Topic: A couple questions about replacing piston rings

[Ira]

Hi, I'm hoping someone know the answers to these.  I want to install a set of new piston rings in my stock k7 engine.  The top end is apart on my bench. The pistons and bores are measured and in spec. They are clean and the cyliners are honed. I have the new standard sized rings. 

Last night I measured the ring end gap, following the service manual - put the ring in the skirt of the bore, made sure it was a uniform depth, and used a feeler gauge to measure the gap. Same with the second ring, and the oil rings.  Move on to the next cylinder, repeat.

1) I cannot find a CB750 manual that lists MINIMUM ring end gap for replacement rings.  The max is 0.7 mm.  I have the Clymer manual and a PDF of the shop manual.  Does anyone have a definitive measurement or range?

I looked up the Wiseco chart and used their "end factor" do do the calculation on my own.  Is this the best I'm going to get?

Wiseco - Most standard engines     0.004    0.005    0.015

 So for my application - Bore is 2425.591 (about 61.61 mm)

Top ring : 2425.591  * 0.004 = 9.702 (about .25 mm)
Second Ring: 2425.591  * 0.005 = 12.128 (about .31 mm)

I measured all of my rings, and they have on average .25 mm gap.  This would mean the top rings are in spec, but the second rings need to be ground.  Does everything sound correct so far?

2) The sets of rings did not come with any paperwork at all. They were sold as "Genuine Parts by OEM supplier" and the brand is T.K.R.J.

I just want to make absolutely certain which ring is the top and which is the second.  One ring dark colored (almost black) and marked on the top with a tiny "R".  The other ring is lighter colored and has no markings or discernible top/bottom. 

Thanks in advance.  I can post pictures of the rings if you want.

[Ira]

Hey, sorry for posting in the wrong subforum. Please move or delete as needed.

[thirsty 1]

I would install as is. The dark ring goes in the center.

[Ira]

Thanks.  The service manual states that the "chrome" ring is the top ring, so that makes sense.  And am I correct that it is not directional?  There's not a letter or dot on either side.

I have access to a ring grinder and may gap the second ring to 0.30 mm to be on the safe side. Just trying to get assurance either way, because this is the first time I've done the top end on this bike.  And like I said, I can't find an official minimum spec anywhere.

[markreimer]

There is a top and bottom on the upper two rings. If there isn't a letter or any markings on either side, you should be able to see a slight bevel on one side of the chrome ring I believe.


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[Ira]

Thanks. After I posted, I went back and looked super closely. There's an "r" on the chrome ring too, it's just hard to see.

[Ira]

One more update:

I talked to a buddy who is a mechanic and works on vintage Hondas.  He was able to dig up the following minimum gaps settings:

0.20-0.40 mm for both top and second rings

0.10-0.30 mm for oil ring

So for future reference, there it is.  All of my rings are in spec.

[70CB750]

Thank you

I remember reading an article on ring gaps.  Seems like you can get away with rudiculous ring gap without increasing blow buy - means to err on the plus side if in doubts 

[bwaller]

0.004" per inch of bore for compression rings is a safe standard for a street engine. Oil rails 0.010. Sorry I still think in inches.

[The Lone Builder]

Hi,

I'm bringing this back to life because I have just spent an hour or more searching for info on which way up my new (3-piece) NPR pistons ought to go.

Letters/markings on top everyone said. But I could find no markings. Bevel up; bevel down .... etc.

Anyway to cut a long story short, I then thought I saw a mark, so rubbed the end of the ring with a grubby thumb and, low and behold, there it was - "N 25". Problem solved. Did the same on the other rings and right enough, the marking was there too.

So seems there is a film of whatever on the rings which hides the markings. So if your having trouble ....

Sean