Topic: Piston to head clearance - decking a block?

[Nikkisixx]

I'm no Google Ninja, mea culpa if this is a dead horse. 

Putting a set of the $100 eBay 836 pistons in a 750 street bike.  Stock rods, polished and peened, with F3 rod bolts.  The block was bit warped and had to be surfaced about 0.010. 

I just mocked up the pistons in the block to check it all out.  The base gasket (Vesrah) is in, and the block is tightened but not torqued.  The piston has a slight dome that stands about 0.070 proud at the center.  Above the rings the piston pops out about 0.010.  It stands about 0.060 proud at the center.   

I'm using a Cometic head gasket which seems to be about 0.075  0.050.  I figure once it all gets torqued the piston will have about 0.050 clearance to the head surface. But, not sure how much those gaskets will compress.

Here is the question:  Is 0.060 enough piston to head clearance?   While we are here, how about a safe piston to valve measurement?

edit: that head gasket is only 0.050

[turboguzzi]

would be interesting to know how you measure

best way is to place plasticine blobs in all suspect areas, turn engine and measure the squashed height of the blobs

that said, 060 is actually quite relaxed, if you have any areas that could do useful squish, youd want to go under 050-040... but thats kind of advanced stuff

060 is an ok number for valves to P too

[Nikkisixx]

would be interesting to know how you measure

For piston deck height : Dial gauge on magnetic base in conjuction with a machinists ruler for a straight edge.  Run the piston up until the center of the dome touched the ruller then zeroed the gauge.  Next I run the piston up like finding TDC and note the height.  Measured at the center of the dome and then the edge of piston at the wrist pin.  Verier caliper for the gaskets.

I hate getting clay in the motor.  I use it only if I need to see where the valve pocket needs some love.  Measuring the thickness of the clay is a guessing game for me and not really in my skill set.   

[turboguzzi]

sorry, but cant really follow your process... and in any case seems like  you are accumulating a lot of error

if you dont want to use clay or plasticine, then use 080 solder wire, thats VERY easy to measure

just make sure you place it symetricaly otherwise piston rock will alter the reading

[Nikkisixx]

Guzzi, thanks for the replies.  1000 words:




Gauge zeroed when the top of dome is level with deck.  Then the crank is rotated further to TDC and the gauge reads 0.061.  Repeated at the edge and it reads 0.010.  Not much room for error, perhaps a thou or so. 

[turboguzzi]

sure, i see, but sorry, that will tell you nothing about your head to piston clearance

the critical area is at the piston edge (specially with a big bore) and since during the casting process combustion chambers can shift off center a bit, you really need to use solder/ plasticine / clay around the edge and measure directly

dont be surprised if there are marked differences around the piston due to the problem noted above

http://kzrider.com/forum/2-engine/410994-gpz750810-racer-running?limit=20&start=180#438549
http://kzrider.com/forum/2-engine/410994-gpz750810-racer-running?limit=20&start=180#438550
http://kzrider.com/forum/2-engine/410994-gpz750810-racer-running?limit=20&start=180#438551

[Nikkisixx]

Hmm.  The combustion chamber is way smaller than the bore and at the edge the flat surface of the head is above the piston.  But, I do see that the dome is convex and even tho it is out of the bore 0.010 where I measured it is obviously higher than that a little further in. 

Let me see if my daughter will lend me some modeling clay...    Putting it in the fridge before measuring is a good tip, by the way.  Thanks Guzzi!

[NitroHunter]

Shoot for .040" clearance between piston and head with steel rods, .050" with aluminum.
You want the tightest squish band achieveable without hitting at max rpm.

[turboguzzi]

ok, looks like your getting it

[NitroHunter]

TG - Nice work, I like that squish band.

[turboguzzi]

seems to work too. getting best power at only 28 degs max ign advance, the edges of the piston/chamber are always super clean and motor refuses to detonation even with 12.5:1  and lotsa WOT

[PeWe]

I double checked this when timing the cam. First clay on piston crowns and later easier piston to valve procedure by tighten the valve adj screws at highest lift until it stopped or about additional 2mm down. I measured the stick out over the rocker arms with vernier caliper.
I found this: point 05: http://www.wiseco.com/FAQs.aspx
Piston - valve IN ~1,5mm, EX ~2mm. Based on what I have found on this forum

[01Thomas]

Hi!
Not to hijack this thread, but what's better about F3 rod bolts?

regards
Thomas

[Old Scrambler]

F2-3 rod bolts are stronger............I'll soon find out as I am using a set on a high-compression build with standard rods.

[Nikkisixx]

I went with solder to check the squish band and found I have a ring about 0.080 wide that has about 0.030 between the head and piston.  From what I've read that is a bit tight.  Me thinks I will be altering the chamber a bit.

[turboguzzi]

would look for  gasket material thats  010 thicker than stock and cut by hand a new base gasket....

[01Thomas]

OK, Could you let me have a part number for the F3 rodbolts and nuts?
regards
Thomas

F2-3 rod bolts are stronger............I'll soon find out as I am using a set on a high-compression build with standard rods.

[Old Scrambler]

http://www.cmsnl.com/honda-cb750f2-supersport-european-direct-sales_model14690/bolt-conn-rod_13213kv0000/

I ordered a few other items so shipping was not a big factor. You may be able to source in the US.  Used units may be OK if you trust the source, ie., a recently running motor.

[Nikkisixx]

would look for  gasket material thats  010 thicker than stock and cut by hand a new base gasket....

Not a bad suggestion but I will be reworking the chamber and milling the head anyways. 

[Nikkisixx]

would look for  gasket material thats  010 thicker than stock and cut by hand a new base gasket....

Not a bad suggestion but I will be reworking the chamber and milling the head anyways.

I looked into getting thicker gaskets.  I only checked the Dynoman site, but the head gaskets they list only go to 0.040 in MLS.  That is no thicker than the Cometic head gasket. Copper is 50 thou but not recommended for street.  Thickest base gasket also the same as what I've got, and would rather not cut my own.

How about milling the pistons flat and nipping it down a bit?  Flat pistons are supposed to improve flame travel.  I don't really like the way the dome kind of pinches off the edges of the combustion chamber at TDC anyways. It creates a little pocket that could trap gasses.  That is the reason to just fly cut a low angle chamfer in the chamber. 

Could make up the compression lost from the domes by milling the head a bit more.  I measured the dome to be 1.5 cc (others have gotten a rather optomistic 3 cc).  Thoughts?

[Old Scrambler]

Your piston profile is already rather flat. Upon final torque of the assembly, the gaskets are 'squished' and are approximately 20% thinner.  I don't know but Ken at Cycle-X may have thicker gaskets.  I have a much more pronounced piston to head/chamber clearance issue on my Salt-Flats Triumph motor. We solved the clearance issue by installing a 1/16-inch aluminum plate at the cylinder base, and then milling the circumference of the head to achieve .025-clearance at TDC with the standard (compressed) copper head gasket. We then cut thinner gaskets in case we find the need to increase compression.

[turboguzzi]

hard to say without seeing your CC, piston, etc. but that pinching might be positive for squish. pics maybe?

[Nikkisixx]

Sort of like Scrambler mentioned, I'm gonna stack some stuff. Buzz at Dynoman reccomends I add one of the 0.010 steel base gaskets to the stack along with the stock base gasket.  Word is that using two gasets will hold up fine.  Thanks for the input fellas!

[NitroHunter]

As long as the pressurized oil passage seals, multiple base gaskets can work fine... up to, and including, 5 of them.  Hey it was cheaper than pistons but the cam chain gets a little tight... 
I'm not sure why the copper head gasket was not reccommended for the street, I used some of them many years, in a lot of different motors, without issue... ??

[Nikkisixx]

Had the same question when I saw copper was verboten for street use.  I've used copper too: on old Triumphs they are standard.  Triumphs have a fire ring though, and the block is just a big chunk of iron. 

I'm going to look at the o-rings pretty hard to make sure they will seal.  And I also ordered some Karropak for the arts and crafts project turboguzzi recommended just to hedge my bets.