crf 250 x reliability

[scotsy]

Service schedule on my gasgas is the same 12hours for rings oil every 12hours 3 races for pistons. If you go by that you'll spend more time taking them apart than riding them if you aren't losing compression and do regular checks and services don't worry about nearly all modern bikes have nicasil barrells and modern oil are so much better than they used to be. The early crf450 did suffer from top end and were basically a 1 season bike for racing they used to revs so hard they put the valves into the head i think thats been sorted now but the race teams used to remove the standard valves and put titanium one in instead. I know a few people who have crf 230's and 250's not heard of any major drama's with them people seem happy with them

Don't know if you got this the correct way around but I'm pretty sure the STD are titanium and they 'S T R E T C H' and eventually even the thinnest shim available won't give you back the clearance so that's when they swop the INLET's for steel ones (TRX or aftermarket 'kibblewhite' )

http://cgi.ebay.co.uk/Prox-Steel-CRF...torcycle_Parts

As well as my 05 450X i have an 02 450R which has just reached the point where next time they need 'shimming' i'll need to swap the valves as there is no thinner shim for the RH inlet than the one thats just been installed.

Ian

me on the '05 450X


my lad Stewart on the '02 450R

[jasonbc]

Don't know if you got this the correct way around but I'm pretty sure the STD are titanium and they 'S T R E T C H' and eventually even the thinnest shim available won't give you back the clearance so that's when they swop the INLET's for steel ones (TRX or aftermarket 'kibblewhite' )

Prox Steel CRF 250 R X Intake Inlet Valves Gasket Kit on eBay (end time 13-Sep-10 14:40:41 BST)

As well as my 05 450X i have an 02 450R which has just reached the point where next time they need 'shimming' i'll need to swap the valves as there is no thinner shim for the RH inlet than the one thats just been installed.

Ian

me on the '05 450X


my lad Stewart on the '02 450R

Ok mate guess i'm thick as f**k, can you explain to me how a valve that only moves up and down operated by a cam lobe can stretch??? Perhaps your shims are soft and ware out prematurly but i can not see how a valve can stretch as theres no tensile loading on it..it's spring loaded to open up after the cam lobe closes it. If a valve stretched it would hit the piston when opened as it would be too long

[scotsy]

Ok mate guess i'm thick as f**k, can you explain to me how a valve that only moves up and down operated by a cam lobe can stretch??? Perhaps your shims are soft and ware out prematurly but i can not see how a valve can stretch as theres no tensile loading on it..it's spring loaded to open up after the cam lobe closes it. If a valve stretched it would hit the piston when opened as it would be too long

I'm not here for a barny mate, but you've got it all back to front again

Shims are not soft....anything but

The life of a valve, the cam lobe OPENS IT and the valve springs CLOSE IT AGAIN. The valve is continuously under tensile loading wether it is open or closed. the continuous opening and being slammed shut by the springs 'stretches' the valve and the clearance between cam and valve stem head 'closes up' neccesitating a THINNER shim but there comes a point where there is no 'thinner shim' and thats when you need new valves

here is a post about it

"Check the valve clearance; this is going to be the two stroke equivalent off adding premix oil to the gas. With running the stock parts checking valve clearance after every 10 hours of run time would be a good conservative estimate offering a “safe bet” type of protection. The stock intake valves are two piece units made of Titanium that are welded where the valve stem meets the valve head. Titanium has a tendency to become elastic and stretch before it breaks so when you see your valve clearance require a shim size 3 sizes smaller (or more) to get valve clearance back in spec it’s time to seriously start looking into getting your head checked. Otherwise the alternative could possibly be broken intake valves and the excessive damage that comes with it.

If you’re planning on aftermarket valves/springs there are two names that really stand out for quality and durability; Kibblewhite and Ferrara. The Kibblewhite stuff can be had through Eric Gorr’s Forward Motion while the Ferrara bits can be obtained through Ron Hamp Cycle.

Another cost effective alternative is to run the stock Honda TRX steel intake valves with new stock springs although this doesn’t get you very far compared to the aftermarket alternatives.

The springs are the weak link in the stock set-up but once addressed, and combined with quality stainless steel valves, valve train life expectancy is increased almost ten-fold over that of the stock parts (estimated 300hrs).

With the steel and stainless steel valves you can lap them to the head, but results will vary and may have detrimental side effects. A good head job done by the above tuners will include a 3 angle valve job cut with a precision valve seat cutting tool called a Serdi. With a competent operator this insures a precision match between the valve and seat for optimal sealing of the combustion chamber. Lapping with Ti valves, stock or aftermarket, is a no-no."

If you still don't believe me go on crf's only or thumpertalk and do a search

I'm only trying to help

Ian

[jasonbc]

I'm not here for a barny mate, but you've got it all back to front again

Shims are not soft....anything but



here is a post about it


Ian

That was the point i'm making about shims that they are harder than the valves or they ware out. The valve springs are a set length and are compression springs so unless the springs stretch aswell how would they allow the valves to travel further than there original stroke, i still find it hard to believe that valves stretch

[scotsy]

The valve springs are a set length and are compression springs so unless the springs stretch aswell how would they allow the valves to travel further than there original stroke

Stick to riding as its obvious to me you've never had a 4 stroke engine apart and have no idea how they work

Ian

[jasonbc]

Stick to riding as its obvious to me you've never had a 4 stroke engine apart and have no idea how they work

Ian

Guess your right, so these wouldn't qualify then

[AlanH]

The valve springs are a set length and are compression springs so unless the springs stretch aswell how would they allow the valves to travel further than there original stroke

The springs hold the valves tight closed as they are fitted in compression. They are longer when not fitted than when they are fitted.

So, when the valve is sitting in the head with no pressure on the top, it is being held in place by the valve spring pulling the top of it and at the bottom it is held hard against the valve seat. At high revs, the valve is being repeatedly slammed shut. The cam lobe doesn't ease it on to the seat completely smoothly. The valve hits the seat with some force and the head of the valve stops. The rest of the valve, spring etc has momentum and this all pulls on the top of the valve in a stretching motion as it all stops too.

After thousands and thousands of repetitions of this pulling motion, the valve can stretch. Sometimes the head fractures and falls off and that creates a right mess.

[scotsy]

Guess your right, so these wouldn't qualify then

OK, i was 'fishing' and it worked..i got a bite

But Jason, you are comparing 'tappet' engines to 'shim' engines i think?

On 'tappet' engines the 'valve clearances' generally get bigger as things wear out but on 'shimmed' engines they get smaller. Titanium valves DO stretch a little and also as you pointed out in an earlier post they 'cup' and eventually sink into the valve seating area a little as well, all of these things 'close up' the gap between valve top and camshaft lobe/bucket. This usually manifests itself in them becoming hard to start and sometimes a 'clacking' noise from the airbox. The stainless steel valve conversions allow much longer intervals between 'shimming' and consequently a longer life for the engine

Save me a slice of the humble pie
Regards
Ian

(sorry again about the 'craic' )

[jasonbc]

OK, i was 'fishing' and it worked..i got a bite

But Jason, you are comparing 'tappet' engines to 'shim' engines i think?

On 'tappet' engines the 'valve clearances' generally get bigger as things wear out but on 'shimmed' engines they get smaller. Titanium valves DO stretch a little and also as you pointed out in an earlier post they 'cup' and eventually sink into the valve seating area a little as well, all of these things 'close up' the gap between valve top and camshaft lobe/bucket. This usually manifests itself in them becoming hard to start and sometimes a 'clacking' noise from the airbox. The stainless steel valve conversions allow much longer intervals between 'shimming' and consequently a longer life for the engine

Save me a slice of the humble pie
Regards
Ian

(sorry again about the 'craic' )

Humble pie accepted and being offered around

[boney]

hi guys
thanks for the replies ive had the honda for about 3 weeks now and have to say im well chuffed with it its a little bit slower than my mates wr250f but the bike has so much more low down grunt he reckons a crf will probably be his next bike ,enough said
and thanks for the masterclass on shims etc
regards dave