Cylinder and case mating

[dmend]

Hello everyone.

I built my first VW 1600cc about six years ago so I'm still a newbie with much more to learn. I've got a 1600cc stock engine that I'm rebuilding and found my cylinders aren't setting on the case square. I had a guy with a machine in his garage mill the decks, took off .010". All four cylinders make contact near the top of the case and are as much as .025" gap in other areas. I suspect he didn't have the case halves square with his machine. Not sure I'd trust him to correct the problem if he couldn't do it correctly the first time and he was 2.5 hours away.

I assume this analysis is correct, though I did put in 11 case savers and it looks like the studs are not inhibiting the cylinder installation. Could there be another reason I'm missing why the cylinders and deck are not mating?

I've spent a day on the phone calling machine shops with no luck for deck milling. Can anyone refer me to a source in northern Illinois, southern Wisconsin, or northwest Indiana?

Other question: The gasket kit I got had four paper gaskets to put between the deck and cylinders. I've had other gasket kits without them. Are the necessary or are they an "if you got them use them" kind of a thing?

Thanks for your help.
dmend

[Marc]

Although it isn't usually an issue on a "stock" build, do check that the very bottoms of the cylinder spigots aren't contacting the main-bearing webbing inside the case.

With new cylinders there'll be no wear ridge at the upper limit of the top piston rings' travel so the only limitations on the piston deck height you can run are concern for sufficient piston-to-head clearance and the effect of the unswept volume on the compression ratio . If the case has been "decked" the piston deck height will be reduced, perhaps enough to be a problem.

Nominal stock deck height is usually around .060" as built from the factory with those paper gaskets. It's generally accepted that .040" is the minimum "safe" piston-to-head clearance - personally I prefer a bit more on an engine that's not going to be refreshed frequently.

The unswept volume has a significant effect on the engine's compression ratio - reducing the piston deck by .015" on a stock engine will bump the C.R. from ~7.6:1 up to ~7.85:1 (which can be enough to raise the octane requirement with a stock cam). When machinework like case-decking and head flycutting is performed, you must determine the resultant C.R. or risk the consequences.

Most builders forego the paper gaskets and use a sealant such as RTV between the cylinder bases and the case. A little goes a long way; too much and you'll have gummy worms clogging up your oil strainer
DO NOT use RTV if you opt for the paper gaskets - with them a very light coating of a Permatex-like sealant is better.
Steel cylinder base shims are available in a range of thicknesses (typically in .010" increments) for setting the piston deck height.

There's a good deal more to discuss, but I'll pause for you to soak this much in and for others to comment.

[dmend]

I did check that the cylinders aren't hitting the main-bearing webbings since it's a new CW crank. I decided to pull a few studs to make sure my observation was correct. I was wrong. Without some of the studs the cylinders set squarely on the deck. As a trial, I gently torqued the heads and that pulls the cylinders down to the deck. If I ever have to put in case savers in another case I'll set the studs with the cylinders in place. So much to learn...

I've got the tool to check the deck height. That's next on the plan. By your description, I didn't know the c.r. was so sensitive to change. I'm not really a numbers/math formula person so I have a tendency to gloss over some of those details.

Things always look better with a good nights sleep.

dmend

[Marc]

With the cylinders snugged down against the case, lay a straightedge across the tops (as in, parallel to the crank) and verify that it contacts them at all four points. The head can conform enough to get away with a couple thousandths gap but ideally there should be none. If this reveals that the cylinders are cocked, only redecking the case will fix it. If they're straight but not equal, it should be possible to compensate by using different-thickness cylinder spacers.
When measuring the piston deck height, stay along the piston centerline over the wristpin so you aren't deceived by the piston tilting in the bore.

I'm assuming that this is going together with stock 85.5mm bore. .060" deck yields 8.75cc unswept volume, add that to the chamber CCs....with typical ~52cc chambers that makes 60.75cc total unswept volume.
To calculate the C.R., add the unswept and swept volumes (the latter is 396.16 for 85.5x69mm) and divide that total by the unswept number.

(396.16 + 60.75) ÷ 60.75 = 7.52

Say that the piston deck is .050", that yields 5/6ths of 8.75, so 7.29cc (making the total unswept volume with 52cc chambers 59.29cc).

(396.16 + 59.29) ÷ 59.29 = 7.68

With repetition this computation becomes easy to do. But feel free to post your numbers here and we can double-check them.

It's obvious that changing the bore or stroke affects the swept volume. It has no effect on the chamber volume, but a change in bore does change the "deck volume" of any given piston deck height...the above examples only work for an 85.5x69mm combination, for anything else there's a little more math involved...

Stock cylinder head chamber volumes run from ~48 to ~54cc so for an accurate calculation there's no substitute for "CC-ing" the heads, the 52cc number used above is only an assumption, typical of the majority of stock heads that haven't been flycut or had a lot of valve-seat grinding.

[dmend]

I got some cylinder shims and was able to level the cylinders. They even looked good with a straight edge laid across them. With a good nights sleep, strong cup of coffee, and my spreadsheet I decided to tackle this math stuff and made a tool to calculate the c.r.. Your assumption was correct at 55.5x69mm. The volume spec on my head is 53cc. The heads have the ledge in them. It looks like at 0.060 deck height I’ll get a c.r. of 7.413.

I see the spreadsheet doesn't copy very well into this system.
jugs mm 85.5 inputs
stroke mm 69 inputs
deck height 0.06 inputs*
mm cubic Centimeters
cyl diameter2 7310.25
x pi 0.785
volume 5738.55
x stroke 69
= swept vol 395959.69 396.0 swept
deck height mm 1.52284
vol * deck height 8738.90 8.7 unswept
head chamber volume 53 Input
Total Volume 458
Chamber + deck vol 61.74
Engine Displacement CC swept vol * 4 1584

c.r. 7.413

total volume /sum of chamber and deck volume
*heads have lip
bore/2=radius, radius*radius*pi 3.141*stroke=displacement of one cylinder, *4
cubic inch*16.38=cubic centimeters

So now that I’ve done all this I’ve order a CB Performance Cheater Cam 2280, 87mm slip-in cylinders/pistons, 26mm oil pump. I have the Weber Progressive 2bbl, header, and the heads are 040s with stock valves and the ports are larger than stock (though not ported) and 1:1:1 rockers. This should give me 1640cc. With a deck height of 0.040 the c.r. is 7.95, at 0.60 the c.r. is 7.6.

With the ledge in the heads do you think its o.k. to set the deck height at 0.040”?


jugs mm 87 inputs
stroke mm 69 inputs
deck height 0.04 inputs*
mm cubic Centimeters
cyl diameter2 7569
x pi 0.785
volume 5941.67
x stroke 69
= swept vol 409974.89 410.0 swept
deck height mm 1.01523
vol * deck height 6032.15 6.0 unswept
head chamber volume 53 Input
Total Volume 469
Chamber + deck vol 59.03
Engine Displacement CC swept vol * 4 1640

c.r. 7.945

What size engine is in your trike?
Dmend

[Marc]

...With the ledge in the heads do you think its o.k. to set the deck height at 0.040”?

No safer than it is with non-stepped heads, unless you open up the I.D. of the ledge to be greater than the diameter of the piston top - and that could compromise the cylinder-to-head seal, especially with the thinner cylinderwalls of the 87s.
If you're looking for 8:1, it'd be better to reduce the chamber volumes by 3 cc or so instead. For example, a .020" flycut should result in ~50.15cc for 7.93:1 @.060" deck (8.11:1 @.050").

This would've been a great place to use AA thickwall slip-in 88s; they require that the heads be opened up but take no machine work to the case. Assuming the heads were also shaved ~.010"in the process, you'd end up with 8:1 @.055" (and 1679cc).

If machinework to the heads is off the table, just shoot for a little more deck height (.040" leaves little margin for safety). At .050" you'll have 7.77:1 on your 1641.

...I see the spreadsheet doesn't copy very well into this system...What size engine is in your trike? Dmend

Well, don't bother pasting all that gobbledegook in on my account. Unless I redo the frame on my trike I can't run dual carbs on it. This winter I'm building an 88x76 (1849cc) engine for it that shouldn't be too big for the 34DMTR 2-bbl but give a little more torque than the present 1585.

[rrb6699]

hey quick question similar to this subject.
I took the head off a block and noticed "new moon" valve marks on the pistons. maybe 1/64" marks. it must have been done cranking with a key because it bent the valve tops outwards slightly. I had the shop redo the head.

other than a slight increase in combustion volume, is it anything to worry about before I install the head?

the clearances seem fine.

[Marc]

Sounds like a totally different subject to me....
What kind of engine is it?

[rrb6699]

woops, sorry. its not a bug. its a daewoo 1.6l,16 valve eTech engine. the post was not meant to go in here. too many windows open. but, I guess if you can comment on that by all means...

also, need to clean the block in the vehicle.
whats the best way to clean it because I'm definitely not going to get it shop ready clean. just scotch bright pad the buildup enough to rough up the sealing surfaces for the new head gasket?

it's a 1.6l 4-cyl 16 valve Daewoo engine. eTech. everything laying flat fortunately.

[Marc]

Typically on an OHC motor if the valves hit the pistons, the cam timing is suspect - the belt may have shifted by a few cogs due to the tensioner not keeping it tight enough (or the belt may be broken completely). Valves come closest to the pistons at/near TDC on the exhaust stroke (not at max lift) and a small change in cam timing can be enough to cause contact. Like the VW waterpumpers, the Daewoo is an "interference" engine (on non-interference motors the valves should never be capable of contacting the pistons regardless of the cam timing) so when you go to put the head back on you'll need to ensure that the crank & cams are in synch. Recommended replacement interval for the belt is 60,000mi, and it's wise to replace the tensioner too....should be ~$50 for both.
That's not the only cause (valve "float" at high RPM or binding in the guide could do it too) but it's by far the most common.
There are drill-driven abrasive pads made for gasket removal - going nuts with those can actually remove material from an alloy head, and polish iron to the point where it loses "tooth" for the new gasket, so if you decide to spare some elbow grease by using one of those go easy. https://www.youtube.com/watch?v=Uy5p-cUge5s
There are also some aerosol products available which can "melt" the gasket as well - I've used those to good effect, in conjunction with careful use of a single-edge razor blade.

[rrb6699]

ok, not to get away from bugs here, but, that's the most intelligent answer I got.

I suspected the valves bent once I got no compression removing a plug. so I removed valve cover to confirm that. then proceeded to remove the head, have it reworked at a shop withj new valves. I spent today cleaning the block surfaces to prep for the head installation.

ill set the cams gears on once I see how they line up from the shop, although, all valves are shut so thats how it came back.

I don't know how reliable these interference engines are, but, they dont seem like cross country engines.

anyway the block is clean and everything except the head bolts have arrived. thats why I ask about the new-moon creases on the top of the piston heads before I start assembly.

[Marc]

...ill set the cams gears on once I see how they line up from the shop, although, all valves are shut so thats how it came back....
I ask about the new-moon creases on the top of the piston heads before I start assembly.

Until you bolt the camshafts down no valves will be open - just make sure that the crank & cam positions are "in phase" before you do...and don't roll the crank over until the timing belt is in place to keep things in synch. Typically the timing marks on the pulleys are all with respect to the point at which #1 piston is at TDC and "ready to fire" (intake and exhaust valves fully shut) - refer to your manual for guidance.

Generally speaking, most OHC engines are "interference" these days - hence the importance of timing belt/tensioner upkeep. Non-interference engines tend to be of the lower-compression, lower-output variety, although there are exceptions where the pistons have valve-relief notches to provide clearance.

As far as your valve-dents are concerned, I'd have to see them in person to be certain but most of the time they're just cosmetic. If you have the pistons out, by all means inspect the underside of the piston crown closely for any bulges or stress cracks, but odds are there's nothing to worry about if they look OK from the top. If there are any sharp edges, smooth them down with sandpaper to eliminate "hot-spots" which might cause preignition. Their effect on unswept volume (and compression ratio) is trivial.

[rrb6699]

o.k.
that's what I suspected there seems to be no damage visible from the piston tops.

I'll smooth down the sharp edges. I have 500-4000 grit abralon. I used 500/4000 over to clean the block.

I'd guess around 1000 should smooth those sharp edges.well enough. gotta watch the sleeves or crank each piston up.