Pistons

[King]

Iwill be doing a rebore (from 40 over to 50 over) on my 51FL cylinders and fitting new pistons. A couple of questions. Are the low compression 7.5/1 ones preferable over the standard 8.5 /1 ones far as ease of starting is concerned and what brands do you use?

Thanks

King

[pan620]

I personaly would use the higher comp. ones,low comp. = low power,starting should not be a problem.I have had good luck with Dixie Dist. pistons,nice price also.

[Panacea]

Hi,sounds like your on the right track with stock compression, I tried a couple sets of wisco pistons trying to get more power out of my 49, and I did notice increased power. Problem was with forged pistons the expansion rate is too dissimilar to cast cylinders. In order to fit them at 2 thousants ,warming up the bike completly before thrashing on it was quite neccisary. The second set was set up at 3 thousants clearance, which seemed to last a bit longer but still not long lived. MW

[Rammy]

Any body tryed the KB hypereutectic pistons , they say that they can run very little clearance . the add in V-TWIN sounds good?

[Cotten]

Nearly any cast piston out of any Tedd, CCI, or whatever distributors' box will most likely be the Dixie offering (I believe made in S. Korea.)
They are very high quality.

(Beware of generic rings, however.)

It is hard to justify the cost of a forged piston unless you intend to build a fire-breathing monster that will be intentionally punished to the brink of disaster constantly. Just the re-balancing is enough to make you scratch your chin. But it's only money.
I have never installed any KB's.
But my late-model associates had no problem shattering their skirts. It is exceedingly rare that a piston would be the weakest link in a motor.
Compression vs. ease of starting is a personal decision, depending upon your mass, vigor, and past injuries. Although I preach stock design, a mild cam with some overlap allows an advantage when kicking. Geez I miss real Siftons.

..Cotten

[mbskeam]

I'm runnin KB's, these are the 8.5:1 pistons and it kicks over fine and has about 6000 miles on them and they are holding up okay.
So Cotten......
I've read your shattering comment before on these pistons what up?........justa wundering

mbskeam

[1950bobber]

I TOO am running a close tolerance KB set in my early Balance Master's flywheel'd Shovel pushing 81-82 cubes (shaved the heads ten thousands). Broke her in easy but kept my rev's above 2000 for the first 1200 miles or so inorder to keep these babies from wobble'ing up the cylinder wall at low R's! Seems to have have worked well so far....I believe if you break your bike in correctly and do so for the first 2000 miles (that's if you did the bottom end as well as the top end), you'd avoid the damage on the piston and the cylinder wall making a broke in scoot a long-gev-ity motor! But make no mistake, close tolerance KB's can break if you're hard on her initially. I know....I screwed up a set before I learned that lesson....$$$$$ and a lot of work later!
Jim in Seattle "1950 Bobber"

[fourthgear]

I too have KB's and so far I have only about two hours run time on them ( scoots on a stand , rebuilding front end ) Mine are 9.5.1 and they seem to rev a little faster and now ( after mbskeams fix on my dist. ) start just fine. I'm not sure if its the Crane cam or the pistons that let it rev. so quickly, but its real crisp, all new rebuilt motor top to bottom my be why too?If you take a KB piston and a stock cast in hand you will notice a big diff. and we all know lightening reciprocating parts reduces strain on bottom end components. Welcome to the twenty first century!

[Cotten]

I'll defer to Jim in Seattle's experience with KB's as my vintage customers had no reason to require them. (Please remember also that Liberty stopped taking in motorwork some time ago.)
And my latemodel buddies in the business strive for max'd horsepower, with longevity dismissed to an afterthought. Still, splitting a skirt with no other apparent cause than harsh break-in is a catastrophic failure that raises some application, design, and materials questions:
Specific to vintage machines (or even later models if they still have ballchecks in their pumps?), a motor that is started with excessive oil in the sump will endure enormous hydraulic pressures until the sump is evacuated.
A blast of oil forced past the skirts and rings to foul the plugs is all too common,...yet cast (and of course forged) pistons survive this wedge of un-compressible liquid over and over and over. If a eutectic alloy has its limits in this area, where is the logic in spending extra megabucks?

....Cotten

[1950bobber]

Hello again! I'm no wizard mechanic and my experience with KB's as I stated, is with a very modern motor. My "balanced" Shovelhead is S&S (rods,jugs, cases), Jims (pinion, sprocket shaft, crank pin), Andrews A grind, CV carb with increased flow headwork. Like I said, she was broke in GENTLE and was babied on break-in as well as rode gently with occasaional higher revs to speed but NO RACING or abusive treatment. The KB's have held up. Are they for a torque monster or a hard ridden motorcycle? That I can't answer...but an appreciated, well cared for motorcycle like my Shovel....then so far I can say no woeful tales....just break her in right, that means 2000 RPM lower limit gear changing for at least 700-800 top end miles...my experience with KB's is that if the motor is lugged or if R's are even a bit too low, the "wobble" created by the not-yet-seated piston could damage the skirt and most likely the cylinder wall. If I were doing a Panhead, I'd stick to cast or forged pistons....I'd find them more dependable and less fragile. Time will prove me wrong or right...KB's are still relatively new and I'd want to see the testimonies on older iron before I'd use them there. Shovels are a bit more sophisticated than our Pans, I believe. And AGAIN....I'm FAR from an expert and have just this limited experience as mentioned. Good luck to all who use them on older iron and keep us updated....sincerely!
Jim in Seattle "1950 Bobber"


I forgot to mention...my Shovel's ratio is approximately 9.5-1. It's just a Shovel you know!!
Jim in Seattle "1950 Bobber

[caschnd1]

I'm running the Wiseco 8.5:1 pistons. Very easy starting. About 15K miles on the motor now and going strong.

-Craig

[Cotten]

Hey Jim!
By "deferring" to your experience,... I was agreeing with you!

...Cotten

[1950bobber]

Cotten...

I took it that way! no problems here!!!
Jim in Seattle "1950 Bobber"

[48moon]

On the subject of pistions...

The guy rebuilding my EL panhead motor just installed new (Dixie) pistons and found the skirt on the front piston just touched the flywheel locking it up. He switched pistons (front to back) and found the same thing. He could only assume the front rod is slightly shorter than the rear one. He also was wondering why 61" pistons have no scallop at the bottom of the skirt like 74" pistons do. As this would have avoided this problem.

Any thoughts?

[Cotten]

My first suspicion would be that you have a latemodel rod which are .030" shorter (post-'73?), but that's not much.
Did you have the rods straightened and aligned after rebuilding them?

...Cotten