I have just finished writing a long email to you and then visited our message board and found that you had eliminated the possibility of ring expansion into the inlet tract as the possibility I had feared. This makes the main point of my email redundent, but I wrote it in a spirit of helpfullnes and as you were, in your email to me, considering your options, then I will copy my private email to you here, in case is has any interest to others. It will also go some way to illustrate why our deliveries tend to be long — we are busy writing emails!!!
As regards the crankcase clearance for the skirt of Silk pistons. SS cases were machined to just take a +60 SS piston which is 3″ dia, but some LS cases were bored less as a +60 piston is 0.0625″ less on dia.
I have found that the location register diameters on cases and barrels are not as consistent as we might hope and therefore when you put a barrel on to a case, try to wag it about a bit to see how precise this location is. This can put the axis of the barrel out of line with the clearence bores in the case. It can also compromise the alignment of the faces for the transfer port covers making it difficult to get them to hold all gaskets equally. I have also seen rebores that were significantly off centre to the true barrel axis, although I would hope not in this case, as we bored it for you and we have a special workholding fixture with accurate datum features. OK I copy todays email to Erik.
Please excuse me if I speak openly.
I admire and completely understand your spirit and wish to do your engine preparation yourself.
Although it is risky to make an assessment on photographs only, I think that a piston ring may have been able to open out into the enlarged inlet tract where the bridges were removed.
If I take the example of Richards engine, he has tried to prove that he can make an engine, based on standard Scott components, that is more powerful than those I make and has used a different type of head and very big inlet tract. As regards this inlet tract, when the piston is at bottom of stroke, the bottom edge of the SINGLE ring can just be seen starting to come into view into the top of the wide inlet port that has had some of the bridges removed to aid gas flow. If we take the ring as nominal 1.5mm and allow 0.7mm for max possible overtravel, we still have 0.8mm of ring held within a complete bore and thus not able to expand out into this wide port. If Richard decided to add a second ring, this would open out into the unbridged inlet port and smash the engine. If it is wished to use two rings, then a small “stump” sections of the removed bridges must be left to hold the lower ring from being able to escape into the port.
I was surprised when you decided to use two rings, but assumed that you had left enough of the bridges to hold in the second ring. If this was a late decision and if this is what has happened, then it has been a very expensive mistake and you have my complete sympathetic understanding.
Please do not imagine that everything I have done over 40 years has been successful. I have made painful mistakes but have learned from them.
This is why the latest Sports Engine has been produced slightly detuned from full race spec, as it will spend its life in California. As a point of possible interest, it took me about one full week to measure adjust and set everything in the engine build so that all clearances volumes and other critical measurements were known, not just guessed at. Paul Dobbs will race it next weekend against the top riders on excellent much developed bikes. It will be interesting to see how it goes.
My opinion is that a well prepared standard Scott engine with iron barrel, can be almost as powerful as one of my sports engines, at considerably less cost.
The major difference is that the Moss crankcase has a lower inlet floor to give a bigger inlet passage. The flywheel is reduced to 8.75″ dia to clear this and I add a weight ring in the flywheel to compensate for this and add a bit more weight than standard. The barrel in iron can be as good as an alloy one as regards gas flow. The other items, High comp head and big transfer port covers are needed. As regards the inlet, then you either need a big carb, approx 34mm, or cheaper is the Y branch inlet manifold with two smaller standard oval flange Mk 1 concentric carbs.
What I used to do in the past was to get a Scott case and have a good welder, build up in the area underneath the inlet tract and then clean it up, so I could lower the inlet floor of a standard case. When I did this, I cut out the front to be able to makes solid bosses so the case could be clamped securely as the standard Scott case is weak here. I then found that if I put a sheet aluminium cover over this front opening, but put it on packers, that there was an advantageous cooling air flow that kept the inside of the cases from becoming too hot. When I take the cover off, then chain threading is much easier. Please also remember that I did not start out to offer these items commercially. They were done by me for me and the criteria was to have only the highest strength and quality. In this case my own castings are from high strength, low expansion heat treated allow and are in strength and performance terms about as far removed from the mediocre Scott items as a Trabant is from a Ferrari.
The main and I believe original concept of mine to increase the cross sectional area of the inlet tract, was to remove the “Spectacles” that are the roof of the inlet tract in the crankcase and to use the underside of the block to perform this duty. The practice of lowering the floor of the inlet tract is useful if ultimate performance is wanted but is not really beneficial unless you use a bigger carb than standard.
In all tuning there is a law of diminishing return. In today’s world culture where many find it easy to exploit others, it is difficult to ask someone the question, “How much can you afford to spend” together with “What characteristics would you like to achieve” If we can ask this openly and get an honest answer, then we can make a proposal that can get as much of the final requirement for the money available as possible. Most folks are very wary of being open about what they can afford, lest we take advantage of them, so they plead poverty. We then make the best proposal we can within what we imagine to be their budget, but well realising that we could have made a much better engine at a higher price.
We have tried to help others by putting as much information as possible on our website and in newsletters, but it is impossible to put 40 years Scott experience and 55 years engineering experience on a site and still try to earn a living. Recently we attended to a customer who had received an engine but said it did not run properly. When we checked out the bike, the first thing we noticed was that all chains were very tight, we say like a bow string! Where do you start in your assumption of the basic knowledge of the average owner?
OK final statements
I think that you had possibly got 99% of a good engine, but made one mistake. It is sad, but we must never be defeated.
If you had either brought the engine and we could have built it together here as a check, or assemble it yourself loosely, to be sure it all fitted together, then send to me and I would measure rod travel and set bump clearances and head volumes and assemble.
Things will be a bit slower here, now that Richard is largely working elsewhere, but while I am still working, I will always try to give extra help to those who try to help themselves.
OK a few ideas here for you to think about