Ajusting CR on 2.5 conversion

Anglocanadian · 11-10-2019, 06:37 AM

Going with the Bruce Anderson 2.2 to 2.5 conversion, he describes taking .04" from the heads to up the CR. I find that this would not leave enough material to angle out the combustion chamber to 90mm.
Is there any reason I cant skim the top of the cylinders ? I have to put the ring grooves in anyway.

Greatly appreciate any advice.

faapgar · 11-10-2019, 06:56 AM

You need a slight bevel in the head for the larger 90mm piston.If it is a 90mm 2.7 RS style you need a longer rod to get the CR to 8.5.JE or others can move the pin location up or down to get your desired CR.Ciao Fred

Anglocanadian · 11-10-2019, 11:01 AM

Yes I have already beveled the heads to 90mm , after calculation the CR I figure removing .04" will give me 8.16:1 which is better than described in the book also the deck height is 2.5mm at the moment so a 1mm reduction would be perfect.
Something that occurred to me was rather than taking .40 off the chain housing to match why not make a custom .40" shim to insert between heads and cam housing ?
I really don't want to go to the expense of new rods.

ed mayo · 11-10-2019, 08:28 PM

We did the cam housing shims years ago, they work just fine.

Geneulm · 11-11-2019, 03:34 AM

Can’t you stretch the rod by machining an offset into the top bushing?

Trackrash · Trackrash's Garage

On my old 2,5 I was able to get to 8.4 CR using Mahle RS P&Cs. .040" was taken off the heads and I omitted the cylinder base gaskets. It was a nice running motor using 36mm ports and E cams.

Anglocanadian · 11-11-2019, 03:04 PM

Thank for the conformation on the shim Ed , it did seem to me to be better than running into problems with the chains.
If cost was not an issue i would send off the rods for adjustment it is the simplest solution.
But my situation is i have access to basic machinery (lathe,Mill and surface grinder no jig grinder or hone) and my labor is free, and taking 1mm of the top of the cylinders is a fairly easy operation. I don't want to drop the deck height below 1mm so i think i will leave the bottom gasket in

Trackrash · Trackrash's Garage

Quote:

Originally Posted by Anglocanadian

Yes I have already beveled the heads to 90mm , after calculation the CR I figure removing .04" will give me 8.16:1 which is better than described in the book also the deck height is 2.5mm at the moment so a 1mm reduction would be perfect.
Something that occurred to me was rather than taking .40 off the chain housing to match why not make a custom .40" shim to insert between heads and cam housing ?
I really don't want to go to the expense of new rods.

Won't that throw off the rocker to valve geometry?

On my motor I just enlarged the three holes in the cam sealing flange so it would align properly. Worked fine.

dannobee · 11-11-2019, 04:33 PM

Re: chain housings, just omit the gasket and use threebond.

Re: rocker geometry. I think he meant 0.040". In that case, no harm, no foul. Regrinding the cam to a lower base circle will take off more than that, and in any case, a lash cap would fix any geometry issue.

Why not take 0.040" off of the bottom of the cylinder instead of the top? Since I'm really lazy, that's what I would do instead of fiddling with the taper and/or groove.

Anglocanadian · 11-11-2019, 06:20 PM

Quote:

Originally Posted by dannobee

Re: chain housings, just omit the gasket and use threebond.

I wanted to keep the chain housing in its correct position so there is no problem or the chain tension

Why not take 0.040" off of the bottom of the cylinder instead of the top? Since I'm really lazy, that's what I would do instead of fiddling with the taper and/or groove.

The barrel's came without CE grooves so I need to machine them in as well on the top.

911pcars · 11-13-2019, 12:37 PM

On a past rebuild/build from 2.5 > 2.7, the cam wasn't centered in the cam housing oil seal. I compensated by milling an appropriate amount from the cam box sealing surface.

Moving further along, with the cam housing/cylinder head/cylinder stack closer to the crankshaft center line, the result created excessive timing chains slack, too much slack for the tensioner range. I ended up sourcing oversize idler sprockets to restore the tension range. Worked fine until I sold the engine at 23K miles.

Sherwood

Anglocanadian · 11-13-2019, 02:37 PM

Quote:

Originally Posted by 911pcars

On a past rebuild/build from 2.5 > 2.7, the cam wasn't centered in the cam housing oil seal. I compensated by milling an appropriate amount from the cam box sealing surface.

Moving further along, with the cam housing/cylinder head/cylinder stack closer to the crankshaft center line, the result created excessive timing chains slack, too much slack for the tensioner range. I ended up sourcing oversize idler sprockets to restore the tension range. Worked fine until I sold the engine at 23K miles.

Sherwood

That,s exactly what i intend to avoid by using the shims to keep the cams in there original position.
In fact I have already made a set !

faapgar · 11-13-2019, 03:34 PM

First you have to consider the age of the motor.Is this the 1st or 2nd or 3rd valve job.When the valve seats are cut and the valve resurfaced things change.I have redone motors where with 10.5 they were actually 7.9 to 1.When you start with an unknown motor you CC it.Then you have a starting point.Assumption does not work well when you play with these.Ciao Fred

faapgar · 11-13-2019, 03:54 PM

Do you ever wonder about the side effects of pulled head studs or broken head studs?You should sign up for Boyt911sc engine rebuilding class in April.Tony has asked me to help.I will do one week with 2 schools.He is a wonderful person who will be 77 soon.No book will ever cover this school.Ciao

Trackrash · Trackrash's Garage

Quote:

Originally Posted by Anglocanadian

That,s exactly what i intend to avoid by using the shims to keep the cams in there original position.
In fact I have already made a set !

Very interesting. I can't wait to see how those work.

What material did you use and how did you cut them out?

Anglocanadian · 11-13-2019, 06:21 PM

I got the local metal supply store to cut me 6 pieces of .04" aluminum and then marked out and spotted through the stud holes. After they are fitted over the studs simply scribe around the areas to be cut out and then mill and dremel away.
I did get fancy and media blasted them for a nice finish

dannobee · 11-14-2019, 09:23 AM

I think the spacers are supposed to look like this

Walt Fricke · 11-14-2019, 02:30 PM

Either should work. I'd consider making 12 singles - should save setup costs at your friendly local hydrocutter?

Anglocanadian · 11-14-2019, 02:49 PM

They actually do hydro cutting at the metal supply shop I got my aluminum from.The time involved in measuring all the stud centers and then the extra cost of the cutting wasn't justifiable for a one off project.
I cant see the point of putting three together , your throwing away half the material and the connecting strips don't add any support .

Henry Schmidt · Henry Schmidt's Garage

A few years back, we were able to make 10.25:1 compression with RS pistons on a short stroke 2.5.
The magic was in the heads.

Old school with a twist