Deck Height Again

Advokat · Post by **Advokat** » Sun Jul 28, 2002 3:21 pm

Once again it is time for my daily question. I assembled my short-block today and it went pretty smoothly all back together and no binding.

Went to check the deck height and my piston top sits a little bit higher than the top of the cylinder at TDC. This is a 1904 (74mm crank) so I knew this could happen.

Question is, how is the best way to figure out what size barrel shims to purchase? I was trying to get an idea of the excess piston clearance with a feeler guage, but this does not seem very precise.

I was thinking about just getting a few different sized shims and going by trial and error until I liked what I saw.

What do you all think?

By the way, thanks for all of the help-every day!

Rick

Post by **sparkmaster1** » Sun Jul 28, 2002 3:29 pm

Figure how far out of the cylinder it's sticking, then go to the aircooled.net tech section and figure out how thick of a shim you need for the compression you want. You will also have to cc the heads also. Tim

Advokat · Post by **Advokat** » Sun Jul 28, 2002 4:47 pm

Sparkmaster, I realize that, but the question remains, how do I best go about accurately figuring out how much is sticking out?

Thanks

Chiba · Post by **Chiba** » Sun Jul 28, 2002 5:31 pm

You could use a micrometer depth gauge, Pointy end of a vernier, vernier or digital height gauge, dial indicator and surface table or use a straight edge and some feeler gauges. It's better to start with longer cylinders and machine them to size.
Don't Stack shims, Don't use head gaskets, Don't machine pistons and Don't use the deck height to dial in Compression ratios, the proper way is to cc the head.
Keep the Deck height to 0.040" and the pistons will Thank you.
That's what i think anyway, half the people on this forum will say the opposite.

Bore" x Bore" x Deck Height" X 12.87 = Deck Height volume in cc

[This message has been edited by Chiba (edited 07-28-2002).]

Murdock · Post by **Murdock** » Mon Jul 29, 2002 7:31 pm

<BLOCKQUOTE>quote:<HR>Originally posted by Chiba:
Keep the Deck height to 0.040" and the pistons will Thank you.
That's what i think anyway, half the people on this forum will say the opposite.

[This message has been edited by Chiba (edited 07-28-2002).]<HR></BLOCKQUOTE>

I'm building a 2275cc (94x82) engine and c/r is 8.5, for a deck height of .040in I'd have to have 70cc heads( per aircooled.net calculater). Maybe I'm misunderstanding what you're saying, as 70cc's sounds like alot?

Bent · Post by **Bent** » Tue Jul 30, 2002 1:53 am

Generally speaking for the lay man .040" is the Minimum deck, you can have much more if and when needed.
Ad , what about measure it at bottomm dead centre to give you a starting size.

[This message has been edited by Bent (edited 07-30-2002).]

danimal · Post by **danimal** » Tue Jul 30, 2002 9:47 am

advokat, the easiest way to do it is to use any borrowed shim that tall enuf to get the jug above the piston... calculate deck height from there.

if you use the right shims, it's actually preferable to running no shims... rimco machines a wide-base shim that protects the jug mating surface on the case much better than the narrow seating surface of the jug itself.

if you've ever overtorqued/overheated a 10mm head stud motor, you might have seen the case damage caused by the jug... it leaves a raised-area welt around where the jug sat on the case.

>>>I'm building a 2275cc (94x82) engine and c/r is 8.5, for a deck height of .040in I'd have to have 70cc heads( per aircooled.net calculater). Maybe I'm misunderstanding what you're saying, as 70cc's sounds like alot?<<<

don't know about your numbers, but getting a tight deck with pump gas on a big motor will usually require the maximum amount of head cc's... in fact, the vw factory now uses stock heads with 58-60cc chambers on their mex motors.

60cc's is definitely possible with cb 044's... do an stf search for the details on allen weiss's motor, you'll see what i mean.

dan
oceanstreetvideo.com

Advokat · Post by **Advokat** » Tue Jul 30, 2002 12:38 pm

Thanks guys. Actually, I did what I thought and what Danimal suggested and used some barrel shims to see where things are and it worked out. Not every cylinder has the exact same deck height, however.

Thanks

Post by **Marc** » Tue Jul 30, 2002 1:16 pm

<BLOCKQUOTE>quote:<HR>Originally posted by Advokat:
 Not every cylinder has the exact same deck height, however.
<HR></BLOCKQUOTE>

Differences in rod length and piston compression height will account for some of the variance, and the left and right sides are often different. Make sure that the barrels are all the same length and that the case was decked evenly (use a straightedge across adjacent barrels, move barrels around to see if deckheights change).
It's not unusual for there to be differences in deck height but you want to be sure you know WHY in each case.

JC-ATL · Post by **JC-ATL** » Tue Jul 30, 2002 3:19 pm

<BLOCKQUOTE>quote:<HR>Originally posted by Marc:
Differences in rod length and piston compression height will account for some of the variance, and the left and right sides are often different. Make sure that the barrels are all the same length and that the case was decked evenly.<HR></BLOCKQUOTE>

Also, though it sounds like you've already closed up the case, it's a good idea to measure wrist-pin to deck size on your pistons, and the length of your rods, and cc your heads before you assemble anything. This allows you to match "long" rods to "short" pistons and match these to the variances in your heads for the best possible combinations. Not absolutely necessary, but it will make a difference in how smoothly she runs, as your compression ratios will be more closely matched.

Good luck.

------------------
_________________________
Fast bikes & slow cars

Advokat · Post by **Advokat** » Tue Jul 30, 2002 5:34 pm

My heads are coming from AJ Simms and I called him today to report this deck height issue as this is pretty important with the work he is doing.

One thing he mentioned about my disparate numbers was the fact that I am using barrel shims (two per jug) and that these are the stamped type that frequently have imperfections and raised spots from poor toolwork. He suggested very lightly sanding them to take off the rough spots and this could easily be responsible for .005 difference.

Thought you might want to know this.

JC-ATL · Post by **JC-ATL** » Wed Jul 31, 2002 6:03 am

I wouldn't use two barrel shims, I'd use one, and get a machinist to mill one for you to the correct thickness.

danimal · Post by **danimal** » Wed Jul 31, 2002 6:03 am

<BLOCKQUOTE>quote:<HR>Originally posted by Advokat:
One thing he mentioned about my disparate numbers was the fact that I am using barrel shims (two per jug) and that these are the stamped type that frequently have imperfections and raised spots from poor toolwork.<HR></BLOCKQUOTE>

BZZT! you are stacking stamped shims?? that is a red-light foul

throw all that junk in the trash, and get yourself some machined shims.

dan
oceanstreetvideo.com

vtec this · Post by **vtec this** » Wed Jul 31, 2002 9:02 am

I would get a base shim that is as thick as the amount that the piston sticks above the barrel. That would make a deck height of 0. I would then use a head gasket as my actual piston to head clearance. then go from there.

this is only what I would do.

Advokat · Post by **Advokat** » Thu Aug 01, 2002 11:48 pm

As usual, thanks again for your help. I ordered a set of the correct size from Rimco.