KR pistons

Racing K Models

Re: KR pistons

Postby 55panman » Sat May 13, 2017 5:59 pm

Yes, for an amateur racer it's not someone pushing your bike out to the starting line and someone hauling the remote plug in starter and maybe an umbrella girl. It's sitting on your bike with it running, maybe after bump starting it all by yourself in full leathers with MX boots, steel shoe, and full coverage helmet, and waiting in the line up queue for 2 heats or races to run before (maybe up to 5 minutes) in 87 Degree heat with 90% humidity before going out to run. It can be dangerously exhausting. One year at the Sturgis Calcutta, I think 1995, in July it was 100+ degrees. I got dizzy and went down on my Trackmaster Triumph. I had a heat stroke. I was lucky. I was at Blackhawk Farms for a AHRMA road race around that time and a fellow rode right off the end of the main straight into the woods and crashed. He was killed. They figured he had a heat stroke. I used to have to bump start my 1977 XR750 by myself before a race as I said in full leathers with a steel shoe and a crash helmet. The fact I'm a good sized guy helped. Needless to say the bike was always in top tune. It would get pretty exciting in a small tight pit to get it stopped after bumping it without hitting anything.
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Re: KR pistons

Postby GuS » Sun May 14, 2017 6:38 pm

Patrick, Panman and Ambike.
Tanks a lot for you answers and help.
You're right about the smaller diameter, it was the top groove that lead me to believe the crown 1/2 way down to the top ring was narrower.
I will never be able to collect the parts needed to get an autentic bike. This not my intension. Plan is as said to get it on track to have fun, and along the way find out as much I can what i got, and assemble it to right specs. So far it has paid off, as the valve timing was completely wrong.
To the radius at the bottom. I see the K/KR pistons in Jerry Branchs engine presented in that article also are radiused at bottom.

The Harley competition racer booklet states 0.007" piston to cyl clearance. In my notes I have noted the Dow metal piston should have 0.016"

I learned from the late mr Steph at fhp board that themperature difference between piston/cyl was 100 deg celsius. Thus boiling a piston in water, then measure the change in size. Has helped me if I didnt know the piston manufacturers spec,and to check pistons befor I bore. I ignore that measurement once and bored for T&O pistons avcording to their specs.... They seized forst run...

Also if you measure the cyls before and after torqing to a stress plate, you will find that this alone will steal 0.001" ( reference is wl cyl with narrower base than th Ks) . But if you ensure correct torqe when boring/honing, can control mixture (not lean) and conrol the ignition timing, you may reduce the clearance and give the cyls a longer life. As a note i run Venolia (forged) with their specified 0.007 clearance in my wl stroker. They make a lot of noise before hot.

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Re: KR pistons

Postby 55panman » Sun May 14, 2017 10:10 pm

I was at my local dealers when the mech. bored my 1981 Sturgis cylinders, around 1985. He showed me the bore before bolting on the torque plates and after bolting the torque plates and it changed by .002. Evo alum. cylinders are like rubber. They stretch by something like .010+ when warmed up.
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Re: KR pistons

Postby EKHKHK56 » Mon May 15, 2017 12:06 am

Most pistons need specific clearance depending on material, manufacturing process cast or forged and lifestyle. Cast pistons grow less than forged at 200 degrees or any temperature. Forged pistons always need more clearance than cast. Depending on type of racing the number goes up. Take ROSS pistons which are the only ones I used except Keith Black and factory in my racing water cooled Chevrolets. ROSS has a chart for piston to wall clearance with the same Forged piston. There are 5 increasing larger clearances starting with Street and ending with Nitro Top Fuel. Circle Track is next I think. Bore size aIso figures in. I run the K Superior Pistons at 1.5 with perfect results gorgeous street. In circle track racing with extended heat cycles you have to loosen it up some to cut friction and heat. Especially if you don't have time to break it in and have to race with minimal break in time. 25 laps running 6,000 rpms allows no room for tight parts to get a break. And it's a vicious circle, the pistons overheat from tightness and tighten up from heat :shock: I always ran an extra 1-2 thou wall clearance over street specs with my stock car, late model junk yard motors so they would be happy right away with extended high rpm running. Castrol GTX. There my secret oil.....
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Re: KR pistons

Postby 55panman » Mon May 15, 2017 10:55 am

Oh Yeah, I didn't think to add that note before. My dirt track engines never get broken in. I live on edge of city limits but still houses every acre. I fire up on frontage road and run up and down 2 blocks going fairly hard through the gears 8 to 10 times. Check head and cylinder bolts. That's my break in. So piston clearance is loose. Go to flat track race run a few moderately hard practice laps and it's ready for the heat races. Oh, my neighbor has a Pro Mod '68 Camaro and 4 houses down is a Modified Stock car racer, so neighbors not too uptight about noise as long as in the day.
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Re: KR pistons

Postby mikeslemmon » Mon May 15, 2017 11:27 am

cylinder expansion is less at .the bottom so a little taper is not necessarily a bad thing
.

a stress plate may not replicate a hot head or a warm set of cases

a lose piston won't hurt the barrel
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