Topic: It's running, kinda.... need some help please?

[BrianAdair]

 
I installed the pistons and cylinders the other day, that went good. Tonight I was going to put the head on to check piston to valve clearance with the clay method, so I didn't use my head gasket. (Bad idea....?)

I don't have a copper gasket so I didn't want to ruin this one just for checking clearances, didn't know if it would or not.

I bolted the head down didn't go to full torque, only about 10ft lbs started installing all the cam towers and cam, rocker arms.

Then I tried turning it over, it would only go so far before the pistons hit the head....
Removed all the cam, towers, rocker arms, and tried it again still hitting, will the head gasket prevent this...?
I measured the head gasket thickness at the chamber rings and it is .056 in, how much will it compress when torqued?
I placed the gasket on the cylinders and measured from the top of it to the top of pistons to where they would meet the head and it is .040 in. I have the Rocky S12 64mm pistons, and the gasket is the Rocky gasket that went with the pistons.
The pistons measure 63.94mm and the chambers on the head are about 59.5mm they vary in about 0.5mm or so.

.040 sure doesn't seem like much clearance between the head and pistons.....? and that's not even compressed...
I did have the head resurfaced, but I don't think that would cause this would it?
The chambers on the head were always around 59mm.

As for the cam, that i was asking about in another post, man the lift on this thing is high I had the tappet adjustment screws completely out and it still wouldn't clear, is it possible that this cam would have required valve stem shortening?
  
This kinda makes me sick feeling after all this work and money I'm putting into it, maybe I'm overreacting?  


Thanks for any input or advice,

[754]

 Turn a piston to TDC, then make sure the cylinder is pushed down.. now measure/check the top edge of piston. Is it above the top of the sleeve?

 You may need a thicker base gasket or trim the pistons.

[BrianAdair]

Turn a piston to TFC, then make sure the cylinder is pushed down.. now measure/check the top edge of piston. Is it above the top of the sleeve?

 You may need a thicker base gasket or trim the pistons.

What is TFC? I know what TDC is, I checked it there, and the cylinders are down all the way, not sure of the measurements, but the pistons are below the edge of the sleeve, at the edges in the center they have a very slight dome, lees than the stock ones, these pistons are .030 shorter from top of pin to center of deck.

I was looking at thicker base, and or head gaskets, on Dynoman.

[mick7504]

If the pistons are below the top of the cylinder @TDC, it seems that the problem is possibly elsewhere.
Will the engine rotate freely with the head removed?

I had a crankshaft counterweight just hitting on the crankcase once and only by a few thou.
It's only ever happened once to me and still don't have an answer as to why.
The crank was from a different 750 SOHC engine but it wouldn't fully rotate.

Mick

[lordmoonpie]

If the pistons are below the top of the cylinder @TDC, it seems that the problem is possibly elsewhere.
Will the engine rotate freely with the head removed?

I had a crankshaft counterweight just hitting on the crankcase once and only by a few thou.
It's only ever happened once to me and still don't have an answer as to why.
The crank was from a different 750 SOHC engine but it wouldn't fully rotate.

Mick

If the piston is below the deck with the head off at TDC or even level with it you shold be OK given the domw is not extreme. Problem must lie elsewhere. I've had this on non-SOHC bikes and as Mick said it could be almost anything. I had an issue with a new gearbox locking the engine up becasue the selector forks fouled the inside of the cases so I couldn't turn it to TDC. One thing I found on the 750 with big pistons though was spark plug length was critical. Look at your spark plug electrodes - have they been closed up by interference from the piston? Might be worth trying with all the plugs out if you haven't already? If you have, the problem must lie somewhere else. Are the barrels seated properly on the cases - if not they might show pistons below the edge until you torque them down?

[BrianAdair]

thanks for the replies guys. The engine will rotate freely with the head off, the pistons are hitting about 1.5mm in from the edge, I didn't have the gasket in when I done this. The cylinders are all the way down and at tdc on all pistons, they are less than flush, at the edges not by much, but I would say around .005 to .010 you can see that they are lower and hang your finger nail on them.

[MRieck]

Is that cylinder decked?? Pistons usually sit pretty low in the stock deck height 750 cylinder. What cam are you using?

[BrianAdair]

Is that cylinder decked?? Pistons usually sit pretty low in the stock deck height 750 cylinder. What cam are you using?

                                                                                                                                                                                            No, it's stock, hasnt been decked. The cam is unknown...... I have a post in the high performance threads, asking what cam it is with pics and all the markings, can't seem to find out what it is.

[voxonda]

Brian, using the same head? Meaning you havent switched from K to F(2) head?

Rob.

[BrianAdair]

Brian, using the same head? Meaning you havent switched from K to F(2) head?

Rob.

  same head, K7 it was resurfaced, and I asked how much he took off, he said .012 to .013....  I was hoping it would only be like .005....  I don't see how this could change it to where the pistons would hit though, the chambers are still the same size as they were before.

[mick7504]

Having a look at the pictures http://forums.sohc4.net/index.php?topic=80296.0
(Reply #17)
The deck of the Rocky piston looks to be marginally higher than the stock piston.

[BrianAdair]

Having a look at the pictures http://forums.sohc4.net/index.php?topic=80296.0
(Reply #17)
The deck of the Rocky piston looks to be marginally higher than the stock piston.

Looks like you are right!
I measured them but was more concerned with the highest point and forgot to be concerned about the edge....

Does anyone know what a safe distance is between the pistons and head at TDC, I don't want to lower my CR by going to much, but don't want to be so close that it would be possible for it to make contact.

Looks like my options are, 1 thicker base gasket, and possibly a thicker copper head gasket. or 2 having the pistons turned down around the edges like the stock ones, in far enough to where they clear into the chambers.

What are your opinions on this?

Thanks for everyone's input!

[754]

 Turn part of the dome flat like the left piston..

[MRieck]

Turning the piston down will improve squish. You can safely run squish at .050 with stock rods. If you have good rods I'd run at .040 or even a little bit less. Measure your deck height over the piston pin otherwise piston rock will give you false measurements.

[754]

 I am thinking now, 4 spacers the thickness of the head.. would be good to pull the cylinder down hard at the gasket, to accurately measure the deck height.. just tighten to part torque.

 Mike if you are measuring to pin how do you keep it on center? If pidton rocks a bit, it still wont change much, above the pin.

[MRieck]

I am thinking now, 4 spacers the thickness of the head.. would be good to pull the cylinder down hard at the gasket, to accurately measure the deck height.. just tighten to part torque.

 Mike if you are measuring to pin how do you keep it on center? If pidton rocks a bit, it still wont change much, above the pin.

I think we are talking about the same thing Frank. I zero the indicator on the deck very close to the edge of the bore and slide it slightly to hit the edge of the piston. I position the indicator so it is over the pin where there is the least amount of rock. Most 750 pistons (stock deck with base gasket) are way down in the hole (like .045 or more).

[BrianAdair]

I am thinking now, 4 spacers the thickness of the head.. would be good to pull the cylinder down hard at the gasket, to accurately measure the deck height.. just tighten to part torque.

 Mike if you are measuring to pin how do you keep it on center? If pidton rocks a bit, it still wont change much, above the pin.

I think we are talking about the same thing Frank. I zero the indicator on the deck very close to the edge of the bore and slide it slightly to hit the edge of the piston. I position the indicator so it is over the pin where there is the least amount of rock. Most 750 pistons (stock deck with base gasket) are way down in the hole (like .045 or more).

   this is how I was measuring the otherday, when I get home I'll try and get better measurments and pictures. Dang .045    not with these pistons...

[754]

Can you not see the diff in domes on the last posted pics?

 The left piston I assume is original, at the top edge of the piston, it turns 90 degrees for about 1/4 inch, then the dome starts.. the Rocky poston is full dome. I noticed this first time I saw the pic.. which is why I say check, check and recheck your fits.. anything you miss, will bite you.

 Trim the pistons or raise the base gasket..

[754]

Your stock piston , I assume is a F as it has a bit of dome or maybe late K model.

 I compared about 5 pistons today..
 against a stock 3pce oil ring flat-top piston.
 I measured from top of pin hole to top outer edge of piston, ie  deck height.
 Stock is 17mm

 Big bore pistons ranged from 15.6 to 16.7mm. They were all lower allowing some room for decking. This included 2 diff 836cc, a 970cc and a 1080cc.
 

[mick7504]

I noticed this first time I saw the pic.. which is why I say check, check and recheck your fits.. anything you miss, will bite you.

This is very good advice to take and keep on board.

[BrianAdair]

Can you not see the diff in domes on the last posted pics?

 The left piston I assume is original, at the top edge of the piston, it turns 90 degrees for about 1/4 inch, then the dome starts.. the Rocky poston is full dome. I noticed this first time I saw the pic.. which is why I say check, check and recheck your fits.. anything you miss, will bite you.

 Trim the pistons or raise the base gasket..

Yes, I see the dome on the stock piston, (one on the left)  When I measured these and compared them I was thinking to only be concerned on the center dome height, as I was thinking this would be the highest point to cause problems.
I understand what you mean now.... Check, re-check and check again.

Your stock piston , I assume is a F as it has a bit of dome or maybe late K model.

 I compared about 5 pistons today..
 against a stock 3pce oil ring flat-top piston.
 I measured from top of pin hole to top outer edge of piston, ie  deck height.
 Stock is 17mm

 Big bore pistons ranged from 15.6 to 16.7mm. They were all lower allowing some room for decking. This included 2 diff 836cc, a 970cc and a 1080cc.
 

Thank you Frank, you have been very helpful!
I am going to remove the cylinders again and measure the Rocky pistons at the pin to the top edge, and also the stock ones, I will post the numbers this evening.
Yes it is a later model 1977 K7 engine.


Thanks to everyone who has help in this post, you guys are great!
I have done small engine work before, as in replacing timing chain, gears, and valve seals on my 360 V8 jeep engine, but I have never really been in this deep with performance stuff, so this is my first time. You guys are helping me learn and I appreciate it very much!

[BrianAdair]

Turning the piston down will improve squish. You can safely run squish at .050 with stock rods. If you have good rods I'd run at .040 or even a little bit less. Measure your deck height over the piston pin otherwise piston rock will give you false measurements.

When you say "squish" does this meen the space between the piston at TDC and the deck of the head?

[MCRider]

Turning the piston down will improve squish. You can safely run squish at .050 with stock rods. If you have good rods I'd run at .040 or even a little bit less. Measure your deck height over the piston pin otherwise piston rock will give you false measurements.

When you say "squish" does this meen the space between the piston at TDC and the deck of the head?

I don't know how accurate this is, i am learning myself. But if others can confirm its correct, its a good visual.
http://www.muller.net/mullermachine/docs/squish1.html

[MCRider]

There is a lot to digest here too.
http://www.rbracing-rsr.com/squishcalc1.html

[754]

 This may help before you pull it apart.

 Lay strips of masking tape on the piston, or get the really wide stuff and cut a few semicircles. One or two layers should work. Then turn motor gently till it starts to bind, dont use a lot of force.. then when you remove head it should show exactly where it is hitting.