Been watching all the info in this thread as I know a rebuild is in the near future. I have a 71 351c with the factory 4 bbl. When I bought it, there was a 2 bbl stuck on the 4 bbl intake and the temp gauge stayed at the hot end of the gauge all the way from Seattle to Portland. Installed a 4 bbl carb, removed the 4 blade fan and installed a 5 blade, removed the brass hat under the thermostat and installed the kit from WCCC (aluminum plate with a hole and a regular 195 thermostat) and a three row radiator. During our open house this last year at WCCC temp in the 90’s and cruising around my temp ran consistantly 190. I checked it with a remote temp sensor at several stops during our cruises.
Not sure if I’m too late to add anything, but here goes.
The Cleveland engine does have small design flaw due to its canted valve arrangement in that the valves need clearance in the piston ring lands, I have copied a pictured from the internet and placed 2 arrows to high light what I mean.
Unfortunately these pockets cause a cold stagnant mass of fuel mixture in the combustion chamber which will not burn during the power stroke, instead it burns on the exhaust stroke on it’s way to the exhaust port causing higher cylinder pressures when the piston is trying to travel up.
Once the engine is running the only way left to compensate for this unburnt fuel mixture contamination of the exhaust stroke is the run the engine Rich, which is not the best way to solve it for a street engine. It is also one of the reasons why the Cleveland could not pass fuel economy and emission standards in the early 70’s.
The way to alleviate this problem is to reduce the pocket depth and radius the sharp edges of the pocket. Another way is to only have one shallow pocket, you should really only need a pocket for the exhaust valve (because the piston chases the exhaust valve at TDC).
Unfortunate when looking for a piston, it becomes difficult because piston manufactures tend to make one piston suits all engines and thus have 2 deep pockets to cover all the different options.
To get the problem solved with the shallowest pocket means doing an engine mock up assembly a measuring piston to valve clearance with play doe, and then slowly grinding and sanding the piston pocket to clearance.
Peter 
So you are saying, get flat top pistons and cut your own valve relief as needed, can you even get the piston with out relief already there?
It’s not too late, I am just getting the Engine cleaned and pressure tested and it has to be bored out. I had no advice or real ideas about the pistons so that’s good to know, so thank you. I’ll be taking all of this info to my machinist and talking to him about it, once they get back to me since they are full of work right now. Since I now know I have to bore it out, I was wondering if anyone had recommendations on boring. It seems that the most popular for power is to bore it .030 over, and that seems to be as far as people go. I’m not going for anything particularly performance oriented, just a fun street car, so should I just bore it maybe .010 over? or just go the full .030? Are parts more available for one size over the other or is it fairly similar? Thanks for any advice.
just bore it little as possible,to clean it up and reseat rings and true it up. same on crankshaft, why cut when you might only need a good polishing? leave as much meat as possible. I only went .010 on bore and polished the crank on mine and still gets warm in traffic and long lights. Also dropped compression down to 10 from 11.5 to run on pump gas.
As grumpyone27 says, only do what is needed. 0.10 inch rebore if possible, original Cleveland blocks are getting had to come by, save some metal for a next time rebuild.
When talking pistons it’s is always better power/economy wise to go for a dished piston.
I sort of like this shape, it about the best I could find on the internet.
The outer rim edge of the dish should be more contoured and maybe the bottom of the dish should not be so flat, but still it is sort of the correct shape. Unfortunately this is not a Cleveland piston, it is for a turbo Barra 6 cylinder ( modern version of the old ford 250).
Hopefully you are getting the idea of shape here. The deeper the centre of the dish the better, but even a 0.080in (2mm) dish is better than a flat top, in fact any dished piston is better than a flat top, and a flat top piston is way better than a pop up piston.
I don’t know of anyone that makes such a piston for the Cleveland,(although I have done very little research on it) the one pictured was cut from a billet by a company that makes them for the Barra 6. They will custom make any piston shape you want and cost not much more than a new piston. Don’t know if they do it for Cleveland’s. It maybe a case of finding something close and just cleaning it up a little with a small hand grinder/sander.
Also the problem here is the deeper the dish, the lower the compression ratio, which is counter productive, if you try to shave the heads and use thinner head gaskets you then put the valve pockets deeper into the ring land, which is also counter productive.
The way to have a high compression ratio and a deep dish piston is to use the Australian 302 Cleveland heads (the 302 Cleveland is a de-stroked 351 Cleveland with small chamber heads). All the 302 Cleveland heads seem to be in Oregon for some reason, I have seen them on Craiglist about a week ago.
Peter
Can you tell us why you think the dished piston design is better than a flat top design(and flat top better than domed? Combustion chamber shape in the head, CR, intended power, and power adders (turbo or supercharging) are at play here. I’ve seen dished pistons used in the past to lower static CR when used in turbocharged engines-gas and diesel-but would like to understand your thoughts on piston design better.
Thanks!
When the piston travels down on the power stroke, the cylinder walls become exposed to the unburn fuel air mixture and in doing so absorbs heat from the mixture thus cooling it down. The first point of contact between the fuel mixture and the cylinder walls is at the top ring land of the piston. With this section of the fuel mixture now cooled it will not burn easily.
So with a flat top piston there is an area (or should I say volume) of unburnt fuel/air mixture around the outer circumference of the piston just above the top ring land.
With a pop up piston this volume is increased.
The only time this section of fuel mixture will burn is when detonation is present.
To solve the problem we need to force the combustion plume down below the outer rim of the piston thereby the combustion plume now burns closer to the cylinder wall cleaning up this unburn mass. Also, by using a dished piston the top outer rim of the piston now displaces the volume which would have contained the cold unburnt fuel mass, again reducing the unburn fuel mass.
Another thing to consider is what happens when the combustion plume touches the piston. When this happens the super-heated plume transfers massive amounts of heat into the piston causing its temperature to rise. The plume is between 2000°C (3632 °F) and 3000°C (5432 °F), (depending on fuel ratio), the melting point of an aluminium piston is about 660°C (1220 °F). The aluminium piston (including any forged alloys) will try to transfer this heat away and in the process will super heat the ring land where the unburn fuel mass is on the flat top piston. The fuel mass will now ignite and this is your detonation.
The plume will always touch the piston at the closest point between the two, this is generally at the centre of the piston crown. By using a dished piston the distance between the combustion plume and the area of contact with the plume is increased.
If the unburnt fuel mass was left unchecked, not only does the engine lose the power the mass would have produced, but that unburnt fuel mass enters the exhaust stream on the way to the exhaust port where it burns and produces increased pressure on the now upward moving piston during the exhaust stroke making the engine work harder.
To try and give some support to what I am saying here is a quote from David Vizard’s book.
“How to build Horsepower volume 1”
“The most popular compression ratio we work with is 8.8:1. Several of our chevy engines with this compression use dish pistons. 186-style closed-chamber heads, and a stock type carburettor. Test after Test has shown that they produce as much as 20 horsepower more than an equivalent engine with an open-chamber/flat top piston combination.”
David Vizard’s description here below does not explain the reason why very well, but I guess that is all he had to go on.
“A piston dome hurts horsepower by restricting flame front travel in the combustion chamber”
I hope that answers your question, but remember the advice I am giving here is for a lean burn, powerful, but economic engine used primary for road use and daily usage.
Each engine design must be considered on what its intended purpose is, if you were running popup pistons it would mean running rich, if you were running methanol you would use a piston dish shaped like a donut and use lots of swirl. You must consider what your intended outcome would be.
Peter
You’ll have no metal for next time if you bore 0.10, you’ll have cooling jacket open where the cylinders were. 0.010" you mean I’m sure.
oops! yes 0.010"
1969xr7vert. yep that’s what i mean’t.
Another recommendation is to use a longer connecting rod, partictuly for a regally used street engine. The way it was done here (Australia) was to use the Australian 302 Cleveland 6 inch rod in the 351 Cleveland, In fact it was so popular the there where pistons specially made for the conversion, one of them is the ACL I first pictured here (to show the eye brow cuts in the piston). There where may other American piston manufactures which also made similar pistons for the Australian market place.
I believe it is also possible to enlarge the pin end of the 302 Cleveland rod from 0.912 to the more common Chevy 0.927 so there is a greater selection of pistons.
The idea of a longer connecting rod has been a fiercely debated topic for years, it does work but not the silver bullet that some people would like to think it is. It is more of a fuel efficiency modification than a power modification.
Rod ratios is all about fuel ratios, that is you would use a short rod for rich fuel mixtures and a long rod for lean mixtures, and a very long rod for part throttle lean mixtures. So for a engine which mostly cruises in the 15:1 fuel ratio, it will need very long connecting rod, the longer the better.
So what could you expect power wise, well about 4.5-5% gain in power if changing from 11.5:1 to 12.5/13:1 fuel ratio and increasing rod ratio to about 2:1, but about 10% fuel economy gain. For a 15:1 fuel ratio (at cruise) it’s about a 20% fuel economy gain.
As an example there is approximately 20% fuel economy gain between a 302 Windsor and the 302 Cleveland, the main difference is that the 302 Windsor has a rod ratio of 1.69:1 and the 302 Cleveland has a rod ratio of 2.01:1.
I am not saying the 302 Windsor is a bad motor, it is my favourite, just that the 302 Cleveland is so far out in front economy wise and always gets over looked. Where a bout’s peak power is very similar for both engines.
(also note the is only 20hp difference between a 2 barrel 302 Cleveland (230hp)and a 2 barrel 351 Cleveland (250hp)in the same car when looking in the ford workshop manual)
If you look closely at different engines you may notice that engines with long rod ratios have reputations for good fuel economy.(Also note engines with square bore/stroke ratio’s also tend to have good reputations of good fuel economy).
By putting the 302 Cleveland 6" rod into a 351 Cleveland changes the rod ratio changes from 1.63:1 to 1.72:1, it will save it’s cost in fuel. It will also run smoother and respond better to the throttle movements.
But if power is your main concern, it would be better to run a stroker crank with short rods and run it rich.
With the introduction of inexpensive stroker kits for the Cleveland the long rod idea was mostly forgotten.
I attribute this modification to Smoky Yunick, but the idea may go back further in history to the 1920-1930"s where engines with 4:1 compression ratio’s had very long rod lengths.
So I guess you maybe questioning why I am talking about fuel economy, well it to offset the deterioration of fuel economy when using a set of lower (high numbered) diff gears, yes I am saying to let the engine rev.