Late Atlas top end refurb and detective work

Being lazy, I'd refit the pistons as they are into the barrels.  Maybe a new set of rings, and hone to improve the finish, but I'd sooner put up with those two grit scratches than go to the trouble and considerable expense of a rebore, with all the ensuing uncertainties about running in, nipping up, piston top cracking, etc etc, plus then having your barrel at the next bore size up.  Spend the piston money on a holiday!

New big end shells wouldn't go amiss.

I've no idea how important perfectly evenly weight pistons are on a parallel twin.  Everyone seems to think it's important, but surely the only issue might be a small rocking couple?  ...which is nothing compared with the inherent primary balance issue with a 360 crank (where either fore-and-aft primary balance or up-and-down primary balance can be matched - but not both).

Can you get a bigger valve guide?  I turned down one from an 850 Commando because a loose guide had rattled the hole bigger than the oversize available.

You don't have a rare and (regrettably) valuable 8 fin barrel for a 500, do you?

Hello well I am doing a Atlas motor  up for a friend  and I found  that Atlas crank are not balanced well so Next stop  is Steve Maney  Norton engineering   for a crank balancing job  , yours  anna j  

According to calculation, if  the head is brought to 200C in an oven, and the valve is cold, the gap starting from perfect fit will be about 2 thou on 1/2" diameter.  As soon as the guide is as hot as the head, this becomes about 1 thou.  So if cold interference is more than 1 thou, they will still need some force to extract when at they are both at +200C.    If the interference is as much as 5 thou, then heating (to extract it) by 200C only reduces that interference by 1 thou to 4thou - which must be hardly worth it.  So I don't know if the books give a figure (I'm sure mine didn't), but it looks like much more than 2 thou will start to give more problems because you'll need too much force to get it in and get it back out even when hot.

I cannot remember - but do the heat insulation washers have a bottom cup to tend to hold the guides in on shoulders?

Someone here pointed out that compression plates upset push rod geometry.

Re rocker spindles etc - Phil H wrote an essay somewhere in the tech pages here all about those issues...

Why didn't the bigger engines have a decompressor?

I do remember I measured mine, making up an alloy bar and turning it down until it was snug to get as good a measurement as I could.  Then I turned down the much bigger 850 type guide to the oversize I'd decided on.  I'll look up my notes, but someone out there must know what the interference should be?  The hole in mine was nice and smooth and round - but too big for the biggest oversize I could buy off the shelf.  Also made up the drift as per the book, and used the same tool to extract the other (stiff) one with.  No oil seals.  Did they arrive with the Commando because of tilted engine resulted in the head not draining so well, or because they pumped a lot more oil up there?

Note that RGM supply +15 thou Colisbro guides. These can be turned down on the O/D to the required oversize to suit your holes once they've been reamed/bored back to round again.

Terry Guy

Hello  Yes  Valve  guides  From experience   I have found that  Colsibro Valve guides are the best  and best fitted by heating the cylinder head  in your kitchen oven  but do not let the other half catch you  and you need a good wood chopping board  to drop the head  on to when hot please use good thick leather gloves  you will need to heat the head twice  first time to get out the old guides and then after a good clean up of the head to fit the new guides by re-heating the head  in the oven  is the best way   and the safest  way   it was OK for me to use my oven as I live on my own  NO other half  to contend with, Ps when you in there clean out the oil ways in the cylinder head          yours    anna j