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Post Info TOPIC: There's a shim under my engine block. What is this?


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There's a shim under my engine block. What is this?
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Hello.

I acquired my TTR 2 months ago, and have been wondering what this shim is for? Is this a big bore kit any of you are familiar with? No engine problems so far. Just curious as I know this bike has been heavily modded and the previous owner also has no idea. Dunno how many hands she's been passed to. 

111_by_nickbeja-dbnzj32.jpg

 

This is what the bike looks like (when clean). Yeah, doesn't look like the usual TTR at all:

20786139_10212936282059925_3573836428120

biggrin



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3 options come to mind...

1 - diffrent piston with a higher gudon pin to crown length...
2 - longer conrod..
3 - has been stroked using the standard length conrod..




..

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Bikes... 06 TTR250 / 09 XT660R / 1977 Montesa Cota 348 MRR / 1979 Montesa H6 125 Enduro...

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petenz wrote:

3 options come to mind...

1 - diffrent piston with a higher gudon pin to crown length...
2 - longer conrod..
3 - has been stroked using the standard length conrod..




..


 

Cool. Would be nice to know exactly which one was done though lol. My mechanic did say it feels more powerful than a stocker TTR250 he worked on before and it might have more compression so I'm leaning more into the 3rd option. If so, my engine effectively has more displacement than stock? I'm just kinda worried if the reliability of this engine might've been compromised by this mod but it seems to run pretty well for a really old abused bike.

 



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Is that a ktm ttr 250?

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threshold wrote:

Is that a ktm ttr 250?


 

Lol. This has to be the slowest KTM 250 then.



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purdoy25 wrote:
threshold wrote:

Is that a ktm ttr 250?


 

Lol. This has to be the slowest KTM 250 then.


 Ha not by as much as you'd think.   Those first KTM250F's were SLOW, sleeved down 400's by memory.   Not anything like the current ones.



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purdoy25 wrote:
threshold wrote:

Is that a ktm ttr 250?


 

Lol. This has to be the slowest KTM 250 then.


 

But also the most reliable KTM ever biggrin



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Looks like the stand works. So starting to doubt the ktm theory

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Lol. This one's a reliable KTM alright, though still in the process of sorting out all its problems since this bike was really neglected when I bought it. Spent around US$600 on repairs and stuff. Replaced friction plates, timing chain, carb manifold, rear shock rebuild, rewired spokes, repaired swingarm linkage, realigned frame, electrical diagnostics... and lots lots more. Right now the carbon brush is toast and needs replacement (needs bumpstart). I wonder what will break next? XD



-- Edited by purdoy25 on Sunday 24th of September 2017 06:40:23 AM

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Did a standard timing chain fit??? If it did, I wonder if the head has been heavily skimmed to increase compression ratio, and the shim is to take up the "slack"?

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mossproof wrote:

Did a standard timing chain fit??? If it did, I wonder if the head has been heavily skimmed to increase compression ratio, and the shim is to take up the "slack"?


 

Yeah I ordered a standard TTR250 timing chain for it and it fit. Does the increased compression do much to improve its performance though?



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I believe so:




Image result for effect of increased compression ratio



Increasing the CR produces an increase in output throughout the rpm range. It is also worth an increase in fuel economy. ... Cylinder pressures and output will increase as the CR is raised, but what is less obvious is that the increase in compression also increases the engines thermal efficiency.10 Oct 2003

Ok, the pic in that link didn't quite work, but the words are the important bit.
Increased CR also makes the starter work harder, and puts a bit more strain on the crank, so longevity might be reduced a bit, but fresh oil regularly and not ragging the bits off the old girl constantly but just making use of that extra grunt when you really need it will make more of a difference. Alternatively, thrash it to within an inch of it's life then thrash it some more, then fix it when it breaks

Anyway, ride safe,
Simon.



-- Edited by mossproof on Monday 25th of September 2017 09:17:50 PM

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mossproof wrote:

 


 

Hmmm I see. Thanks for the info!

I read somewhere that higher compression ratios also make it easier for the engine to pull taller gear ratios? I do feel like my bike is geared too low with a 49T rear though I haven't checked the front yet. It can climb steep-ish inclines at 6th gear with no problem and I don't really need all that pulling power (conversely, I would like a bit more top end and mpg gains). Being a supermoto and all, it probably would make more sense to run 43-44T rear and maybe a 15T front since I'm more into just cruising around town and the occasional ride by the twisties. 



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