Valves and Guides
After dismantling and vapour blasting the heads I set about measuring some of the parameters. The intake valves looked in good condition and had barely perceptible movement within their guides. The exhaust valves on the other hand were not so encouraging and had quite a bit of play within the guides. I used a digital micrometer and small ball gauges to take measurements of these.
The measurements are as shown below- the intake side is within specs as per the Clymer manual however the exhaust stem diameter is narrower than the range (6.945-6.960mm) and the valve guide is also outside spec (supposed to be 7.0). These measurements were for the right cylinder head but the results were quite similar for the left.
| Stem Diameter | Valve Guide ID | Clearance | Range | |
| Intake | 6.97 | 7.00 | 0.03 | 0.025-0.055 |
| Exhaust | 6.92 | 7.04 | 0.12 | 0.040-0.070 |
The intake valve system seems to be in good condition however both the exhaust valve and the guide should probably be replaced. Despite this, I have decided to leave the valve guide as is for the time being and only replace the valve. The new valve diameter is 6.99mm which makes the clearance 0.06 with the old guide. I will run the head like that for the time being and leave learning to replace and hone valve guides for another day. I will definitely do that job on my next project the R75/5.
Lapping the Valves
The new exhaust valves and the old intake valves were reseated (lapped) into the head. For this procedure I used Goodson’s coarse and fine lapping compounds and a traditional wooden lapping stick. I gently put the valve into the cylinder head and applied a thin smear of WD 40 to the stem. Then I applied a thin smear of lapping compound to the seat surface of the valve. The valve was then ground in a back and forth motion against the valve seat in the head. The key here is to listen for the changes in sound- at first it sounds very course and then as the compound is worn out the sound becomes smooth again. I repeated this process many times, cleaning the old compound off with a rag each time. I was also very careful to not get any compound onto the shaft of the valve. You are trying to achieve a nice satin appearance to the seating surface (not shiny). After using the coarse I moved on to the same procedure with the fine paste. The final seating was tested in three ways- using a dye, looking for light in the chamber and putting some water in and testing the seal. When I was satisfied the seal was perfect I moved on to reinstalling the valves in the head.
Reinstalling the Valves
I used new springs and keepers to reinstall my valves. I applied a smear of assembly lube (moly paste- I used Honda M77). The valves were then gently inserted into the head with a twisting motion. The spring retainer was put in place on the head and the new spring installed followed by the spring retainer. The blue markings on the new springs went downwards (against the head) although I don’t believe there is any critical orientation to this spring. Then the usual struggle with a spring compressor ensued before I was able to get the new keepers into place on the valve stems. I found a little smear of moly paste on the stem grooves to stick the keepers in place and a magnetised screw driver helpful with this procedure. It’s fiddly.
Pistons and Rings
The pistons were heavily coked. As described elsewhere I cleaned them up with a lot of elbow grease with WD40 and red scotchbrite. I also used scotchbrite-like attachements on my Dremel. After cleaning up there were some pits on the surface of the pistons but they didn’t look too bad overall. I removed the old rings and meticulously cleaned the ring grooves using 1000grit sand paper and WD40.
The new rings were from Goetze and were quite easy to install. The orientation of the rings is important (they have a right way up which is clearly described in Snowbum’s website) however the Goetze rings are conveniently marked with a the word TOP in small yellow letters on each ring. I also did the two pistons one after the other which allowed me to use the other as a reference as I worked. I was able to put the new rings on quite easily with my fingers although I wasn’t able to get the old oil scrapers off without breaking them as you can see. After the rings were installed I measured the gaps using feeler gauges and they were within spec. The Goetze instructions were very clear.
Next up will be resealing the cylinders during reassembly of the bike! Stay tuned.
