KR pistons

Racing K Models

KR pistons

Postby GuS » Fri May 12, 2017 4:19 pm

Hi all Im new here.
I have a mix and match bike with the major parts being KHR and XR parts plus some other parts of known and unknown make. Am at present doing a very slow process of finding out what I got, with a long term plan of getting the bike on the track.
Hope I can share my findings here, and I can get som help identifying the various parts.
First question is regarding pistons.
In Gordon Jennings article from June 65 on Jerry Branch and KR tuning, there are pictures of what is said to be one original and one modified pistons.
Both the original and modified piston seem to have narrower diameter at the dome going half way down to the top ring. Jerry states in the interview he is using standard K pistons as they are lighter than the KR pistons. All K/KH pistons I have seen are only aftermarket stuff and they are all straight with no smaller diameter at the top.

Link below is to Beauty of Speed webpage where this picure can be seen
http://www.beautyofspeed.com/data/doc_cycle-world_jerry-branch/cycle-world_1965_jerry-branch_03.gif

I posted the question above at FHP teck talk, got some answers, but none had knowledge of such piston design (Im not talking about the chafered crown modified by Jerry Branch, but the narrower diametre crown on both pistons in the article). I recently pulled the cylinders off, and found a piston that look excactly like the one claimed to be a standard K piston. Interesting is there is not only a narrower crown, but there is an additional groove going round the dome. A groove similar to a ring groove, narrower only 1.15mm/0.045 inch wide. Studying the Gordons article once more, Im sure those pistons got that same groove.

Question is if any of you got knowledge of this piston? Is it a standard K piston or a racing piston? See image below.

The 1955-62 K model parts manual lists pistons with rings 22251-52 replaced by 22251-52A
The May 1954 Dealer net price list for Competition motorcycles lists
22101-48R solid skirt alum for WR, KR and KRTT,
22251-48R solid skirt Dow metal for WR, KR and KRTT
22101-54R solid skirt alum for KHRTT

Image

Image


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Re: KR pistons

Postby thefrenchowl » Fri May 12, 2017 5:05 pm

1) Nobody remembers what made a KR piston different from a K/KH piston apart from the groove

2) Don't loose sleep over it and use K or KH pistons, they are around...

3) What you think is a stange smaller diam at top is in fact the right way to make a piston, smaller at the top where all the heat and expansion are and wider at the bottom where it's much cooler. All K, KH and KR pistons have a smaller or bigger 45 degree chamfer on top so that flame front travels faster behind the top ring to seal it.

4) My KHK had OEM pistons with grooves when I got it, assumed KR ones, but can't see anything different with std pistons...

5) By using deduction, I can tell you with certainty that KR pistons are shaped like K pistons, no radius on bottom of skirt to clear the flywheels. Yours have, so are intended for KH stroke, hence likely KHR pistons if you believe groove on top is specific to race pistons. I'm not that sure so won't say they really are...

6) If one is building a mongrel motor, bit more or bit less mongrel won't make much difference...

Pick up your best bits, assemble them and enjoy the ride...

Patrick
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I'm the one who has to die when it's time for me to die so let me live my life the way I want to...
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Re: KR pistons

Postby 55panman » Fri May 12, 2017 6:16 pm

If you're talking about the article with Branch where he actually cut the dome down at an angle running toward the combustion chamber?? Don't do any fooling around with the dome leave it as is. Also be sure you fit the pistons loose enough especially if you do want to put it on the track. Patrick what do you seat piston clearance .005 to .006?? If you set them at modern clearances they will seize. Ask around for some more guidance on that.
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Re: KR pistons

Postby 55panman » Fri May 12, 2017 6:18 pm

Oh, and there was some theory on that extra groove at the top. I'll have to search the recesses of my memory to see if I can remember where I saw it. I have a few pistons like that.
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Re: KR pistons

Postby thefrenchowl » Fri May 12, 2017 7:23 pm

I run my methanol burning supercharged KHK with Superior pistons at just 1.5 thou clearance, never had a hint of seisure...

All the groove pistons I have on the shelf (4) are full of very hard carbon deposits... My guess is they don't do much...

Patrick
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Re: KR pistons

Postby 55panman » Fri May 12, 2017 8:48 pm

Patrick, that a alcohol burner which runs a lot cooler. I think most dirt track guys said to run a lot looser. A Kansas or Ohio 1/2 mile in the middle of July gets awfully hot with about 100% humidity.
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Re: KR pistons

Postby thefrenchowl » Sat May 13, 2017 4:25 am

My KHK engine was build as a street engine 1st, with twin DC carbs, with these clearances.

Image

Engine had about 3000 miles of street use before I methanoled/supercharged it.

I ran all my sportsters the same clearance with cast pistons.

Only exception is the XLRTT which has S&S forged 3700 series and I put 3 thou in there.

I know everybody says cast and forged need more than that but I tend to disagree with all of them!!!

Patrick
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I'm the one who has to die when it's time for me to die so let me live my life the way I want to...
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Re: KR pistons

Postby ambike » Sat May 13, 2017 4:54 am

Yes, the reduced diameter at top is correct & effective.

The relieving should aid gas pressure to expand the compression rings. As installed, ring tension alone does not provide the sealing forces.

In a flat head design, the groove might create a path for the incoming charge to offer extra cooling.

I've seen two-stroke engines with secondary transfers extending below the ring set. The idea being such port geometry directs a blast of cool premix onto the ring(s) from two sides.

Patrick : What type of oil were you running ? Castor ?
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Re: KR pistons

Postby 55panman » Sat May 13, 2017 10:51 am

Patrick, thanks for sharing your experience and observations. I will definitely try talking to more guys that have built KR race engines before I put mine together this year. I've always leaned to looser piston clearance on my dirt trackers with never a problem or oil burning problems. Gus said he plans to put it on the track. As I said a Midwest dirt track race gets VERY hot in the middle of the summer. And depending on rider attendance and the organization you can spend a lot of time idling in the starting que with the motor idling. Those cast iron cylinders get awfully hot. Even my XR. After a couple of heats and races I'd have to make sure my left leg was well away from the engine or I'd burn my leg trough my leathers. Not at all like street riding.
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Re: KR pistons

Postby ambike » Sat May 13, 2017 4:11 pm

Around here, I get the impression few have raced much, let alone with big-bores on longish dirt tracks .

Racing is a lot different than hauling-ass down the freeway WFO dodging POS mini-vans full of drunk illegals with 20 filthy keeds.

I stopped doing events long ago, but the comments re waiting in the queue on a blistering day ( getting stuck actually ) are on the money.

In North Texas, our favorite half-mile known as The Devil's Bowl, will live up to its name.

At this stage of the game, I'd recommend riders wanting to compete on two wheels to focus on road racing. Run the old iron, but include at least one modern bike not requiring constant tinkering like a whining bitch. Yeah, leave her at home to mow the grass. Name-brand tracks are faster, cleaner, safer, better attended, and easier to run faster. Costs more in equipment and preparation, but at the end of the day very satisfying. Bonus = On pavement the bike stays clean.
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