BHPian Motolysis recently shared this with other enthusiasts: Question 1: Does anyone know if the Gen 1 Duke 390 comes with undercut valve stems? Both the exhaust valves have a narrower diameter at the head. The following are images better explaining this with an uncleaned valve and a partially cleaned one. Unclean: Partially Cleaned: As you can see, the bottom half of the stem is not the same diameter as the top. Two of the valves, one Intake and one Exhaust, both on the same side, were also leaking when tested with petrol and compressed air. I checked the cylinder head,d and there is no warping. Which brings me to. Question 2: Should I simply get new valves along with the seals and springs (as it’s a relatively cheaper affair) Question 3: Do I need to measure the current shims in order to get the proper sizes for the new shims to adjust the clearances post valve replacement? (All the clearances were out with current shims) I have attached a few images of the Piston and the Cylinder. Piston Side 1 Piston Side 2 Piston Top Question 4: The cylinder seems to be in decent shape, to my very limited knowledge and experience. Can I keep using it? There are no deep scratches that catch fingernails. The Piston, on the other hand, has some scuffing marks on both sides, and I have yet to check both the cylinder diameter and the piston, as I don’t have the required micrometer. I was thinking of changing the piston and the rings, getting it as a set. Which brings me to the next query. Question 5: After digging around a lot, I found out that the Cylinder has a Nikasil Coating and shouldn’t be honed. Will using new rings and a piston be okay without honing? There are some visible cross-hatches on the cylinder wall,s but I am not sure about the slight discoloration marks and whether deglazing is needed or not. Could it lead to the rings not setting properly ?BHPian BullettuPaandi recently shared this with other enthusiasts: Quote from BHPian Motolysis:I have a few questions Disclaimer: Not a Duke owner; just adding what I know. 1. If you’re referring to the taper on the bottom of the stem, it is there on most valves by design. I believe it is to direct more air to the perimeter/circumference and aid in cleaning. Nevertheless, look up for the specific part in your catalogue, and see if any pictures/diagram of the same are available to verify. 2. Unless the valve is bent out of shape (highly unlikely as the piston head doesn’t show any sign of contact with the valves), what you really need is perhaps a re-seat. You can roll the valve on a flat surface and see if it rolls like it is straight, but the proper test would be testing if it holds water/compressed air after a re-seat. Look up ‘lapping of valves’, either buy the compound and do the same yourself or reach out to a mechanic for assured result. Odds are that you won’t need new valves; the grime on the unclean picture looks like a lot and may have been enough to loose compression. Should the lapping and re-seating fail, buy the specific OEM part with the part number from the catalogue, or a known-to-fit after-market one. 3. Parts catalogue should be available for your bike, and with it there is no need to measure the existing part- shims in this case. Look up for the part and buy the same with the number. I’d fix the valve issue first and then see if I could still use the current shims (presuming the grime on the valve causing lack of seal, just might’ve meddled with the adjustability). But, if you’re sure the shims or gone, or if you feel ‘might as well’, it is worth switching while you’re at it. Either way, using the catalogue is better than relying on measurements. 4. You can try using lens cleaner or petrol to clean the deposits off of the head while you’re at it. But from what I can see, it looks good to me. The compression test after the valve job would attest for the same anyway. 5. I think such discolouration is fine- especially on such high compression engines that are known to run lean and hot. Then again, I’ve never taken the cylinder apart to do anything. So I have no clue on this. If not answered by other fellow members, just ask a reputed mechanic. I would suggest reaching out to one for the lapping job anyway, as it is a bit of a chore. Kudos to you for doing this and wishing you the best.Check out BHPian comments for more insights and information.
