Big Bore Kit, or not

[Peter Burger]

I currently work on the top end of the motor for my 64 C car. The clearance between the pistons and cylinders is out of spec, so I am faced with replacing both. At this time I have an 86 mm big bore kit in the car.

I have access to a used - within specification - Mahle 82.5 mm Piston / Cylinder set. A friend of mine recommends to go with that instead of a replacement AA Performance 86 mm Aluminum Cylinder Kit. He is certain that on a car with an otherwise standard setting – cam, Zenith carbs, etc. – there is no difference in performance and that the lower mass of the smaller pistons might even be advantageous for the long term health of the motor.

Maybe some of you could share their opinion on which way I should go.
Thanks
Peter

[Dick Weiss]

Peter,

First of all, the big-bore kit you have are cast iron cyl's. w/just aluminum fins; NOT fully aluminum like the early chromed cyl's. The AA (or other mfg's) offered less cost compared to the OEM Mahle kits, but make sure the used set passes specs; Since your car is a C, is the Mahle set a C (or Super) w/8.5-to-1 compression and NOT an A/B Normal 7.5-to-1? Also, the existing C camshaft still intact & the correct carbs (dates)--look into the venturiis for 28mm size?

As far as performance, a little extra torque & HP w/a big bore kit, but higher 9.3-to-9.5 compression will be common unless you add extra cyl. base shims under them. If any cyl. head fly-cutting work was done, you'll need to do a chamber-cc check to determine the compression ratio.

Dick

[Dick Weiss]

BTW Peter,

How much clearance differences between the cyl's. & piston skirts? Is a ring-land showing up @ the top 1/4"
which is the most wear (several thousands) during piston reversal? The bores should be straight w/in .001".

[Wes Bender]

A friend of mine recommends to go with that instead of a replacement AA Performance 86 mm Aluminum Cylinder Kit. He is certain that on a car with an otherwise standard setting – cam, Zenith carbs, etc. – there is no difference in performance and that the lower mass of the smaller pistons might even be advantageous for the long term health of the motor.

I am just as certain that he is wrong..........

[David Jones]

There is no replacement for displacement, except re-engineering and that costs more money usually.

[Dave Wildrick]

I have seen you drive in the annual Hill Country Rallye, and you tend to drive in, shall we say, a "spirited" manner. I do not think you will be happy with the performance of a smaller displacement motor.

[Wes Bender]

Even if you keep the Zeniths (which is what I did), put a big bore kit on it (I used Shasta), replace the C cam with the street version of the SC cam and run 9.25 to 9.5 C.R. and you'll be a happy camper IMHO.

[Mike Wilson]

To me, going to 1720 cc's is a no brainer.

Mike

[Peter Burger]

Reading all the responses I think I know where your vote is.
....and David - thanks for your assessment

Thank you for the details Dick - Some information and a few questions concerning your comments.
- The venturies on my carburetors read 28-32. To my knowledge that is the standard for a C car
- How do I determine whether the P/C are for a A/B Normal or a C (Super) car.?
The casting on the cylinder reads 82 ZD2.
- The clearance between the Piston and Cylinder is on average 0.0060" (0.0025" at the skirt)
I certainly can see a different wear pattern in the top 1/4 of the cylinder wall though I
do not have specific dimensions.

No matter which way I go, and I start to lean towards the big bore kit, how would I do a cylinder chamber check and what numbers should I see?
How do I determine what cylinder base shims are needed?

Thanks
Peter

[Jim Nelson]

Even if you keep the Zeniths (which is what I did), put a big bore kit on it (I used Shasta), replace the C cam with the street version of the SC cam and run 9.25 to 9.5 C.R. and you'll be a happy camper IMHO.

That works out to be a great combination, in practice.

[Dick Weiss]

Peter,

The Normal A/B piston top is flat and long skirts. The Super piston has a raised dome w/a chamfered edge and slightly deeper valve cut-outs and similar skirts. The S90 piston has a higher dome and still having a chamfered edge, but shorter skirts to clear a counter-weighted crank--same on an SC piston, and finally--the C piston has a little raised top w/a rounded edge--same edge on the SC piston. All would be 82,5 but different compressions.

The cyl. heads should have a cc # stamped on the face between the chambers; A plastic plate would rest (& a little light grease to seal it) on the cyl. seating face. Add a pair of spark plugs to close the chambers & obviously all the finished/set valves are installed incl. the springs! Feed fluid thru a hole in the plate (water or--?) using a barrett measuring tube having marked cc's @ least 70 to fill it full to the underside). Chamber matching should be 1cc
(or less?) on all 4.

If you still want to use a big-bore kit, go to it but keep the compression no higher than 9.3-to-1; Pending the deck hights to be calculated along w/the chamber cc's will determine the ratio by adding extra cyl. base seals and premium fuel may not be needed, unless 'pinging' occurs--recheck the distributor timing; Carb jettings should be OK, unless--?

Dick

[Dave Wildrick]

No matter which way I go, and I start to lean towards the big bore kit, how would I do a cylinder chamber check and what numbers should I see?
How do I determine what cylinder base shims are needed?

Thanks
Peter

When I rebuilt the motor in my 65C coupe in 2010 (using an AA big bore kit), I had Jack Staggs rebuild and cc the heads. Using those volume numbers and Harry Pellow's formula in Secrets of the Inner Circle, I determined the number of cylinder base shims (and their thickness) I needed to get to the desired compression ratio, which was something like 9.25:1. It's been 7 years, so I don't recall the specifics off the top of my head.

I think Ron LaDow, Cliff Murray, and others who do a lot of this may have a more complex process involving calculating deck height (using plastigauge or clay, etc.) and other parameters I have no experience with. Maybe they will chime in.

[Ron LaDow]

[...]
No matter which way I go, and I start to lean towards the big bore kit, how would I do a cylinder chamber check and what numbers should I see?
How do I determine what cylinder base shims are needed?
Thanks
Peter

http://www.precisionmatters.biz/pdf/com ... -ratio.pdf
BTW, I HATE adjusting the CR with shims; totally screws the quench area.

[Wes Bender]

Ron, I've got a copy of your C. R. spreadsheet on my computer, but is it available anywhere else? That's what I used when I did my latest rebuild and it works great.

[Norm Miller]

Peter,

If you go with the AA big bore cast iron cylinders bring the piston to top dead with one shim and add shims to achieve a .075 to 1mm clearance between the top of the piston and the top of the barrel.
Do not buy the "aluminium" set as it wont fit.

Norm