Top end rebuild of spare engine questions

Hi everybody,

I apologize in advance for the long post, but I need some help.

I have a few questions to which I have not yet found an answer on the web.

Background: the current engine is a 600cc. Runs very rough, even when the carbs are synch-ed. I have adjusted the valves, and it still runs rough. Compression measured dry is within specs, and wet is higher (around 145 psi dry, and 170 wet); this would indicate that the rings are a bit tired. The engine vibrates and makes lots of noise upon accelerating harder (high frequency noise like loose pieces of metal inside a can). I adjusted the cam chain tension, the noise is still there. When traveling at high RPM the bike consumes a lot of oil, which may indicate that the valve guide seals and/or rings are tired.

To me, all this indicates that I should probably rebuild the top end. It so happens that I recently bought a cheap spare engine, whose top end I am rebuilding now. All is documented on my website, where I keep a journal of what I do. Here are the issues though that confuse me.

The spare engine has a bore of 61mm and appropriate pistons whereas the factory default calls for 58.5mm. It appears somebody
modified it. Questions:
* if I replace the whole top end: head, cylinders, pistons, valves, would compression not be lower? Since the volume of the over-bored cylinders is higher, but the stroke is the same?
* how would this affect performance?
* would I have to also change the crankshaft to modify the stroke to be longer?

About the spare engine's rings. The gap in the service manual is something like: standard 0.2 and 0.3 mm, and limit 0.7. The piston rings of the spare engine have a gap around 0.4 mm. So they are definitely not within "standard" specifications, but still not beyond "limit".

Does this mean that the larger gap will end up in consuming more oil, but it is still ok since it is not beyond the limit?

Any pointers are much appreciated and I thank you all in advance for any and all help.

 

With the larger diameter piston, you have a larger volume of air at any given time during the stroke. So - you could have a higher compression ratio, depending on you free volume of air at TDC.

I think we need to know how long your stroke and your original compression ratio - or the free volume at TDC. This will be needed to calculate the new compression ratio.

It could be the same, higher, or lower.

By the way - you have an FJ bike, not an XJ?

 

Thanks for your advice. I permitted myself to post this question thinking that the FJ and XJ have the same engine. However, I will refrain from posting in the future if this is a problem.

 

Like already has been said, the swept volume is larger. Compression is calculated by chamber volume at BDC and TDC, and you now have a larger chamber to fill, and therefore have more area to compress.

Probably will not be able to tell seat-of-the-pants-wise. Might be worth 1 hp at most.

No.

Generally a larger than normal top ring gap will not increase oil consumption, but may increase blow-by and lower dynamic compression. But .4mm is not bad, I wouldn't worry about it.

 

You could post it under the Other Bikes Forum if you like. We're all bikers here and enjoy troubleshooting so bring it on!

 

I wasn't saying it wasn't okay to be in this forum, I was just making sure you knew where you were! :lol:

Actually, I went out riding just now and thought about this. I think you would raise your compression ratio.

I just did a calc from your posting.

Looked up on the internet, your stroke is 56mm.

From that your piston travel volume (from BDC to TDC) is 163cm^3. With four cylinders, you went up to 650cc! Congrats! I think you should be able to feel that in the pants!

Now consider that the bore was enlarged, where the piston travels, but the top head was not. So, the amount of air you suck in during the downstroke has increased. BUT, the volume you compress it to (in the top head) has NOT changed.

I also got off of the internet that your original compression ratio was 10:1. With the increased volume, your compression ratio is now 10.9:1.

So, you might want to think about running higher octane gas! I think that having a higher compression ratio will increase your power, but it will also increase the stress to the engine.

One thing to do if you want to avoid this would be to run a higher head gasket thickness than normal.

If you want to do this, give me the original thickness and I could calculate the new thickness you would need to offset the higher compression ratio. In other words, with a thicker gasket, you could retain the extra displacement, but have the same compression ratio.

 

Great!

I found a formula for calculating the compression ration:

CR = ((pi b^2 s) / 4 + Vc) / Vc

where:
pi = 3.14
b = bore
s = stroke
Vc = volume of combustion chamber, including head gasket

Substituting CR = 10, b = 58.5, s = 55.7 for my current engine, I get Vc = 16626.24

Now, replacing with the new bore, of 61mm, I would get CR = 10.78 : 1 compression ratio if I use the new head and pistons.

This is just approximate, but it may be indicative that I might need to run higher octane meaning, more money for gas.

Richard, the problem with the thicker head gasket is that I won't be able to get one. I posted before here about making my own and most people discouraged me. The one I get from the dealer I think comes in just one size.

 

yep , i agree with most whats said , and also in this factor when they rebore one can have a choice of compression ratio's depending on the maker of the pistons as one has to fit new pistons too ya see , so more than likly its a standard comp ratio [which means the head of the piston is machined to compensate for this factor +/-] to keep it to the original spec

4mm seems fine to me too unless you got plenty of da money then why not!

 

That is interesting about the piston shape, does it look different ohmega?

 

I have some pictures of the piston heads here:
http://fj600.blogspot.com/2006/05/new-engine.html

Hm, probably the right way to find out whether the compression ratio will have changed or not after I put the spare head in would be to actually measure the volume and calculate it for real.

 

for thicker gaskets

can't u put 2 gaskets? :roll:

 

I think I was wrong about needing a thicker gasket. If the compression is higher due to bore size, in order to lower it I would need a thinner gasket right?

In any case, stacking two gaskets doesn't give one much control over the thickness.

Thanks for the input!

 

A thicker gasket would give you a higher compression ratio. This is because it increases the volume when the piston is at BDC and at TDC, but it increases the TDC by a much larger percentage. This would lower it. As far as having two gaskets, I don't know if that is a good idea.

Head gaskets are supossed to compress a certain amount. If you have two, then I would guess that it would need to compress even more (more torque on bolts). This would mean getting closer to stripping the aluminum.

 

Doubled up gaskets will lower compression. You're making the combustion chamber volume bigger with stacked gaskets, not the swept volume (as you do when the block is bored).

I know people who have stacked gaskets with no ill effects, but I personally wouldn't recommend it.

 

You are right Joel. I contradicted myself. It would lower compression ratio.

 

Oh, I get it!!! I looked back at the formula for the compression ratio.

Vs = pi b^2 s / 4 = volume of air swept by the cylinder

Vs + Vc
Then CR = --------------
Vc

where again
b = bore diameter
s = stroke
Vc = volume of compression

And it's clear that if the swept volume Vs is made larger by a bigger bore diameter, then CR is larger. On the other hand if Vc increases, then the compression ratio decreases. To see this, let X = Vs + Vc, and Y = Vc

Imagine that we add quantity Z to Vc, thus, the new compression volume is Vc + Z. Therefore, the new compression ratio
X + Z
CR1 = ----------------
Y + Z
X
where as the old ratio was: CR = -------
Y

We're trying to answer the question: is CR1 smaller than CR ?

which is the same question as: CR - CR1 greater than zero?

we do: (X/Y) - (X+Z)/(Y+Z) = XY + XZ - YX -YZ = Z(X-Y)

So, when is Z(X-Y) greater than zero? When both Z and (X-Y) are greater than zero. We know z is greater than zero, because we enlarged Vc by Z. And we know that X-Y is greater than zero, since X=Vs+Vc is greater than Y=Vc.

Therefore, if we enlarge the volume Vc above the piston, either by using a thicker gasket, or by removing some of the material around the valves, then the compression ratio will decrease.

The question remains by how much to decrease it such that it compensates for the added compression that results from the bigger bore. But I can calculate that easily.

Thanks guys, this is a great forum. I know now what options I have to do the top end rebuild correctly.

 

Quick way......to check if the heads are the same is first clean chambers , then keep level then with a measure device fill flush with water , then do other head.... then compare...

bigger gap in combustion camber =lower compression

narrower gap is opposite