My Cammy Engine Blog

[paul.wirdnam]

They have arrived!

[brenton.roy]

And...

[Eero.Korhonen]

We are waiting for box to open, Paul.
Br, Eero

[paul.wirdnam]

And...

...I'm broke!

[paul.wirdnam]

I had time to inspect the cranks and all looks OK....at least the inner races do not free slide on to the crank pins as before, although I did ask them to fit them...which they haven't. The edge on the inner facing part of the race will need chamfering to clear the radius / hard chrome at the very base on the pin....but I was planning on doing that anyway to ensure the races pushed hard up against the crank.

[paul.wirdnam]

Re-assembled the cranks today with new ball bearings and the inner races of the big ends fitted. Glad to report that it all went smoothly.

But in checking out my pistons, I think it would be very unwise to re-use them --- the gudgeon circlip grooves are too badly damaged / worn on a couple of them. As a last resort, I guess I could use gudgeon pin "buttons". Has anyone any experience of using these on a cammy? Brass or ali?

[paul.wirdnam]

Well, after lots of useful advice on this Forum and the Yahoo Group, I was all set to use aluminium buttons in the end of my gudgeon pins --- this would allow me to continue using my current pistons without the use of circlips. Just to re-cap, my current pistons are STD and my barrel was re-lined back to STD in 1997....it shows very little wear and I'm therefore reluctant to remove metal from the bores just so that I can use the set of +20 pistons I already have.

But having had yet another look at these pesky pistons, I really don't like them! I wish I could remember where I got them from in 1997...but 17 years ago is too longer ago for my old brain cells. In the photos below, I show one of these pistons (#1) along side two other types of 4F pistons I have (#2 and #3) --- these last two are +20, so can't be used as-is without a rebore.



In the next photo, you can see that #1 has probably been turned down to suit the 4F; there is no relief on the left side for the gudgeon pin, but there is some on the right side. The oil holes are not uniformly drilled.

I like #2, mainly because I think 3 compression rings might be better than 2! And those additional rings around the diameter inspire me!

By the way: markings on the inside of the pistons are:

#1 : NSA 2741
#2 : No markings at all
#3 : Heplex 4162 (correct Hepolite number for 600c 4F)



Looking from below, you can see that the gudgeon pin boss on #2 is lubricated via the two visible drillings. On number #3, there is a diagonal oil hole drilling from the oil hole on the outside of the piston. But #1 has no oil drillings at all to the boss; that can't be good for a floating gudgeon pin, can it?



Another thing I don't like about #1 is that the central hole though the gudgeon pin is way off centre (about 30 thou inch!). Not only is it off centre but it is not even drilled parallel to the outside of the gudgeon pin....so my test ali button that I made would not sit square in the pin. And the pins in #1 and #2 are significantly heavier than the pins in the correct Hepolite piston (#3).



Rant over. But I'm not going to re-use these #1 pistons...have ordered a set of STD JP pistons which should arrive today.

[Simon.Gardiner]

A belated comment on loctite, I've had no luck with loctite 648 (Iwhich I think is the same spec as 638 but to higher temperatures) on singles crankshafts, it just disappears.

I guess air-cooled motorcycle engines have higher extremes of temperature/stress.

(Hells teeth, that cammy four is weird, but beautiful.....)

[paul.wirdnam]

I order the JP STD piston set on Monday 27th and they arrived the next day --- well done F.W.Thornton!

And having checked piston-barrel clearances etc, I'm very pleased I chose to go with new pistons. At the base of piston and barrel, and at right angles to the gudgeon pin, the clearance is between 4 and 5 thou of an inch and is virtually the same as my old pistons, so I do not expect any problems. Just need to make sure I glaze bust the bores.

The piston ring gap, at 14 thou, seems a little on the large size. I've always worked on the formula of 3 thou per inch of bore for an air cooled, non-racing engine. The 600cc engine is just over 2 inches, so a gap of at least 8 thou seems about right, but maybe the cammy needs a larger gap.

But overall, I'm very impressed with the JP set....just hope they run as well as look:

[brenton.roy]

I guess that when the price is forgotten about, they'll be reliable and one less thing to worry about.
Even if there was no real problem, I would have been concerned about the differences between pistons and pins.