I am helping a buddy who is computer literate rebuild a carbureted motor with 250,000 miles. The motor is in very good condition but going to be installing new cam bearings, rod bearings and main bearing and finally new piston rings. As you can see by the pic, the cross hatch marks are still there. Not sure if they are from the factory.

I also came across this on Hastings website:

http://www.hastingsmfg.com/ServiceTi...efinishing.htm


My question is after re-honing, should I use cast, moly or plasma piston rings and why?

Thanks

Waid

 

Waid.I know you didn't ask this,but you insure the rings seal you need to measure the bore(s) in multi locations with a dial bore gauge to be sure just honing is enough. They have to be round.

 

Everything is still in spec. I measured the piston to wall clearance with feeler gauge and it is 0.003". Even the valve to guide clearance is still in spec. There is a tiny piston bore ridge. Since I dont have a dial bore gauge, I took a straight edge and placed it in the piston bore against the ridge. I was not able to get a 0.0015 feeler gauge under the straight edge next to the ridge.

Waid

 

If we have a block come in and has a ridge that can be felt with your finger, it gets bored, we dont waste our time to see if it will clean with a touch hone. measure it with a dial bore gauge, your methods are inaccurate.

 

Use whatever is cheeeeepest.

Not much sense in wasting any more money on it than necessary; he'll be back there again soon.

That pattern isn't factory; otherwise it wouldn't be still visible in that DEEP ring wear.

The RIGHT question to ask right now is, if it didn't work last time, what makes him think it'll work this time.

Note the blowby at the top left of the pic, where the last ring "job" failed to even touch the cyl wall over there. It's not round. Zero chance of a truly "successful" outcome.

 

The circumference of a circle is: c = pi * d or the diameter is: d = c / pi.
Since I am only interested in the difference in the diameter and I don’t know the exact diameter of the piston bore at the top and bottom, the piston gap itself can be used to calculate diameter. Even if assume a piston bore diameter of let’s say 4.000 and subtract the piston bore gaps, the answer remains the same.

Piston bore diameter at the top:

The piston ring gap at the top is 0.045”, therefore, the diameter at top is: d = c / pi. Which is: d = 0.045 / 3.14 = 0.014”

Piston bore diameter at the bottom:

The piston ring gap at the top is 0.035”, therefore, the diameter at bottom is: d = c / pi. Which is: d = 0.035 / 3.14 = 0.011”

Therefore, the difference between top and bottom is 0.003”. This will not tell me the exact diameter of the piston bore at various palaces, but it is a very good indication that my piston bore taper is acceptable.

Thanks

Waid

 

.003" taper is a very good indication that your cylinder needs to be bored.

 

Where did you find that 0.003" taper is bad?



According to Hasting (Which I am no were near and that my calculation is correct - I am at 0.003" and looks like I can go up to 0.012"):

http://www.hastingsmfg.com/ServiceTips/ring_gaps.htm

 

OMG. Does a simple rebuild really,really,require all this??. Look buy a pizza for lunch for your local machine shop and ask them to measure the bores as what was suggested. It shouldn't take a truck to run over you to do the job right!!!. Gezzzzz