dommi 99 tappet clearances

Hi Barry

You have gone down a path that many others have visited. ie. An SS type of camshaft in a otherwise standard engine. My advice is to keep the 6 & 8 thou clearances. Especially if you are using a standard 99 head with the early 2" valve springs.

Your camshaft will tend toopen the valves early and close them later. Consequently, if you close up the clearances you will risk the valves floating and bouncing off the pistons at medium to high revs. The later SS heads had much stronger valve springs to overcome this but still kept to the bigger clearances to avoid problems.

Oneitem worthing checking on your engine are the camshaft followers. If you have kept the old style rounded base type then try and get hold of some with flat ends to replace them. The later flat base ones work much better with the SS camshaft and helps the engine tobreathe more efficiently. You will get more power throughout the rev range but not lose on fuel consumption.

The bad news...... More engine power results in more heat for the cylinder head to dissipate. Which the pre 1958 heads are not very good at. After 1959 the heads had more finning to sort this problem. Which may be why your cylinder head is not too happy running with this fancy camshaft. So fit a later head or use less throttle or put up with the lumpy running when the engine gets too hot.

My suggestion is to get hold of and fit one of the early 60s 99 heads. Nobody but the purists will notice. You will get heaps of extra power andless hassle.

Just a couple of additional points, longer duration cams usually require a higher compression ratiofor them to work correctly. This is due to thedynamic compression ratio being loweredas valves open earlier or close later, in effect the charge cannot be compressed until the valves are both shut.

If you fit a sports cam with standard compression ratio, anything lower than about 8.5 :1, you will probably be disappointed with the results with no increase in power and perhapsmore fuel consumption.

By replacing the cam followers with flat ones, you will extend the valve timing (lift remains the same) and this will exacerbate the effect.

Just some things to consider.

Regards.

Les H

If I was doing this when I bought my1959 '99' in 1965 it would be acceptable. It's now 51 years old with the original crank (now .020" undersize) head, barrel (+.040") and conrods. It still has the SS camshaft I fitted in 1965, swapped out with the original "QR" version, (but I would like to go back to the "QR"!) and I've had the cam followers reground to a flat base - this is easily possible and not risky as the radius-based followers have a thicker hardened section at the centre - machining them flat just means that is the same thickness all across, as per the "SS" type (Standard from 1960 onward). My point?

It's a 51 year-old bike - I'd be mad to expect and try to achieve a "sports" performance! Where would I use the extra power anyway, even if I was willing to risk it blowing up? If I wanted a little extra acceleration in traffic around town, or touring runs I'd just fit a smaller engine sprocket - I'm never going to thrash maximum revs out of it! I doubt that it would be forgiving if I tried!

The phrase "horses for courses" spring to mind. If I want a roadburner that I can thrash day-in and day-out I wouldn't DREAM of using an old classic - I would buy a modern "retro" bike such as a Hinckley Triumph. If I want an immaculate, historicold classic that will turn heads when it's run or parked anywhere, get people to admire "how they used to make them"and maybe win shows - then I've got one! (when It's finished anyway!) IMHO,it can't be both!

When you're 19, you're immortal; when you're 65 you're sensible! This doesn't mean you're dead from the neck down!

-Discuss!

I am only 61 so fortunately by your criteria I am not yet old enough to be sensible. Now retired I am having almost as much fun as when I was a student in the late 60s. (Don't ask what's lacking.) I intend living life to the full for as long as possible; who knows how long good health will last? As far as motorcycles are concerned, this means that I will ride my Nortons as I did back then. Well I will when they are going. The Atlas bust its crank and I am only half way through getting the 99 on the road. No matter, parts are readily available and I am a much better mechanic now. Both bikes are cafe racers. Almost nobody rode standard Dominators in the late 60s so why now? Meanwhile (close your eyes - heresy follows) I have the Ducatis to play with. See you at the Ace!

PS Set clearances to 6 and 8 thou. They won't rattle as much as a Ducati clutch.

Previously wrote:

When you're 19, you're immortal; when you're 65 you're sensible! This doesn't mean you're dead from the neck down!

-Discuss!

Aha Alan! At least someone's awake! My post was intended to be controversial (in case you hadn't guessed!) so I was hoping to awaken some passions! I'm a Rock'n'Roller so I will never grow old! I have 17 guitars and about 7 amps(with two more yet to build) so it will take me many years to get through them all! Especially if I go live gigging again! It's a shame the forum isn't used much - I've longed for one for years!

I'm actually 64, but knock off at least 10 years if you're a rocker! I've just boughta roadworthy Navigator so I will bedipping my toes inthe water as soon as I've made it safe. No leaking fuel over dodgy wiring connectors!

My point is that, to an extent, our old bikes are like us - they won't run the 4-minute mile like they could when they were young no matter how much we wish it! Push the performance much above moderate and I guarantee that something will let go! As I've said elsewhere, most of my mechanical experience is with Jaguar XK engines - now there's an engine-and-a-half if ever there was one! A real big cat, but don't push too hard if you want a long life! Can you imagine the force needed for a con-rod to go out through theSIDE of a cast-iron block? Not my Jag but one I had to rescue once - they had re-used the original con-rod nuts & bolts on a rebuild! Phew! Like the garage that refitted my Brother's Triumph 2.5 PI's prop shaft and re-used the original nyloc nuts (and didn't tighten them up either!). He was lucky that it only came adrift at the back end by the diff or his car might have lost its entire back-axle! Those were the days eh?

With 1950s or older bikes we are already on a very severe uphill battle with rubbish petrol - in its day my Dommie only ran properly on 101 Octane! Now they keep adding more ethanol instead of nice, safe lead. I don't want my engine to end its days as a firework display so I have to work with what I've got - that means NO 101 octane! My belief is that there's no adequate additive to counteract the horrible low octane fuel we have to put up with so there's no point in going for hi-comp pistons and twin-carbs on my 99 "Special", like it had ex-works. I am certain it will look and sound the part and have decent acceleration with the single carb and low comp pistons I have for it. When it and me were young we could legally go over 100mph on the road!

I'm still 25 inside! LOL!!!!