Milwaukee Midget

[Milwaukee Midget]

Okay - dialed in TDC confirmed, Intake lobe at 102 degrees.

.003 clearance between the valve and the piston at their closest rendezvous.

That's not going to fly.

I'll check the other four - I'm expecting similar results. 

[Graham in Aus]

Hi Chris, brain numbing stuff cam timing! I always manage to adjust it the wrong way just as I'm getting close!

So, I read this as you are looking for full inlet lift at 107 dgrees Atdc? .340 at the pushrod is a quite some lift, are you using 1.5 ratio rockers? this would mean around 0.500" at the valve? Would you share what cam?

How did you determine the valve to piston clearance? soft springs and the dti on the valve cap?

Am I right in saying that getting to 107 will help the  situation a bit, as the piston will be that bit further down the bore at 107 Atdc?

Just sharing your pain!  

Keep working at it!!  

Edit, Just re-read and noticed the figure on the degree wheel, you are aiming for (and have now got) full lift at 102 dgrees, looks like some piston machining is in order  

What checking clearance (valve lash) are you using?

Interesting cam spec.............My guess is an APT SPVP5......You want that thing to rev...
don't you!

[Milwaukee Midget]

Interesting cam spec.............My guess is an APT SPVP5......You want that thing to rev...
don't you!

You're not fooling me, Graham.  That's no guess - that's a knowledgeable assertion by a man wise in the ways of the A-Block. 

Just for the record, I'm checking all my tolerances with 0 lash and no head gasket.  I want the margin of error on the safe side, as I'm running a chain, rather than a belt drive.  I have a Cometic head gasket - .029 thick, and with .010 lash, and I'm at about 1/2 of where I'd like to be on clearance.

So yeah, looks like those sleepless hours I spent last week putting pistons on con-rods will need to be redone.  Nothing like paying for two balance jobs on the same set of pistons. 

If this were an academic effort, it would be called "tuition".

The cam spec calls for 13.5+ static CR, which would be easy on a 1275.  In a 970, it's tough to achieve.  The upside to my combination is that with the Rod/Stroke ratio I'm using should still be able to achieve a decent dynamic CR - something I hope to calculate tomorrow. 

By the way, David Wizard, the guy who pioneered the scatter pattern cam design and confounded APT, is planning on doing a series of lectures in Australia next year using his new book as a basis for the lectures.  He'll be lecturing in Minneapolis next weekend - and of course, the class is already oversubscribed.

[38flattie]

Well, at least you know the problem- now you can fix it!

Chris, couldn't you just order thicker head gaskets? Seems an easier fix that milling the pistons, and custom gaskets would probably even be cheaper.

[Stan Back]

With his short stroke it probably would change the static CR too much, unlike the 6-3/8 Caddy stroke you're working with.

[Anvil*]

The reward for all your effort is when it transitions to showing up to an event and just having to warm up before running a new personal best (with no last minute thrashing and chasing problems). You get to relax and just have fun, maybe spend time helping someone out.  It's a worthwhile investment of your time.

[Milwaukee Midget]

Thanks, Anvil* - although given my speed at thinking things through and my klutzy nature, anything more than pointing someone to the nearest porta-potty would probably be more harm than help. 

Let me throw this one out to the brain trust.  I know a lot of you have worked with unusual combinations.

Everything I've read tells me .080 clearance on the intakes, .100 on the exhaust.  The valves are completely vertical to the pistons, the rods are steel, and the valve train is very stable.  Might I be able to tighten up those specs a tad, and if so, what combinations have worked for you?

I calculated my dynamic CR this morning, with a .029 gasket, it's ~ 10.0:1, which is pretty steep, on a 13.5:1 static, so I can lose a bit without too much worry.  I want to avoid having to recontour the combustion chamber - at least for this year.

I haven't ruled out Buddy's idea - Cometic makes one for my app as thick as .120, but at what point does a gasket become a shim?

Chris

[38flattie]

With his short stroke it probably would change the static CR too much, unlike the 6-3/8 Caddy stroke you're working with.

Haha! Same either way Stan, thicker gaskets, or trim the pistons. Least amount of compression is probably going to be an 'eyebrow' in the pistons for the valves.

Chris, can you do the 'eyebrow', or order some custom made gaskets in .060, or similar? I had all my gaskets made to order, in any thickness I wanted, but they are copper....

[Cajun Kid]

Chris  try the Cometic MLS  head gasket around .090  or so.

Charles

[jdincau]

I read back through the previous couple of pages and I am not sure so I will ask. Did you try setting the cam with the .050 lift opening and closing figures from the cam card?

[Peter Jack]

If I remember correctly the split is different on 1 & 4 than it is on 2 & 3 to compensate for the siamesed port. Is the cam put in straight up?

If the pistons have any sort of a dome I'd sure be machining the pistons rather than using a thicker head gasket because there would be a significant difference in the compression ratios you arrive at using one method over the other. If you do machine the pistons be sure to massage the resulting areas after so there are no sharp edges or points to start detonation.

Good luck. Going at this slow and steady with lots of measuring is definitely the way to go.

Pete

[desotoman]

Depending on piston head thickness you could always use this tool from Isky.

http://www.iskycams.com/pdfcatalog/2004-05/page17.pdf
Scroll down to valve tool.

Tom G.

[Dr Goggles]

The valves are completely vertical to the pistons

Eyebrows aren't going to help our friend here if I read correctly ,zero cant.

[desotoman]

The valves are completely vertical to the pistons

Eyebrows aren't going to help our friend here if I read correctly ,zero cant.

Well if that is the case you could always weld a cutting bit to a valve and do it that way. That is if you did not want to pull everything apart.

Tom G.

http://i93.photobucket.com/albums/l55/Kevin_Doe/RX-7/FlycuttingProcess2.jpg

[Milwaukee Midget]

I read back through the previous couple of pages and I am not sure so I will ask. Did you try setting the cam with the .050 lift opening and closing figures from the cam card?

Here's a copy of the cam card -



What I did was I averaged the degrees at .050 drop from either side of full lift, and right now, it's at 102.

This is turning into a great lesson on applied geometry and nonlinear arc correlation - oh, and reading the entire instructions.

I needed to build the lash into the measurement, and rereading the card, it occurred to me that I've got more lash available than I originally calculated.  I don't know where I came up with my original .010 figure - probably my MGB - but with .018 lash and no gasket, my tightest clearance becomes .023.  The balance of my clearance can be handled linearly by either going with a thicker gasket or machining the piston.

Chris  try the Cometic MLS  head gasket around .090  or so.

Charles

Doesn't look like I'd have to go that big, maybe about .070, but it raises the question as to what issues might arise with a headgasket that thick?  I would be able to maintain a static CR in the upper 12's with that.

A huge plus would be that it would be a retrievable fix if it didn't work.

And thanks for the input, Charles, Buddy, Stan, Doc, Tom, P.J., Anvil*, Graham - it's ALL appreciated, trust me.