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xlh59 · #51 26th June 2022

Thanks Rick,

thats what I thought ... but how does this go along with all the other big bore / drag bikes in the past? The Axtell pistons have the same problem of a huge dome, or did everyone work with 12:1 or above compression ratios?

You will have to adress the same topic with your new engine. I am considering if it is worthwhile to have custom pistons again (with a wider squish and smaller dome)? -- I just wanted to avoid that.

Cheers, Ralf

needspeed · #52 26th June 2022

The way I see it,
establishing a squish area is constrained by 2 things. One is the target compression ratio of 9:1 and the other is the desire to use off the shelf pistons.

So if off the shelf pistons have too much dome volume to provide the wanted compression, then I see no way to run the piston deck close to the head to get any squish.

Unless you can machine the dome smaller and /or increase the valve pockets. I doubt that is doable.

Increasing the size of the head chamber is not the way to go either.

I don't know what pistons are currently available. Were there ever stroker pistons specifically made for a 80 inch shovel with 3.5 bore?
Or do you have to use a 3-7/16 + .060,.070,.080?

The 50cc dome volume you mentioned does seem like a lot for a stroker piston. I have an old S+S 2800 piston and the dome measures very close to 43cc.

If you have a piston that decks too close to the head at TDC then you can deepen the counterbore. You probably have to open the diameter for the fire ring? What about the head bolt pattern?

I skimmed through this thread to see what was done. It's basically the same thing you're doing though slightly smaller bore. Same stroke and cylinder length with off the shelf pistons. He may have mentioned what compression ratio was used, if so I missed it.

http://xlforum.net/forums/showthread.php?t=1208660

To get EXACTLY everything you want, custom pistons may be needed.

Squish is good but you can still have a strong engine without it. There is not a lot there to be had at that bore size.

At 5.75 cylinder length I doubt you'll get it into a stock frame. It will be tight even if you shorten the cylinders.

xlh59 · #53 26th June 2022

Thanks for your feedback needspeed and thx for pointing to that post (which is a kind of reference for me anyway)

-- but I checked the infos in that post with my chart.

If my chart is correct, that engine with the 2800 series pistons and 43cc dome would need cylinders with 5,75 lenght to get to a static ration of (around) 9:1 which should be the target for a street engine? -- This on the other hand implies a quite tall distance between head an piston deck of (around) 3/16" -- so obviously the Dr. didn´t "mind the gap"

. Although this engine is different because it uses no fire ring ...

My last build has cylinders with 5,55" lenght and there is definatly more than 0,1" more room if the head of that one rear rocker box bolt is shortened. So I guess even an engine with 5,75" long cylinders could still fit. Most likely you wont get the rear head of if the fire ring is used.

For me, the current S&S stroker pistons have a deck height of 1,1" so almost that 0,1" shorter. I would also have a set of 2900 pistons which would be even better, but unfortunately it turned out they are to small in oversize plus, if I change to the next oversize, I am stuck to get the 2900 again ...

Cheers, Ralf