Hey all,
I'm in the process of reassembling my NC 250 and remembered that the engine originally had an aluminum spacer between the cylinder and the case. The spacer is about .050" thick. The manual doesn't mention it at all. I'm planning to clay the head this afternoon, so I'll know what the clearances are soon, but I'm wondering about the spacer? What it just used is some models to drop compression, or is it actually a ship for setting tolerances? The motor came out of a scrambler, if that makes any difference.
Cylinder Spacer?
Moderator: ajleone
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DewCatTea-Bob
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- Joined: Sun Nov 01, 2009 10:53 am
- Location: Near SE side of Lake Michigan
Re: Cylinder Spacer?
" the engine originally had an aluminum spacer between the cylinder and the case.
but I'm wondering about the spacer? What it just used is some models to drop compression, or is it actually a ship for setting tolerances? "
____ While that thick optional cyl.spacer can & has been used to lower compression (so that less able-bodied riders could more easily kick-start SCR-models), it's other useful function is for dealing with mismatched cyl.jugs & pistons ! ...
The various narrow-case models came with three different pistons and also three differing cyl.jug heights ! _ So for a (non-stock) cyl.jug/piston-combo which requires it, that cyl.base-spacer is needed to avoid & prevent piston to cyl.head interference (after the cyl.head is fully torqued-down).
__ If ya do the clay-smash test (between the 'squish-band' areas of the piston & cyl.head), with the added cyl.base-spacer in place, and your clay-test then shows at least 1.5mm of clearance,, then ya really shouldn't need the spacer under the cyl.jug .
__ I've put engines together with less than .25mm/.010" piston-to-cyl.head clearance, but then ya need to stay aware ! _ Cuz after a few hours of engine-running followed by a re-torque of the cyl.bolts,
piston to cyl.head contact is then quite common !
DUKE-Cheers,
DCT-Bob
but I'm wondering about the spacer? What it just used is some models to drop compression, or is it actually a ship for setting tolerances? "
____ While that thick optional cyl.spacer can & has been used to lower compression (so that less able-bodied riders could more easily kick-start SCR-models), it's other useful function is for dealing with mismatched cyl.jugs & pistons ! ...
The various narrow-case models came with three different pistons and also three differing cyl.jug heights ! _ So for a (non-stock) cyl.jug/piston-combo which requires it, that cyl.base-spacer is needed to avoid & prevent piston to cyl.head interference (after the cyl.head is fully torqued-down).
__ If ya do the clay-smash test (between the 'squish-band' areas of the piston & cyl.head), with the added cyl.base-spacer in place, and your clay-test then shows at least 1.5mm of clearance,, then ya really shouldn't need the spacer under the cyl.jug .
__ I've put engines together with less than .25mm/.010" piston-to-cyl.head clearance, but then ya need to stay aware ! _ Cuz after a few hours of engine-running followed by a re-torque of the cyl.bolts,
piston to cyl.head contact is then quite common !
DUKE-Cheers,
DCT-Bob
PLEASE NOTE... If this-post is not-yet signed-off with '-Bob', then I'm still in the process of completing it,, and if not also included with 'DCT' near bottom as well, then I may edit this post's wording at a later time. - Dct.Bob
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ccambern
- Posts: 53
- Joined: Fri Apr 16, 2010 5:18 pm
Re: Cylinder Spacer?
Ahh. That makes sense. With an aftermarket piston and no spacer, it has .075" squish. I just got off the phone with Bill at Boretech (he did the head and jug work) and he suggested .050" squish would be about right, so I'm going to try to come up with a thinner gasket.
Does anyone use hyomar between the head and cyl? The dry fit seems kinda iffy to me...
Does anyone use hyomar between the head and cyl? The dry fit seems kinda iffy to me...
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