Why are my pistons so tight?

[Gcraay]

I'm rebuilding my engine and have gotten as far as the rocker arm section. The thing is, when I put my new cam chain in, it seemed to fit, but the pistons don't exactly move that easily. I see other guys do it, and as they pull on the chain, it moves the pistons nice and easy. I can move them some, but not around and around, and fairly tight. I lubed up the cylinders just as I have seen on U-tube, but still tight. Please tell me I'm over reacting and that I don't have to take it all apart.
I went with the second over sized pistons, and had the cylinder walls bored out by a local shop. Could he have made a mistake? I got the specs from the Haynes book.

 

[DIRTY DOG]

did you check the piston to cyl wall clearance befor you installed the rings??
also did you check the ring end gaps?? if they were in spec then it should be ok

 

[scabber]

You might have an oil scraper ring not installed properly and wedged between the piston and cylinder wall. It can and does happen. Not good and could ruin your new bore job.

 

[weaselbeak]

It's not that big of a job if you are that worried about it. I would go back in.

 

[Gcraay]

Crap! I hope you wouldn't say that. They could have moved when I installed them, but I;m not sure.

 

[Gcraay]

No. I trusted that the bore job and the fact that everything was new, it would just be in spec. I guess I'm pulling it apart!!!!!!!

 

[Gcraay]

You know, I didn't remember the crank turning that easy even before I dropped the jugs down. After I installed the chain that is. Man, if I'm cracking the case again, you'll here some choice words coming from North west Indiana. Mostly four letter.

 

[scabber]

If you installed them from the bottom and used the taper push in method no ring compressor. It could be jammed. If you need a wrench to turn it time to look.

 

[scabber]

If it turned easily after you installed the pistons before you installed the head it might not need to come apart. Once the head is installed valve spring pressure makes it harder to turn.

 

[fredintoon]

Hi Gcraay, because they are virgins?
Seriously though, if you have any doubts about how the pistons were assembled, better that you tear it all down and find everything was OK than you fire it up regardless and frag the motor.

 

[DIRTY DOG]

Hi Gcraay, because they are virgins?
Seriously though, if you have any doubts about how the pistons were assembled, better that you tear it all down and find everything was OK than you fire it up regardless and frag the motor.

yep!!!!! better to recheck than to rebuild it again

 

[TwoManyXS1Bs]

You know, I didn't remember the crank turning that easy even before I dropped the jugs down. After I installed the chain that is. Man, if I'm cracking the case again, you'll here some choice words coming from North west Indiana. Mostly four letter.

Perhaps binding in the primary area? Clutch basket setup?

Anxiously awaiting loud sounds from the far northeast...

 

[Gcraay]

Okay, now these were installed with a compressor, they have been lubed with some good thi k Lucas oil treatment stuff, real slippery.
Should they slide in easy, or be snug? They do move, but with force. I still need toget my clearance measured, but I know they fit in, I tried before the rings were on.

 

[TwoManyXS1Bs]

What are your ring gaps?
What are your skirt clearances?
Are the pistons and bores identical, or does each piston have its own specific bore?

 

[NONclow]

I went with the second over sized pistons, and had the cylinder walls bored out by a local shop. Could he have made a mistake? I got the specs from the Haynes book.

In my experience even if the manual says one thing, the pistons can vary slightly, in diameter and weight. Measure the pistons FIRST! Then bore/hone the cyls to the proposed clearance to match the pistons. Double check ring end gap too.

 

[Gcraay]

You know you were supposed to say that you'll be fine. The pistons are new, the bores were from the book. Question, if the piston measurement is not say, 25.50 but 25.47 according to my digital micro meter, is that just rounded up, or will the measurements be spot on? I know 2x4's are not really 2x4.

 

[XSLeo]

Your Micrometer gives the exact measurement.
In the Clymer book page 67, start reading under PISTON, PINS, and RINGS.
Read the whole section, it explains how to measure the pistons, bores and ring end gaps.
Leo

 

[scabber]

If you took the pistons to the machine shop and they did the bore job to the factory clearances for the pistons chances are they are good. The pistons should not move freely once the rings are installed and be snug if they did the bore job right. If you can push the piston up and down by hand probably you are overacting.
If you didn't check the ring gaps or the skirt clearance I would check and confirm that they are correct if the machine didn't fit the rings more reason to check.

 

[peanut]

did you check that the crank turned freely before fitting the jugs and pistons ?

Thats a real critical check.

You need to be certain that all the crank bearing locating nibs were located in their respective holes before torquing the crankcases together. I had to pull mine apart to re-seat one of the crank bearings ,luckily before I tried to bolt the cases together .

If the cases were bolted up with one of the bearing nibs out of its hole I would imagine the crank would be really difficult to turn if at all ?

As two many mentioned another vital check is that you have clearance all round behind the clutch basket, or the back of the clutch basket is going to bind on the crankcase. There is only about 1mm clearance here so fitting the wrong washer or missing one out is critical.

My suggestion would be do the easy stuff first .Check the clutch .It would only take a few minutes to remove it to check.

 

[Brian902]

Gcraay,
Sometimes, some basic information is all that is needed.

Here is the clearances for the 1978 year and this would apply to most models and newer.

Measure piston clearance by installing piston in cylinder and inserting a feeler gauge between the bottom skirt and the cylinder wall. Piston is bigger in that area than at the top where the rings are. Be 90 degrees to the pin and about 1/2" up from the skirt bottom for best measurement.

Measure rings by inserting into cylinder and then use a piston to push them down about a 1" so the sit nice and square in the cylinder. Use feeler gauges to measure end gap.

Anything less than lowest spec and you are in trouble and need to fix it. Too much over spec is not good either.

Good luck.

.