Piston Scuffing on jnr Dragster

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qkdragster
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Re: Piston Scuffing on jnr Dragster

Post by qkdragster »

chris, I have been building /freshening 25-50 jr engines a season for over 15 years ... first off ... they all scuff , some worse than others... DO NOT open your clearance up to .005 or more (as suggested ) ... on the short skirt JE pistons , .002-.0025 is perfect , On the Wiseco pistons .003-0035 is dead on ....dont go tighter than this though .... your honing stones are way too coarse .. a 360 stone is as coarse as you want to finish , I prefer a 400 for final .....also your cross hatch angles are not correct , move your stones up and down more to achieve 45 degree angles to horizontal... to level of crosshatch will allow the oil to build on the cylinder and can cause oil consumption , but too steep of crosshatch will cause oil to move down the cylinder and causes piston and cylinder scuffing , so correct angles is very important.... when I sleeve a block I run the sleeve down to just above the bearing to avoid any aluminum to aluminum contact , leave a stop ledge in the block with your boring bar so the sleeve has a solid stop when you drop it in..then machine the top to zero deck clearance.... On a BZ block I prefer a .004 press fit, and on a Rhino or PM-1 or similar I like a .006 fit .. 45 minutes at 400 degrees in my block oven and she'll go right in , but dont dilly dally >>> or it'll stick part way down and you better have a 2 lb hammer and a plate of steel ready to drive it in !!!! after boring , leave yourself .003-.004 to hone to size... use a torque plate ... very important,, after honing , wash the cylinder down with wd-40 and a clean rag till the rag stays clean .. THEN wash the block with HOT soapy water , dry and wipe again with WD-40 till your ready to assemble ... I have a lot of customers who use alot of different oils .. I have seen time and time again , that the engines on Klotz oils seem to come in with the least scuffing for some reason .(I am not an oil guy ) ... run an air filter ... not just an Outerwear sock.... but most of all remember , they are all going to wear , thats Alky for you ....if you have any questions lemme know .. good luck Paul @ Wheelie Motorsports Jr Dragster Engines...
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Re: Piston Scuffing on jnr Dragster

Post by qkdragster »

another thing to remember, upon initial start up of the day , a Raptor block , is going to expand faster and more than a BZ, a BZ will expand more then a PM-1 and a PM-1 will open up more than a Rhino... (the thinner the actual block the more and quicker your piston to wall will increase after warm up ) hence open up clearance a little more with a thicker block.. When you get to the track and fire up for the first time of the day , set the idle down LOOOOOOOW, and let the engine warm up SLOOOOOW..... ESPECIALLY ON A COLD DAY ).. remember the piston will expand first from the combustion heat and a thick walled block like a PM-1 or a Rhino will expand alot slower .. then your .003-.0035 clearance will disappear , you have no room left for oil and you will scuff the piston/cylinder ... If you open up the clearance too much , after it warms up , the piston to wall gets out of hand , the piston rocks too much , hits the head or rounds the piston rings off, then your done.. I have even seen the short JE pistons turn over on their side after warm up when setup at .006 cold !!! so set your clearances where I suggested , run good oil and warm up LOOOW and SLOOOW and your engine will last alot longer ....


dont run your ring gaps too tight either ... a ring that butts ends will kill a cylinder , a ring gapped too wide hurts nothing ..... .004 on a Raptor stock bore might be OK , it is not on a 3.540 bore Rhino.... .0012 is better there .. cya
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Re: Piston Scuffing on jnr Dragster

Post by MadBill »

qkdragster, I assume that very basic piston/ring design is rules-dictated, since it's missing a dozen features of modern race engine ones?
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Re: Piston Scuffing on jnr Dragster

Post by robert1 »

We turn ours just over 10,000. It's a kart running 12-15 lap features. If the cylinders looked like what you guys say is normal we'd have to stop half way through the first race and rering it.
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Re: Piston Scuffing on jnr Dragster

Post by nicwoods6 »

Call calico and DLC the skirt and re-install with JE/CV piston at .0035

Or call CP and have them make you one with a second top ring. it will help locate that high pin piston and make it go straight up and down In the bore.


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Re: Piston Scuffing on jnr Dragster

Post by qkdragster »

MadBill , no there are NO rules dictating the piston at all in basic E.T. class racing for Jr dragsters .. there are really no rules at all as long as you use the approved engine blocks, the internals are wide open. Only rules on engines are in the outlaw 330 style engines which are dictated then by bore and stroke size... the others are ruled only by e.t. ..... based on age of the driver ... 12.90 - 8.90 and 7.90 seconds e.t ... with the older kids allowed to go 7.90/..... but basically that is all we have to use on the market at this point, Wiseco used to make all the pistons years ago , then JE got into the market ... all the pistons are set up with 1 compression ring and 1 oil ring, no skirt coatings are std.. I have toyed with using different skirt coatings over the years and it definitely does help, but bottom line most customers dont want to pay for extras like that .. a piston like these in 3-3.500 bore area is $100.00 plus and they seem to think that is bad enough .. LOL.... and yes NIC the high pin location does cause rocking , along with the short skirts , thats exactly why we use the small clearances , NOT the .005-009 some were recommending , thats outlandish in this scenario...and we didnt say that was normal ... we said all engines will get scuffed to SOME extent .... until we get a better piston , all I can do is recommend to the do it yourself rebuilder is to have your cylinder bored/honed by a professional shop to the correct clearance ... with the correct finish ... and run good oil, keep it changed daily... and run a good air filter ... and warm these things up slowly.... the pic in the original posters article shows a poor quality hone job ( honed with who knows what kind of hone , but not the correct one ) ... a situation where there is aluminum to aluminum transfer from block to skirt, and as mentioned , the bottom of the cylinder should be debarred at minimum or chamfered a touch.... we used to Nicasil coat the cylinders on the kart engines and that really helped ... but it also takes a special hone that a do it yourself rebuilder wont have ... of coarse a Kart engine using a Briggs Raptor style block is a crude almost caveman type deal when compared to these thick Jr dragster billet blocks ... they expand so much when you start them up , minimum clearance isnt much of an issue ....and ring seal isn't very good either , but those are a rules dictated class , like the Jr Sprint Briggs World Formula overhead valve engines .. we have had good successs with the Jr sprint stuff , winning almost every race we entered with our house sponsored car this season , I think 19 of the 23 races , including the biggie .. the Tulsa Shootout , down at the chili bowl ... so there is alot to learn with all these different style of engines , each one has its own rules , set ups and problems.. like I said , ask your engine builder questions, if you dont get the answers you want , find another engine builder .... your engines dont have to look like that .In this pic , the engine on the far left thats still disassembled , was one somebody set up with about .008 clearance ..... the skirts collapsed and it rolled over in the bore ... They dont run very well like that .... [-X Wheelie Motorsports Jr Drag Racing Engines ...Image
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Re: Piston Scuffing on jnr Dragster

Post by vwchuck »

So the point of the short skirt piston is to enable you to NOT be able to run reasonable piston to cylinder clearance which in turn cause massive piston scuffing. :?
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Re: Piston Scuffing on jnr Dragster

Post by chris b »

Right, I've found a source for a for a new liner, so im going to fit the new liner next week. The hone marks on that cyl are not mine, this is an engine I have stripped but not built before. I am getting a much nicer finer finish than that, with 280 grit, but I shall order some 400 stones for this on. The options are to either bore to 1/8" from the bottom of the bore, leaving a step for the liner, making shure that the id of the step is still bigger than the piston. The Piston will still come out of the bottom of the liner by 1/8 ", but will be a damn site better than the .7" that I have at the moment. The second option which i prefer is have the liner end a fraction below the piston skirt at bdc, this means there will only be a lip for the liner to meet on one side, (above the bearing). It's a JE piston in it. All the air cooled quads and bikes I've ever built have always had between 2 and 3 thou. 5 seems massive.

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Re: Piston Scuffing on jnr Dragster

Post by af2 »

chris b wrote:Right, I've found a source for a for a new liner, so im going to fit the new liner next week. The hone marks on that cyl are not mine, this is an engine I have stripped but not built before. I am getting a much nicer finer finish than that, with 280 grit, but I shall order some 400 stones for this on. The options are to either bore to 1/8" from the bottom of the bore, leaving a step for the liner, making shure that the id of the step is still bigger than the piston. The Piston will still come out of the bottom of the liner by 1/8 ", but will be a damn site better than the .7" that I have at the moment. The second option which i prefer is have the liner end a fraction below the piston skirt at bdc, this means there will only be a lip for the liner to meet on one side, (above the bearing). It's a JE piston in it. All the air cooled quads and bikes I've ever built have always had between 2 and 3 thou. 5 seems massive.

Chris
If you are saying the liner is going to hang down, I would not do it. The piston skirt hanging means nothing to a point.
The air cooled quads and bikes you worked with in the past do not use the same material as JE and needs clearance...

I remember a shop setting the clearance on a Wiseco piston the same as stock and could not figure out why they were seizing until I told him to read the directions, 2618 and 4013 are 2 different pistons even being forged and need different clearances..
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Re: Piston Scuffing on jnr Dragster

Post by qkdragster »

Chris I always bore down enough that the sleeve is flush with the bottom of the existing aluminum block , the only "stop" I use is in fact above the bearing... you can(and I do " ) go in behind the side cover and countersink a small 10/32 socket head screw and thread it into a tapped hole in the sleeve. only leave it thru a little , when the boring bar goes thru it will leave it flush , , so will the hone, your piston has no skirts on the sides so no issues , and the screw will keep the sleeve from turning if you got the press fit a tad loose ...
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Re: Piston Scuffing on jnr Dragster

Post by Brktracer »

I'm brining this post back up because I'm having a similar problem. This engine has always had this problem (and others) since purchased new from the builder. It now has full ARC rotating assembly, Wiseco piston.
It got hone, rings, rod, piston over the off season. Only has about 20-30 runs at this point. Ring seal is gone - blowing oil out the breather. Cylinder walls scuffed, piston really doesn't look terrible but shows signs of contact and scuffing. Surprisingly, it looks like the problem area is just under the oil ring. Rings look fine.
What is causing this and how do I fix it so it will last?

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Re: Piston Scuffing on jnr Dragster

Post by oldjohnno »

Possibly totally irrelevant but.. a few years ago I had severe skirt scuffing with a big bore two stroke. The problem was exacerbated by the very large intake port which reduced the bearing area of the thrust wall greatly. I switched from a synthetic oil to a castor blend (Shell M) and the scuffing disappeared instantly and totally.
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