Hitchcocks Forum

After finally getting my cylinder head off I have taken off the barrel as well. There was a lot of soft carbon on the piston and in the combustion chamber which I have cleaned off. I`ve checked the end gap on the rings and they are well within tolerance. I`m wondering now whether I should change the guides while the head is off. This bike did use a bit of oil. Or do you think I could get away with just fitting some of the guide seals that Hitchcocks sell. I`m going to fit the larger exhaust valve while I`m at it as well.

this is the clearance in MM between valve and guide.







the valve seat will possibly need to be slightly regrind to fit the oversized valve

Replace the guides. False economy not to. Seals won't help if they are worn, the seals will just wear out quicker! REgards, Mark

Service limit ONE millimetre. I don't think so. More like 0.1mm

was wondering how long it would take for someone to notice



it is indeed 0.10

Heavy oil consumption is usually associated with a worn piston oil-ring and land.


Remove the oil ring from the piston and inspect for any polished surfaces on the top and bottom surface that align with the 'bridges' between the top and bottom rails of the ring, next sit the ring externally into the ring groove and attempt to 'run' the ring all the way around the piston. The OEM piston tends to 'close up' on the oil ring groove, fore and aft, wearing the ring and crimping down on it to the extent it become trapped. Fitting a new oil ring always shows up the defect.


Then eventually the oil ring fractures, and the piston snaps in half through the ring groove.
Then you are in for a FULL rebuild.
I have literally DOZENS of dead 500 piston I could show...

Mauri, I am used to working with very close tolerances and smoke fitting things together, using a scraper. 1.0 mm is like a mile in that world.

In full agreement with Tim here and just to add, I have seen some VERY worn valves and guides come out of some very clean running Bullet engines which didn't use much, if any oil. The tops of the guides are high enough to be out of the way of most of the oil flow in the cylinder heads. another more common cause of oil consumption is found where some over zealous 'porting' work has been carried out, resulting in hairline cracks and even holes from the inlet rocker box into the inlet port, just behind the valve guide, but as already stated above, the piston rings are the main culprit most of the time and sometimes the Indian made rings will do it from new, so quality rings are the way to go.

Out of interest, Ivor, what is the advantage of fitting the larger exhaust valve?

Hi to all. I believe that by fitting the larger exhaust valve you reduce the chance of the standard valve not sealing properly as it is a bit small compared to the valve seat. Someone will correct me no doubt. About the rings, I`ve placed them all in the bottom of the barrel and measured the end gap and they are all within tolerance. Perhaps, as suggested, I should replace the rings and forget the guides. The problem with this engine was a high petrol consumption and medium oil use. I top up oil every 200-300 miles and only get low 50`s mpg.