston turn over deck, even with the long stock piston.

John: that may be an idea! I actually have a Wiseco in my Royal Enfield dragbike, that I did remove some material from, but then again? It's not a raceengine, so if nothing turns up, maybe I should just get a cast EVO piston and do the plate under the cylinder thing:)

Anyone got a 1200 EVO Sporty piston they could measure?. Doesn't have to extremely accurate, just an approx measurement of the pin height:)
 
#6 ·
John: thanks for the pin size measure btw. Believe the BSA is 5/8. Anyway there's plenty of meat in the small end of the conrod, so absolutely no worries here.
 
#7 ·
what your after is whats called compression height - the correct way it is measured is 1/2 the rist pin diameter is used , and then added --- the measurement you get from the top of the rist pin hole to the top of the piston / then add 1/2 of the pin diameter = compression height

i believe i have a 4 5/8 stroke piston - i have a set of 3 1/2 pistons coming in for a shovel that is 4 1/2 stroke and they are domed but the top as being cast is not very thick so you wont be able to fenesse them - i will look

if you call a piston company they will want the compression height that is the key measurement / then the bore size / then the ring thickness evo has 1/16 - 1/16 - 3/16 control rings as they will ask that as well and the pin diameter you are using also the rod top thickness come into play and the never ending valve ressions you might need -- have all that info available it will save you a ton of time if you do

most piston makers in the usa will make 4 as a small order - like egge pistons in calif

http://www.vitalvintagecoalition.com/vintageaftermarketparts/vintage-automotive/egge/
 
#8 ·
Have you looked at Jr Dragster pistons? Most are all side valve Briggs and Stratton style.

I know they have 3.5" pistons and small pins. Been awhile since I had one apart. I think JE makes several degrees of it.
 
#9 ·
When the piston pops up how close to the top of the cylinder is the top piston ring? If the ring is below the top of the cylinder deck, I would fly cut the head to allow for the pop up if the head has enough meat, relieve the cylinder and that will bring the compression down. I think the kr racing Harleys at Daytona in the last of their lives were going like 150 mph and had less than 5 to1 compression?
 
#10 ·
Thanks John! Of course compression height! Sorry about that. Sometimes you mess up and get to sound a bit confusing when out of your mothertongue:) I were pretty sure about it being called pinheight, but did state that I were after the center of pin to top piston measurement:)
A rough measurement from a 4-5/8" stroke piston would be great, so yes, please do if you have the time for it:). My guess is it'll be right around 1-3/16", but it would be nice to know for sure, and confirm my theory about a 4,5" stroke piston being perfect.
Ordering a special piston (or 4) would really be taking it to far, for what I'm using the bike for.

Jaws: thanks for the advice. I'll have a look at the catalogues and see what is available.

Flatman: A splendid idea! Wonder why I didn't think of that? Have seen how the pistons pop up on some of the highly tuned sidevalves. I have roughly 5 mm's down to the topring, so definitely doable. I'm thinking maybe a 1 mm basegasket, and let the piston pop 2 mm's up into the head. I could cast or machine from billet a head with enough meat in it. That would also give me the possibility to relocate the plug slightly, or maybe even twinplug it for a racey look:). Thanks, this is what I'll do if nothing available turns up:)
 
#11 ·
Fun trivia: I believe the highest recorded speed ever for a sidevalve M model BSA, was about 108 MPH, at Bonneville, running N2O. I've heard he didn't make the return run due to blowing the engine up:) If I can make mine run 70 I'll be happy:)
 
#13 ·
3 1/2" bore big twin evo pistons have a compression height of 1.375" for the 4 1/4" stroke.

a 3 1/2" bore 4 5/8" stroke evo big twin piston will have a compression height of 1.1875"

a 1200 sportster with the same bore of 3 1/2" and a stroke of 3 13/16" also has a compression height of 1.1875"

is it possible to remove 0.0625" from the top or bottom of your cylinder?

just guessing but if you run only with a sealer and not a base gasket, you could drop the cylinder roughly 0.040" without any machine work depending on gasket thickness. the 0.0225" deck height may not be that big of an issue with compression ratio loss at the bigger bore you will end up with.
 
#14 ·
Backing up what HRC said. I have a first hand story that ends with the area under the dome which material was removed, for balancing, is where the piston melted. Was a side valve motor also. OT Model B ford in a belly tanker at Bonneville.
 
#15 ·
HRC and Jaws, thanks for chiming in.
Dan r, great info thanks a lot. 1,1875 being the same as 1-3/16" shows me my calculation was spot on.
Choosing between a 1200 Sportster, a 4-5-8" stroke EVO or a stock EVO piston was made very easy for me. Called my dealer today, and all he had in stock was a set of OEM EVO in a slight oversize, that I got a real good deal on, because they had been there forever.
I'll do the plate under cylinder thing, or maybe let the piston protrude into the head. If there's room for it it'll take my compression ratio from 5:1 to 6,3:1 so not exactly going totally wild.
Jaws: forget the long distance excuse, and get your butt to Bonneville again in August. I'll buy beer:)
 
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