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Post by oldbritguy on Aug 27, 2020 20:17:29 GMT 1
A question on a subject which is a common adaptation on the LC motor
When converting a 250lc to 350 (top end swap over) has anyone experienced additional vibration from the motor as a result of the bigger and probably heavier 350 pistons. I am making the assumption that the crank on each model although the same item is balanced for whichever pistons are fitted.
On that has anyone found it necessary to have the crank rebalanced as a result of this? (The motor I have just built for my LC is a 250/350 conversion)
I am also building another 250 motor with a 370 conversion (66mm blaster pistons) from scratch and I am expecting the barrels and pistons back soon. I don't have any 250 pistons to weigh but imagine they will be a bit lighter than the new ones arriving soon with the cylinders.
I might be looking too deep into this but thought I would throw it in here for discussion.
John
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Post by flames on Aug 27, 2020 20:29:21 GMT 1
I had never even thought about that. Are cranks balanced evenly for weight along the whole shaft then? I thought they were balanced each side so as long as the 2 pistons were same size as each other it wouldnt matter?
Are they not balanced without pistons fitted anyway?
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Post by midlifecrisisrd on Aug 27, 2020 20:43:25 GMT 1
Engines were designed as 250's as far as I know so any 350 top end will be heavier than intended
There is not enough mass in the crank webs to offset the rod and piston weight
Hence mine having tungsten weights in it 😏
One thing to note though that I noticed the other week
Picked up a gen std ypvs piston and a Jap TKR std piston
The gen is noticeably heavier
Steve
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Post by tony2stroke on Aug 27, 2020 20:44:20 GMT 1
Your biggest worry there is the pistons being the same weight for either side, I have had different pistons from different batch numbers that have had as much as 9 grams difference, this was on RD200 and made my bike vibrate like crazy, the 250 and 350 cranks are not balanced for 250 or 350 independently but used for either, to the best of my knowledge.
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Post by Tobyjugs on Aug 27, 2020 22:17:12 GMT 1
Ive just checked my notes for this question. I weigh the pistons when i build an engine; Old 250 piston 54.00 mm 140grams Prox piston 64.00 mm 186 grams Vertex piston 64.50 mm 198 grams Wiseco piston 68.00 mm 212 grams Kees's 350cc LC uses standard 4LO crank and my 400cc LC uses standard 350 YPVS crank with 68.25 mm pistons. I can't feel any difference between the two. I will say they both feel a little lumpy around 6000 rpm but 7000+ they both smooth out. The 250cc LC engine just feels more silky. (I like these engines, shame they cant make big HP) I did look into balance factors a bit and was surprised to find out there is a huge difference in percentages depending wether the piston is moving in a vertical plane or a horizontal plane.  |
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Post by Tobyjugs on Aug 27, 2020 22:39:43 GMT 1
Engines were designed as 250's as far as I know so any 350 top end will be heavier than intended There is not enough mass in the crank webs to offset the rod and piston weight Hence mine having tungsten weights in it 😏 One thing to note though that I noticed the other week Picked up a gen std ypvs piston and a Jap TKR std piston The gen is noticeably heavier Steve Do you know what the balance factor you crank uses? |
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Post by oldbritguy on Aug 27, 2020 22:53:54 GMT 1
Thanks guys for your thoughts and although I am no precision engineer, and yes Tony, I would expect some difference in balance between them all given the different piston weights.
As to how much this will affect the running of the motor and vibration into the bars, I think I will just wait and see when I get the thing running. 🤔
Interesting note on fitting additional weights to the crank webb Steve. It will be good to hear what notable difference It makes when running.
This will now be another annoying thought running in the background for a while 😁
John
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Post by tony2stroke on Aug 27, 2020 23:35:09 GMT 1
My problem arose with 2 gen RD200 standard Yamaha pistons with same part numbers, but the batch numbers were different, I did not expect any trouble, but after fitting them the bike started to vibrate badly, I thought that didn't happen before, so I checked the weight to find 9 grams difference, I then removed metal from the clearly thicker underside of the piston crown, put them back in and vibration went away, lesson learned.
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Post by JonW on Aug 28, 2020 2:23:21 GMT 1
As Steve says these engines were always designed to be 250s, so from the factory they are unbalanced but Yam decided that was ok, and time has proven them right as we get a lot of miles between rebuilds. I also have a tungsten weighted crank in my 421 as I figured it would help... but thats all it does. The only way to really fit a 'balanced crank' is to get the pistons you will use, weigh and make them equal, with the rods and bearings. Then put weights in the crank to correctly offset that weight. The problem of buying a generic 'balanced crank' (like I have admittedly) is that its not actually balanced to your setup unless you are very very lucky indeed... but, it is probably closer to being balanced than the old yam 250 crank which is why i got one. I will say that my 421 revs up fast (nikisil cyls tho) and seems very smooth for a yam 2T parallel twin, dunno if thats the crank but it cost enough to have me tell you that it is.  |
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Post by midlifecrisisrd on Aug 28, 2020 10:05:24 GMT 1
Engines were designed as 250's as far as I know so any 350 top end will be heavier than intended There is not enough mass in the crank webs to offset the rod and piston weight Hence mine having tungsten weights in it 😏 One thing to note though that I noticed the other week Picked up a gen std ypvs piston and a Jap TKR std piston The gen is noticeably heavier Steve Do you know what the balance factor you crank uses? Absolutely no idea 🤔 Probably just TSS bollocks 🤣 Steve |
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Post by reedpete on Aug 28, 2020 15:24:14 GMT 1
The cranks themselves with rods are balanced ....and race cranks are lightened and balanced even more carefully.
The pistons balance themselves vertically (Assuming the same weight) but create lateral imbalance.
So 250/350 doesn’t matter regarding the crank but the 350 will create more lateral load.
However, the engine is designed very carefully to ensure the fixing points and one sided dampers allow these Lateral loads to resolve themselves. The harmonics being in the sweet spot when the motor is in the power band. All this and without a layshaft.
When it’s out of resonance in the mid revs then obviously a bit lumpy.
The TZ has resilient mounts so that all this can play out without feeding back into the chassis. The road bike engines are a compromise given the requirement to have bolted exhausts rather than slip joints.
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Post by Tobyjugs on Aug 28, 2020 17:20:36 GMT 1
Not so long ago I was looking at a Kr1 block thinking how well made it looked compared to the yam twin inline.
More compact, balance shaft and huge intake into the crank space. I was wondering if it was possible to mount a TZ monoblock on it.
I digress I liked the fact it had a balance shaft.
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Post by reedpete on Aug 28, 2020 21:50:43 GMT 1
Found what I was looking for...
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Post by Tobyjugs on Aug 29, 2020 0:02:16 GMT 1
Hi Piet that looks interesting but i cant read it can you send me a clear version?
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Post by reedpete on Aug 29, 2020 15:06:45 GMT 1
 This is a little easier to read but if you know how to upload the pdf then be my quest 😁 |
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