Tolerances

I am getting close to sending my engine parts out to the machinist, need some insight on tolerances to give him.
Engine is a very late 72 750 and not a Combat.
Going through a bunch of books and doing searchs these are what I have come up with.
Pistons to bores (cyclecraft pistons) .0035-.0040 on skirt
ring gap (hastings) top .010-.012, middle .008-.0012
valve guides (bronze) .001 interference fit in head
valve to guides (black diamond) .002 inlet, .0025 exhuast
rod big ends .001
I was going to have him use a 180 grit hone on the cylinders.
Any thoughts? I live in Houston and it does get hot here in the summer time.

1. I Love Nortons.

2. I have absolutely no idea how to set up for an engine rebuild.

3. If I ever need a rebuilt engine or other highly technical work done on my bike I will send it to someone who Loves Nortons and KNOWS A TON MORE THAN I about them.

As a kind of a variation on what Herman said, there people who are good machinists and there are people who know a lot about Nortons. And then there are people who are good machinists and who know a lot about Nortons . I've had a gifted machinist screw up a head for me. You said you're sending the stuff out anyway, Memphis Motor Werks is a good choice.

rpatton said:

As a kind of a variation on what Herman said, there people who are good machinists and there are people who know a lot about Nortons. And then there are people who are good machinists and who know a lot about Nortons . I've had a gifted machinist screw up a head for me. You said you're sending the stuff out anyway, Memphis Motor Werks is a good choice.

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there are also people who think they know a lot about Norton's and and think they are a good machinist /mechanics

and yes memphis motor works is a good place, in addition to CNW or Phil at Fair spares ...

go with some who has a good / recognized rep and don't be afraid to ask for a little feedback as many on the forum have used these guys (and others) and can give you their thoughts/experiences

1. Measures match my list for Combats so good enough for non-Combats.
200 to 400 grit is standard bore finish. But shops equipped for standard finish.
2. Houston don't get that hot from Norton view point d/t the hiGH Humidity.
3. Houston is a hot bed of vintage clubs Nortons well represented. I'd hook up with them if in doubt of machinist. Big D is in Dallas if they'll talk shop with you.
4. Take your time to break in a tight built engine and if thinking there be some extra heat loads like 80+ mph freeway runs with the wind in late summer, can open up piston clearance to .005"-ish and ring gaps a few thousandths more.
5. If desire extra endurance processing have parts cyrogenic tempered and dry friction coated before any assembled. Can't do head of seats squeeze to pop right out. ugh.

The red flag for me was his saying "I am getting close to sending my engine parts out to the machinist, need some insight on tolerances to give him."

Why would a well qualified machinist who knows Nortons need to get tolerances from you?

I planning on using Rea's machine shop here in Houston. They only do motorcycle and marine work and come recommended by a number of people that are into Brit bikes in the area. If I have a good feel for what the numbers ought to be from the guru's on this board, and their recommendations are pretty much in line with them, then I'm going to feel okay about using them.

Hard to argue with the recommendations to go to someone who already knows how to set up Nortons, like Leo Goff, CNW/Jim Comstock, Phil Radford, etc. On the other hand, the numbers look reasonable, except for a couple.

Like hobot said, 180 grit is too coarse a hone for the final finish. I use a 280 grit hone to finish the bores on my race engines with either plain or chrome top rings, and then run a plateau brush hone through for the final finish. I use a much finer finish for moly rings, but you don't see those for Norton pistons very often. I've only used them for certain pistons in 920 race engines. For a street bike that gets a lot of mileage, you might want to go with a 320 grit final finish, and break it in properly.

That's just my experience, mostly with race bikes, and I'd defer to whatever someone like Leo Goff recommends for street bikes.

The other one is the valve guide fit and guide clearance. If you are using bronze guides, you can tighten up the clearance a bit. I install the guides at .002" interference fit in the head, and set the valve-to-guide clearances at .0010 - .0015" for intakes and .0015 - .0020" for exhausts.

If you're using stock iron guides, I'd just use the numbers in the factory service manual.

Ken

Thanks Hobbit and Icrken, the rings are Hastings cast iron. I just had a look at their website and they recommend 220-280 grit for final finish.

Cyl wall finish/piston ring type is standard stuff and any machine shop that does competition/high performance engine work will know what to do re wall finish vs the ring material. However, it is important to choose someone who is familiar with air cooled engines since some clearances will be larger than they will be on a water cooled motor. ALSO, this stuff changes as time goes on - a standard "spec" of 10 (or 40) years ago may not be the best spec now. For example, it was "common sense" for probably 100 years that the top piston ring gap is narrower than the and ring. Today, that is not the case and many ring manufacturers and performance engine builders now recommend the opposite, having found that the narrower top gap promotes ring flutter and poor sealing at max pressures.

But in any case, as others have noted, using a known builder of Norton motors is probably the best way to go if you can.

If the bore is mirror and no ridge, I would not even hone it. I would fit total seal rings dry with just a light coat of oil on the piston skirts & ride it.

I sort of agree with that though I would check the bores for taper; if no taper then I would do the same as far as just installing rings BUT you do need to at least hone the bores with the appropriate stone to accommodate the new rings.