Piston Crown Stamp Meanings - help needed

[xs79]

Hi there,

Great forum, and for a long while I've been reading up on putting my 79 XS special back together.

Just gotten to a confusing point. To replace the piston rings...the crown has '0.05' stamped into them. What does this mean? Overbore? I can't seem to find any connections.

Can someone please help me out and tell me what should be stamped atop the pistons to help me obtain the correct rings!

Thanks in advance,

xs79

 

[wherearewe]

Perhaps this excellent post by another member (XSLeo) will help you.


On a stock piston, the numbers stamped into the top of the piston is the piston size. When they make pistons they don't all come out the exact same size. They measure them and match them by size. The sizes are all 74.xxx mm. The xxx part of the number is stamped in the piston. If you only find two didit then clean some more it's there.
With a matched set of pistons they bore the cylinders to match the pistons. All bors are 75.xxx The xxx is printed on the cylinder sleeves below the jugs.
Say the piston says 955 that's 74.958. The sleeves sat 010. That's 75.010. I have set wqith those numbers. Now subtract the piston from the sleeve and you come up with the piston to cylinder clearance when it was assembled. In this case, 75.010-74.958= .052, that's right in the middle of the .050-.055 spec.On oversize pistons the are marked with the oversize in mm the first over size is 25 as in 75.25 mm, second is 50, as in 75.50. Your 1.00 is 76 mm or fourth over. The 3 might be some other companies third over.
I would measure them. Pistons and bores.
The stock sleeves can be bored out to 12th over. That's a 78 mm bore. This makes the 650 cc engine a 707 cc engine.
If you don't have tools to measure, most any machine shop does and will measure your parts for a small fee.
Once you find out what you have, then you can decide what to do.
Hoo's Racing has those 707 cc pistons.
Leo

 

[XSLeo]

Do you have a good dial caliper. They can be bought for around $25 at a lot of places.
You can get a electronic caliper that can read out in metric as well as SAE.
With a good measurement we may be able to decipher what you have.
Leo

 

[XSLeo]

I might also suggest taking your pistons and cylinders to a machine shop and get them measured. You can then use these measurements to determine just what your piston the cylinder clearance is. If the clearance is with in spec of the .050 to .055 mm then your pistons will be ok to use. If they are outside that spec you will need to get new pistons and get a rebore to match the new pistons.
These measurements will help you determine all this.
Your repair manual explains all this.
Leo

 

[5twins]

You may not have cleaned the piston top quite well enough because the numbers on a stock piston don't show a decimal point. Maybe what you really have is "005" on there, not "0.05". But that doesn't make sense because .050 to .055 added to that wouldn't put you over 75mm.

 

[XSLeo]

Pics of your piston tops showing what your looking at would help.
Leo

 

[jay760]

If it was 0.05 you would have almost 1mm clearance, normally 9** sure they aren't 0.5 which would be 2nd over size