| Author | Message | ||
     Berkshire |
Saro, you 'da man!!! I hate that expression, but it's just too appropriate here. Well done! Do you think your friend could supply the deck height numbers? With (rod length + stroke/2) being identical for both XB9 & XB12 (and assuming same cases & jugs), it seems they should have identical compression height. This points to the blast having a 0.168" taller deck, and me needing a 9 piston instead of a 12...  | ||
| Ezblast |
So if you needed lower comp for say a turbo - a XB piston may be the ticket - but we are still searching for a non aftermarket hi comp alternative? EZ | ||
| Ezblast |
OK - so my grey hair is really blonde - lol I ment visa versa - that a Blast piston would be low comp for an XB, and the 9 would be hi comp - how high is the question? EZ | ||
| Gearheaderiko |
While I'm much too tired to digest most of this right now, I do have to say that the Blast and XB combustion chambers are very different. You may be able to use a flat top 'Blast' piston with an XB head for low compression, I doubt you'd be able to use an XB piston with a Blast head for high compression.  But the rest of this stuff is starting to make sense!I think I'm waking up a little... It should be noted that it was mentioned about my checking clearances, clay, etc. when I put my Blast together. I didnt because the whole point was to have everything "off the shelf". While normal assembly was done, checking piston ring gap etc.,there wasnt much point in checking everything else since it was supposed to be a 'stock' parts swap. Must sleep now, work tomorrow.... Discussions like this is why I love Badweb! | ||
| Ezblast |
Has anybody checked - if only a tad of milling the piston top is involved... EZ | ||
| Berkshire |
Okay, it turns out this was a case of somebody having a brain fart on the day I ordered - probably preoccupied and/or sleep-deprived getting ready for Sturgis drags... The CORRECT part, an XB9 10.5:1 piston, will be sent out in the morning. EZ, an OEM XB12 piston might work for a turbo blast, but an aftermarket forging with reverse-dome would be a much better choice. A Blast piston dome would probably hit the flat squish areas on an XB head! You could put an OEM XB9 piston into an XB12 (well, you'd need two of them, lol) and get 12:1 compression, or XB12 pistons in an XB9 with a turbo ...assuming there's a place to PUT a turbo on an XB. Erik, assuming the vendor sends you the correct parts, it SHOULD just be a bolt-together deal, with the exception of quench clearence. IIRC, quench clearence of 0.025" or less runs a high risk of piston/head collision. The tighter it is, the more effective it is, and somewhere around 0.060" it pretty much stops working. OEM's want things to just bolt together (interchangable parts), and they know all "identical" parts aren't EXACTLY the same, and they REALLY want to avoid piston/head collision! So they aim for quench clearance on the "loose" side. As a result, a few "tight" motors will have good quench, most will be mediocre, and a few "loose" ones will be more likely to have detonation issues. Aftermarket pistons will usually have a slightly taller compression height in order to close the gap a little, but most likely you'll need to assemble without a base gasket if you want to get it down to a sweet 0.030", and you may also need a thin head gasket and/or a little milled off the top of the cylinder. Nice pic! Is that a stock XB9 piston? Here's a 10.5:1 XB12 slug to compare:   Dome height is only 0.085", and dome minus valve relief net volume is reported to be +3cc. | ||
| Fast1075 |
I really like the quench area on the XB head...but I like the open chamber of the Blast head...less shrouding. Pretty piston. WARNING!! UPCOMING TECH for the experience disadvantaged who may be contemplating their first engine build. All of the combustion area surfaces of that beautiful piece of aluminum need to be thouroughly deburred leaving no sharp edges...before it goes in the motor...sharp edges get glowing hot..glowing hot=detonation..bad on gasoline...deadly on nitrous or turbo. A piece of sandcloth and a scotchbrite pad will take care of that in minutes...do not do anything to the ring lands or skirt area. | ||
| Ezblast |
It occurs to me that the Blast connecting rod must be quite long in relation to the stroke. I think the the ideal rod length for a high revving 3.125" stroke would be somewhere around 5.75", but that might take some flywheel clearancing, which would mean lightening, which would also be good for revs. The shorter connecting rod would weigh less, as would the shorter pushrods - again, reduced reciprocating mass for better revving. The cut-down cylinder would drop a few ounces too... Combining this with the previously proposed larger cylinder/head bolt pattern for a 4-inch bore would be an obvious choice. The answer to this is the RR piston/cylinder at 4.15 - Brian N. said it would be a simple matter of filling/welding in the old mounting holes and with just a tad of offset - re-drilling,tapping and boring for the larger cylinder, since there is no rear cylinder clearance to worry about. If your tore all the way down, this could be a fun way to go. EZ | ||
| Gearheaderiko |
XB9 '10.5' CP piston, .015 over. Blast piston is also 10.5 CP piston .015 over. "Erik, assuming the vendor sends you the correct parts, it SHOULD just be a bolt-together deal, with the exception of quench clearance. IIRC, quench clearance of 0.025" or less runs a high risk of piston/head collision. The tighter it is, the more effective it is, and somewhere around 0.060" it pretty much stops working." I'm not sure if that statement is discussion or trying to enlighten me?! Again, the whole original point of throwing on an XB top end is 'off the shelf' bolt on performance. Not blueprinting or obsessive parts matching like a high$$ racebike. Just Average Joe bolt on. If you rebuilt a stock Blast (or XB) engine you'd hardly spend the time checking all the things you mentioned. Rebuilding the same engine for stock class timed trials would be different entirely. Parts that were not 'stock replacement' were checked for clearance as well as parts matching to be sure new parts sent were correct. The XB12 piston sure doesnt have much of a dome. Is that an HD stock piston or? | ||
| Berkshire |
That's a 10.5 CP piston Fast, have you seen heads with the 15-degree machined quench areas? Similar squish area, but without the shrouding. Some of the edges on the piston (above) look sharp, but that's just the picture. It was deburred and pretty much edge-free as delivered, with radiused corners and chamfered edges on everything - feels smooth, like they finished it off with a buffing wheel or something. There are some ridges around the reliefs where they meet the flat top. They kind of rise up to the dome, looks like the dome was oval shaped originally and cutting the reliefs pretty much took out the ends of the oval but there are four slivers of it still left - I think they look like they'd kind of shroud the valves, but that would only come into play near TDC. If the XB9 piston has them I may dremel them off. Erik, That was just general information to show the reason a person might want to check piston/head clearance. There's no reason to check it if rebuilding a stock Blast motor because the piston & head are mismatched anyway. If rebuilding a stock XB motor, yes I would check it - first, to make sure both cylinders were the same, and also because it might turn out that I could easily have tighter quench, which might make just enough difference to allow me to use mid-grade fuel instead of premium for commuting, or better yet, run leaner and/or kick up my ignition module to the next advance curve for more power! Also, the compression ratio would be increased, again giving more power. There's an additional reason for checking it when doing an XB-headed Blast... the Blast is weird - taller deck, longer rod, and originally equipped with 15-degree pistons and a 10-degree head. Hopefully the difference in deck height is *exactly* the same as the difference in rod length, so the piston would come up to the same place in relation to the head as it would in an XB - but considering the piston/head mismatch, it seems likely they would have put in a little extra deck height so the piston wouldn't hit the head. If that is actually how it's built, then corrective action will be needed to get the same compression & quench the same piston would give in an XB9. I understand your point about using a bolt-together factory top end, but with a different bottom end some things may be slightly out of spec. Also, remember - this is the same factory that put the mismatched head & piston together and said "close enough!" LOL | ||
| Gearheaderiko |
So, I garner from all this is that XB, XL and P3 pistons are all interchangeable, except that a matched head is needed. The differences are taken up by con rod length and block 'deck' height. I'm inclined to believe they got it exactly the way they wanted it, not just close (though I know how engineers like to use terms like, close enough, approximately, give or take, etc.) What isn't in question is that an XB9 top end (cylinder, piston and head) can be directly grafted onto a Blast engine (I've done it, as have others and with cams its good for at least 7hp over stock) we just now know why! "I understand your point about using a bolt-together factory top end, but with a different bottom end some things may be slightly out of spec. Also, remember - this is the same factory that put the mismatched head & piston together and said "close enough!" Does HD only use engineers straight out of community college, with a barley passing grade, on a sports scholarship? I disagree. That statement indicates that maybe Blast parts are machined on a bench grinder and everything is 'close enough' if it goes together. Parts bin bike is more accurate than 'mismatched' head. 70mpg indicates something was done right! The XB9 top end bolts right on and as with a stock XB, P3 or XL there is room for improvement when it comes to strictly performance. And there is a ton of info on Badweb for doing just that! | ||
| Berkshire |
It's not lazy engineering, it's just manufacturing efficiency. Higher precision means higher cost, and they have to draw the line somewhere. There is no such thing as "exactly" in engineering or manufacturing, it's just a question of how many decimal places are needed to get the job done. Let's say they specify the acceptable tolerance for cylinder height, deck height, and rod C/C length to be +/- 0.0005", then they'd probably end up throwing a LOT of parts in the recycle bin, and that equals a lot of lost money and finger pointing! A tolerance of +/- 0.005" would reduce production cost and waste, but when you add everything up that means that there *could* be up to 0.030" difference between one motor and another. In that case, and assuming minimum safe clearance of 0.035" (and ignoring gaskets, etc.), they might design for 0.050" nominal clearance, so motors produced would end up having anywhere from 0.035" to 0.065" actual clearance. This is a much more "plug-and-play" way of doing things, which is what a manufacturer wants. With a little luck, the parts will usually be closer to the ideal measurements than to the maximum allowable deviation, but the point is that there is going to be some variation. In the case of the "close enough" Blast head/piston combo, it has nothing to do with manufacturing quality - again, the issue is cost. They already had a proven part "in the system", so why not use it and cut out all the engineering, documentation, and endless memos and chit-chat that it takes to get the simplest thing done at a big company. With the intended compression ratio, quench effect wasn't needed so it didn't matter if the piston didn't match the head - it was close enough. | ||
| Gearheaderiko |
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| Bandito90 |
I've not posted in a long time,But here is where my project stands.I've took the rocker off and clearanced it a small bit.I'm going to take a compression test and air leak test as I seem to be getting a ton of blow by.I had originally had to send the 515 cylinder back as the piston would barely go in.I got it back assembled the bike very carefully using the manual.It seemed to be all right,but now it just blows air and oil out the breather.I'm changing from a modified air box to something smaller soon.Any suggestions on what to do?If I have to I'll pull the top end down have the cylinder checked and possibly have it bored locally and just put an over sized piston in. | ||
| Gearheaderiko |
515 is an oversized piston (.063 over) and I dont believe they make one bigger. Do you have the rings in correctly (not upside down)? Is it possible oil has filled up the crankcase from sitting? Did you change the oil pump drive gear? Was the gear in very good shape if you didnt? Are the cams in correctly? Lifters pumped up? Stock or adjustable pushrods? Compression should be up around 200 psi. I'me sure I'll ponder this all day and see if I think of anything else! Let us know how it works out. | ||
| Bandito90 |
The rings were installed by nrhs and the drive gear appeared to be fine.I'm still running the stock push rods and lifters,But want to replace all that eventually.The cams are installed correctly to my knowledge.Trust me I took a long time to line the marks up. | ||
| Gearheaderiko |
Please dont get me wrong,I dont mean to insult, just looking for answers. 1st: Whats the compression test at? 2nd: If you didnt change the oil (or did), drain the crankcase (small allen plug-DONT overtighten). About a half quart should come out. If a lot comes out, thats why your blowing oil.A lot also may come out if the bike has sat. Drain the frame and crankcase and refill with 1 1/2 qts. 3rd: Did you change or modify the valve guides. If not then there is likely interference. If you used behive valve springs then the rocker box clearance is good. If you used traditional high performance springs, I hope you left enough clearance. If any of these things werent done correctly, then the valves may be hanging up (or not opening) due to various issues and theoretically you 'll blow and suck almost as much through the crankcase as the head. Did it start? While NRHS is reliable, everyone has a bad day (I've had minor problems with them), so thats no guarantee the rings (or piston) were set up correctly  | ||
| Bandito90 |
I modified the valve guides.And also clearanced the rocker.I will drain the oil again.I did make the mistake of over filling it after it sat a week or so.I did the first few starts as described on the nrhs site,Everything seemed to be fine.I'm going to borrow my buddies compression tester this week and check that first and drain and refill the oil and go from there. | ||
| Buelliedan |
Bandit, I noticed you said you modified the valve guides. what and how did you do it? Have you checked your guide clearnces? You still have stock heads with oversized springs correct? As for your rocker boxes. Did you install new umbrella valves in them? Did you make sure they are installed with the umbrella valves by the breather holes? (Message edited by buelliedan on October 07, 2008) | ||
| Bandito90 |
I modified the valve guides using the instructions on the top of the thread.I forget what the clearances were,but they are within the specs of the procedure at the top.The is all stock except for the springs and the seals.I'm eventually going to get head work done.As for the umbrella seals I'm not sure what your referring to,But I'm sure I did not replace them. | ||
| Buelliedan |
This could be your problem. In the center section of your rocker boxes are whats called umbrella valves. they should be replaced about every 2 years and if you don't re-install the center section correctly you will have TONS of blowby. | ||
| Bandito90 |
Thanks,I'll refer to the manual and replace it as the bike is a 2001. | ||
| Reuel |
Umbrella valves? Never heard of them! Could that affect gas mileage? Going to get my manual ... | ||
| Buelliedan |
I am feeling like a total dumba$$ right now. Blasts do not have umbrella valves in them. Disregard everything I said please. One thing that can help is to get your breather out of the air cleaner vacum. I took mine and routed it to a small filter which helped with blowby. (Message edited by buelliedan on October 07, 2008) | ||
| Fast1075 |
I have yet to actually see the inside of my motor...how does the crankcase ventilate in so far as the actual path for the gasses. The piston displaces air in and out of the crankcase and ultimately through the "pcv" and vent hose. It would seem to me that like a old school smallblock chevy if anything interferes with oil drainback from the top end,it would fill up with oil and oil would be blown out the vent...it would be worse of course if there was blowby down the cylinder... Curious minds want to know...my motor pukes absolutely no oil from the vent, even though with it idling, there is a good deal of aspiration through the vent tube....guess mine has really good drain back. Is there a baffle of some sort? In any case, I will be installing my cams this weekend and will learn a little bit more about the motor then. | ||
| Ezblast |
Which cams? | ||
| Reuel |
Oh, crap! I already ordered those umbrella valves! Now I have to cancel my order. I think they've already charged my credit card. Kidding!!!  | ||
| Bandito90 |
Well now I know why it was not mentioned anywhere in the manual when I had the engine apart! | ||
| Fast1075 |
The cams are B-50's...i bought em brand new on e-bay for 81 bux  | ||
| Ezblast |
You should be fine - take a look at your oil gear while your in there - replace with SE edition if it looks at all iffy or just because you can. EZ |
