andys2
L plate rider.
Posts: 41 
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Post by andys2 on May 12, 2014 7:55:39 GMT 1
Can anybody tell me what the differences are between the 250 and 400 crankcases? I allready know that the 400's are rubber mounted, so if I had a 250 bottom end, could it be rebuilt into 400 crankcases to make it rubber mounted?
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Post by arrow on May 12, 2014 19:37:54 GMT 1
Are the 400 cases deeper around the crank area due to the longer stroke?
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Post by howard on May 12, 2014 20:28:06 GMT 1
All 400 crank cases are also rubber mounted , 250 are solid .
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Post by spondon440 on May 12, 2014 21:28:43 GMT 1
No
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Post by muttsnuts on May 13, 2014 13:46:38 GMT 1
no, you can't use a 250 crankcase for a 400 engine, the crank is much bigger on the 400, I also seem to think that the hole for the barrel sleeves to slide into are bigger on the 400 as well, but if you have both you can easily check that rather than relying on my memory which is like an etcha-sketch these days !!  |
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andys2
L plate rider.
Posts: 41
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Post by andys2 on May 14, 2014 19:34:20 GMT 1
no, you can't use a 250 crankcase for a 400 engine, the crank is much bigger on the 400, I also seem to think that the hole for the barrel sleeves to slide into are bigger on the 400 as well, but if you have both you can easily check that rather than relying on my memory which is like an etcha-sketch these days !! That's not good. Here's the reason I'm asking, I now own this bike:-  It's an aircooled 250 bottom end with 350LC barrels. The bottom end is solidly mounted to the frame and the vibrations are horrific, I was hoping that I could simply rebuild the bottom end into a set of rubber mounted crankcases. Looks like I may have to go full 350LC engine and change all the engine mounts on the frame  |
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Post by muttsnuts on May 14, 2014 19:54:22 GMT 1
why not machine the crank cases so that they will take rubber mountings ?
the good news though is that 350LC barrels fit the 400 crankcase as the sleeves are virtually the same, you just need to trim a little off the bottom of the sleeves - see my turbo 2 stroke build thread as that is what I've based my turbo engine on, although it is highly modified and a one off engine, the barrels are from a 350LC, but has a set of "one off" sleeves fitted
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andys2
L plate rider.
Posts: 41
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Post by andys2 on May 17, 2014 10:59:29 GMT 1
I'm 99% certain that there is not enough meat on the lugs to machine them for rubber mounts, they seem to be much smaller than the 400 ones.
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andys2
L plate rider.
Posts: 41
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Post by andys2 on May 19, 2014 18:30:13 GMT 1
no, you can't use a 250 crankcase for a 400 engine, the crank is much bigger on the 400, I also seem to think that the hole for the barrel sleeves to slide into are bigger on the 400 as well, but if you have both you can easily check that rather than relying on my memory which is like an etcha-sketch these days !! But I'm going to use the 250 crank in the 400 cases, I've checked and the crank bearings are the same for 250 and 400 so I don't see why it shouldn't work?? |
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Post by arrow on May 19, 2014 19:02:28 GMT 1
no, you can't use a 250 crankcase for a 400 engine, the crank is much bigger on the 400, I also seem to think that the hole for the barrel sleeves to slide into are bigger on the 400 as well, but if you have both you can easily check that rather than relying on my memory which is like an etcha-sketch these days !! But I'm going to use the 250 crank in the 400 cases, I've checked and the crank bearings are the same for 250 and 400 so I don't see why it shouldn't work??
Reduced crankcase compression perhaps? |
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Post by muttsnuts on May 19, 2014 19:50:20 GMT 1
But I'm going to use the 250 crank in the 400 cases, I've checked and the crank bearings are the same for 250 and 400 so I don't see why it shouldn't work??
Reduced crankcase compression perhaps? ah good point, its the other way you can't go, put 400 crank in 250 cases - doh, I should learn to read shouldn't I ! |
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andys2
L plate rider.
Posts: 41
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Post by andys2 on May 19, 2014 22:38:54 GMT 1
But I'm going to use the 250 crank in the 400 cases, I've checked and the crank bearings are the same for 250 and 400 so I don't see why it shouldn't work??
Reduced crankcase compression perhaps? That's a good point, never thought of that. Then again I bet the person that built this engine in the first place didn't either and it seems to run OK, sort of!! |
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Post by muttsnuts on May 20, 2014 8:40:00 GMT 1
yes your crank case compression will be low, by how much I have no idea as I've never had cause to measure the capacity of each......might be worth doing some checks if you can before building it up
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Post by 4l04ever on May 20, 2014 11:53:28 GMT 1
I would build a 400 bottom end and put the 350 top end on. Make a nice 400LC! :-)
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andys2
L plate rider.
Posts: 41
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Post by andys2 on May 20, 2014 12:43:18 GMT 1
I would build a 400 bottom end and put the 350 top end on. Make a nice 400LC! :-) I looked at doing that, but a search on here revealed that Muttsnuts is already doing it, and he had to have the 350 barrels relined to get the port timing corrected. Not bothered about going to that much effort at the moment but maybe in he future. |
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andys2
L plate rider.
Posts: 41
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Post by andys2 on May 20, 2014 12:44:34 GMT 1
Anybodyknow of a 400 bottom end for sale? I don't have enough posts yet to get access to the for sale section.
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Post by 4l04ever on May 20, 2014 12:56:15 GMT 1
I don't think the engine would work very well with a 250/350 crank in a 400 casing as there would be a big gap around the crank webs.
You could make a 350LC bottom end fit with a few spacers. Could probably fit the TZ style water pump on the LC casings too. You could then run standard LC ignition :-)
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Post by muttsnuts on May 20, 2014 15:29:42 GMT 1
I would build a 400 bottom end and put the 350 top end on. Make a nice 400LC! :-) there's a shed load of work that needs to be done to make it work, beleive me I know ! |
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Post by angelgrinder on Dec 21, 2014 13:30:39 GMT 1
Very interesting. I'm thinking of doing something very similar (see my post 'Defector from Aircooled RD forum building LC/AC hybrid').
Ok, you've answered one of my questions, in that I've now seen several AC engines with TZ water pumps. Thing is, I remember trying it back in the 80s and the TZ pump looked like it should go straight on the RD oil pump drive but the mounting holes didn't line up. I ended up with a bit of a bodge that leaked gearbox oil. Maybe there's a different model of TZ pump that I should have found?? Or maybe I should have just modded the side casing properly?
I had a 350B back then and I remember the 350 (mine and others I rode) being very smooth so I don't see why the vibes on the LC Top end should be any worse. Maybe your crank is out of balance? overbored with heavier pistons? Any other suggestions?
I would like to find out what people think about the vibration issue as I was going to go with 250E cases but I have the 350LC cases and could simply (??) adapt the mountings on the AC frame and go with the LC rubber mounts.
On that, has anyone got any experience of fitting tie bar mountings on the AC frame? Is there room without removing the crosspeice to which the sidestand bracket is welded?
With 400 cases, I guess you (we) could fill the gap with aluminium rings or just something like Devcon?
What say the cognoscenti??
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Post by guiri on Dec 21, 2014 19:55:46 GMT 1
your bike will have some vibes... but shouldnt be unbearable..
400 bottom ends dont come cheap..if you can find one..
i think you should check your engine tolerances first see if something a miss.. might be a lot cheaper/easier in the long run.
john.
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Post by muttsnuts on Jan 16, 2015 11:37:11 GMT 1
your bike will have some vibes... but shouldnt be unbearable.. 400 bottom ends dont come cheap..if you can find one.. i think you should check your engine tolerances first see if something a miss.. might be a lot cheaper/easier in the long run. john. Your not worng about the 400 bottom ends, I have several of them and I know exactly how expensive and rare they are, same for the cranks.......... if you think some LC stuff is hard to get hold of, try building a 400 E/F engine from scratch ! - anybody got any 2R9 barrels, I need a couple of sets ! |
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