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Old 07-13-2020, 02:19 PM   #1
Griff
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Default Stroker question

I've had my stroker (Flathead power knuckle) for 20 years now and I need to calm it down a bit as it's a bit too angry, it's a 96" high compression.

So the question is, what effect will fitting 8.5:1 pistons in it have, as opposed to the 10:1 that are in it now?

I don't really want to go to the expense of new flywheels etc, so just thinking of options

Thanks

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Old 07-13-2020, 06:58 PM   #2
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Default Re: Stroker question

My (non-Harley) experience of choosing lower compression was only a little bit mixed, almost all positive. Dropping a Bonneville down from aftermarket 10.5:1s to stock-ish 9.0:1s made the bike a lot nicer to ride, day to day. I now think that I was imagining things, but back then it seemed that while acceleration was real distinctly not as keen, the torque was fatter, especially you got into the torque at a lot lower RPMs.

I then changed cams, from Harmon & Collins (more recently, Megacycle) 1060s -- hey, it was a long time ago to the 1969 stock Triumph 3134s. The quickness diminished again, while the bike got friendlier still; now really nice to use in the real world, it also broke less. At that point I'd begun to very much appreciate and respect the stock engine's characteristics, and mid- to upper-40s horsepower was really not too little to get along with.

All this longwinded tripe is just to say: I suspect you'll be happy with lower compression. Best of luck.
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Old 07-13-2020, 08:10 PM   #3
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Default Re: Stroker question

agree - the knuckle we did last was a 4 5/8 stroke 3 7 /16 bore and we made it 8 to 1

guy had 10 to 1 pistons for many years like you but today the gas is not good iron top end

you wont be sorry - easier to kick and smoother ride
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Old 07-13-2020, 10:47 PM   #4
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Default Re: Stroker question

Increase your final gear ratio. Takes power off the bottom end and gives you a lower rpm on the highway.
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Old 07-14-2020, 10:27 AM   #5
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Default Re: Stroker question

Quote:
Originally Posted by johnjzjz View Post
agree - the knuckle we did last was a 4 5/8 stroke 3 7 /16 bore and we made it 8 to 1

guy had 10 to 1 pistons for many years like you but today the gas is not good iron top end

you wont be sorry - easier to kick and smoother ride
Who's pistons did you use for that John?
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Old 07-15-2020, 12:34 AM   #6
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Default Re: Stroker question

Did that years ago on a shovel with 4 1/2" wheels after running 10:1 for a couple of years. The bike became easier to start and saved money on tickets. The bike ran great and still had plenty of go up a go. Ran the same engine for years after and never had a problem.
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Old 07-15-2020, 08:24 PM   #7
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Default Re: Stroker question

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Originally Posted by joe49 View Post
Who's pistons did you use for that John?
S&S has helped in the cast piston as its the same piston for 3 strokes

so the longer stroke has the least compression and the short has the most

can if you need the number but i believe its 4 3/4 stroke - 4 5/8 stroke and the shorter 4 1/2 all use the same pistons and the long stroke it sits down in the cylinder - all 3 have the same deck height piston
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Old 07-15-2020, 09:51 PM   #8
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Default Re: Stroker question

thanks John.
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Old 07-16-2020, 05:38 PM   #9
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Default Re: Stroker question

if piston to cylinder clearance is in a usable range... easy... shim the cylinders, stroker plates. it has been done to lower the compression for over a hundred years. a 0.062" shim will lower the cr roughly half a point. an 1/8" shim will get you closer to a full point (slightly more really) manifold fit and pushrod lengths change but are pretty forgiving at the 0.062" lift.

if the pistons and cylinders are worn, yeah, bore and replace pistons. if you go that way, weigh the old and new for a weight comparison. a few grams one way or the other won't be that big of a deal, once past 8-10 grams different think hard about rebalancing the flywheels.
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Old 07-17-2020, 12:16 PM   #10
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Default Re: Stroker question

You can lighten pistons that are heavier then what's in it. That is if it didn't shake your teeth out before.
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Old 07-19-2020, 06:32 AM   #11
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Default Re: Stroker question

while that idea seems correct - the pistons we have been seeing are lighter then the ones from years ago - and that changes things - the question becomes is - the balance was it done by S&S at 60 percent instead of the factory 57 percent - or something else

when it doubt we hand it the engine back if they cheap out and say its fine just install them - its not fine and no one remembers the conversion about the balance when the mirrors keeps falling off
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Old 07-20-2020, 01:13 PM   #12
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Default Re: Stroker question

Quote:
Originally Posted by joe49 View Post
You can lighten pistons that are heavier then what's in it. That is if it didn't shake your teeth out before.
yes, but within close limits. maybe a few grams can be removed before strength is altered. swapping wrist pins can remove the weight as well. tapered bore or thinner wall thickness pins will remove weight, but again, within limits.

Quote:
Originally Posted by johnjzjz View Post
while that idea seems correct - the pistons we have been seeing are lighter then the ones from years ago - and that changes things - the question becomes is - the balance was it done by S&S at 60 percent instead of the factory 57 percent - or something else

when it doubt we hand it the engine back if they cheap out and say its fine just install them - its not fine and no one remembers the conversion about the balance when the mirrors keeps falling off
i'm only guessing here, but do most people think S&S balances each individual engine as needed?
they do what is known as an average balance. for example they weigh up a hundred sets of pistons then average the weights. they weigh up a hundred sets of rods and then average the weights. they weigh parts in bulk and then average the weights. they then balance flywheels to those averaged weights. some engines will be close but they will not be an exact perfect 60% or whatever balance factor you may want or think.

having balanced flywheels for many years, i have had the opportunity to weigh and check just how close S&S balance work is. some are pretty close, some not, they are never exact. again, having done balance work myself i have noticed that a few grams one way or the other from perfect is not really a problem. it's a problem when weights get past the 8-10 gram amount per piston. remember that piston weights are added together so your looking at 16-20 gram range total when figuring a balance factor.

another misconception many may have is that a balanced 45 degree twin with a single crankpin is going to be right at all rpm ranges. no 45 degree single crank pin engine can ever be balanced to spin throughout the rpm range they run and be smooth at any and all rpm, not going to happen.

i know we all have felt different bikes that shake at one speed and then run smoother at other speeds. what's going on here? as a rider, your going to feel vertical pulses of vibration much more than horizontal pulses. horizontal pulses on a v twin like a hd go front to rear. that front to rear pulse will be transferred into and somewhat (never completely) absorbed into the driveline. balance factor differences for the most part change what direction those pulses go at a given rpm. through the years the factory oem balance factor was changed yes. the biggest reason for those changes was because of what average traffic speeds tended to be at the time. bikes built many years ago traveled on mostly rough dirt roads, they run lower rpm than todays bikes run. changing balance factors adjusts the compromised rpm range that an engine is expected to mostly run.

very long story short, keep piston weights close when doing replacement if the engine was balanced to that weight amount to begin with.

if the engine shook more than you liked or thought it should before, balance it but understand, all balancing on a hd will be a compromise adjusted for an average rpm.
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Old 08-11-2020, 10:16 PM   #13
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I see no mention of camshaft. I'd start by looking at current cam and comparing to what's out there.

Then, yes, if you want to tone it down more, go to the lower compression pistons. Stick with a similar piston, and I wouldn't worry about balance factor.

I might experiment with the thickest base and head gaskets available, but I wouldn't do stroker plates just to lower compression.
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Old 08-12-2020, 07:02 PM   #14
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Default Re: Stroker question

Late in the game, but piggybacking on Scott's thought...

I put together a mildly warmed-up flattie, and I feel these bikes certainly do like a little more compression than stock, but I could never find really great totally wild hotrod cams.

I suppose it's just a matter of time before a set is available, but I'm fully on board with all the things said by Mr. McKelvey.
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Old 08-12-2020, 08:13 PM   #15
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Default Re: Stroker question

leineweber cams is the hot cam company of choice if your looking for a real camshaft

everyone in the know with a real drag bike has his cam or they are losing

has a web site forget the shovel cam i think its L6S maybe but his springs are out of control for the street do not use in a street bike
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Old 08-12-2020, 09:43 PM   #16
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Default Re: Stroker question

That's true, John, they make some nutty, nutty cams. But last time I inquired with them they did not make anything for BTSV.

https://www.leinewebercams.com/PG5_KNUCKLEHEADS2016.pdf
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Old 08-13-2020, 03:50 AM   #17
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Default Re: Stroker question

Didn't elfors make his cams i don't remember (i know its a 45)
and i just read that scott is making custom ones ab grind whatever that is?
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Old 08-13-2020, 09:49 AM   #18
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Default Re: Stroker question

yes he did.
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Old 08-13-2020, 10:12 AM   #19
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He put a lot of work into that bike and maybe was dissatisfied with the outcome ?
Bought a Honda ?
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Old 08-13-2020, 04:00 PM   #20
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Default Re: Stroker question

then black widow cams he has tom siftons original prints and the legal rights to toms stuff
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