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piston to cylinder wall tolerance  Rating:  Rating
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 Posted: 11-16-2013 01:47 am
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Ricardo
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I just got the CR pistons from the store as I'm doing the 2.2 upgrade on my engine.  My question is:
   1-  What would the clearance be between piston and cylinder wall after it's bored and honed, in other words, the pistons are 3.7775" bore size and according to the enclosed paper the cylinder bore clearance should be  0.0025"; isn't that too close a tolerance? To those of you that have done this successfully ...What was the final CYLINDER bore diameter in your engine?

 

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 Posted: 11-16-2013 02:51 am
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Jensen Healey
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The forged pistons expand less than the stock cast pistons. Sorry I don't have documentation but I had my liners bored .030 over by Barry Spencer. He pulled the liners and used a custom jig to hold the liners during the machine work. They came back super clean on the outside so cooling isn't an issue.

I bought .030 over JE pistons from Delta. No problems in 15,000 miles.

The stock bore is 3.75" so the bore should be 3.78". I would trust JE and ignore stock clearances. They have been making pistons for the 907 for a long time.

Kurt

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 Posted: 11-16-2013 01:59 pm
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pbahr
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Kurt,

3.78-3.75 = 0.30.  Do you really mean 0.030" (thirty thousandths) over????????

YELODOG

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 Posted: 11-16-2013 03:54 pm
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Jensen Healey
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Hi Pete, yes, .3 is more than 1/4".

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 Posted: 11-16-2013 07:12 pm
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pbahr
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Looks like I don't know how to subtract !   3.78-3.75 is not 0.30, but 0.030.  I don't think the piston to cylinder should have a 0.030 difference.

Just checked YELODOG build data and it shows (J.E.) piston to cylinder have 0.0035 diameter difference.

Just to be sure, piston to cylinder diameter difference should be about 0.003 for the 907, and not 0.030.

YELODOG

 

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 Posted: 11-16-2013 08:14 pm
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Ricardo
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Thanks YELODOG, So it would be like most engines, piston to wall clearance between 0.003" to 0.0035".
 Thanks a lot to all of you.
 After about 10 years I finally decided to rebuilt my JH, ALL OF IT....Lets see how fast I can do this since I have to procure a suitable bell housing, either a W58 or a T-5 transmission, Have to replace or reinforce the rails at the unibody chassis, etc
 I'll be asking for advice to all of you, God knows I will need it.
 Thanks again to everyone, from the Sunshine State.
     Ricardo

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 Posted: 11-17-2013 02:33 am
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pbahr
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Ricardo,

Sounds like you should find a parts car to help with the project...........

Answers to any questions should be forthcoming on this Forum so that will be a great help to you.

YELODOG

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 Posted: 11-17-2013 02:51 am
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Jensen Healey
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Agree that the clearance should be 0.0025 with forged pistons.

Kurt

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 Posted: 11-17-2013 05:26 am
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Primordious
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There are a several factors involved with setting the correct piston to cylinder wall clearance, but the most important piece of information comes from the piston manufacturer. Different grades of aluminum have different expansion rates when heated. Just because a set of Brand A Pistons work well with a 0.0020-0.0025 cylinder wall clearance, it doesn't mean Brand B pistons will even run with the same clearance. If they have a higher expansion rate, they could lock solid in the cylinder bore.

Generally when you order pistons for any engine they are ordered by finished bore size, not what diameter piston you want, and the piston size you receive will be marginally smaller based on the recommended piston to wall clearance.

Here's an example;
Let's say you have a cylinder with a 4.00" finished bore and your replacement pistons call for a piston to wall clearance of 0.003". When you put the micrometer on the piston skirt about a half inch below the wrist pin holes, the reading on the micrometer should in theory give you a measurement of 3.997". If your bore is the correct 4.00" you're golden.

Oval, egg shaped, or clover leafed bores can give you fits on ring sealing and clearances, so good machine work is vital.

Side note on new pistons & rings... many years ago I tried rebuilding an engine (my first one) and not thinking I installed the rings with the end gaps basically lined up right above one another. Fouled plugs and high levels of oil consumption led to a second tear down and a lot of mocking by certain friends. On a positive note, I did remember to actually check the ring end gaps to make sure they weren't too tight. What ever brand of rings you buy, be sure to check the data sheet and see if they are pre-gapped, or if they are file to fit rings.

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 Posted: 11-18-2013 09:38 pm
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Ricardo
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there will be no oval shaped cylinder walls since they will be bored and honed.
 Thanks Yelowdog and everyone else for your advice, the paper FROM THE MANUFACTURER states that the clearance at  0.5" up from the bottom of the skirt is to be 0.0025", but as it was pointed out by you guys (And I've done with all the engines I've put together before) a clearance of  0.003" to 0.0035" is much much reasonable.
Rings at 0.016" to 0.019" gap and at 45 degrees ring gap to ring gap.
Also I had a parts car from which I took everything I could before sending an empty carcass to the wrecking yard.
 So now I have a spare 4 speed, probably a 5 speed (if I can find a reasonable w-58  or a Tremec), an extra 4 speed complete rear end Stromberg carburetors and the intake manifold and several other parts.
 Someone had posted that he was adapting a four speed bellhousing to accommodate a t-5?  Would like to know how did that turn out.
 Also, Which T-5 was used?, there are several variations and they were mounted in different manufacturers's vehicles.
 Any one replaced or reinforced the "chassis" rails?  How and with what?  Mine are still fairly good, but showing the age and I am proactive, not reactive to this would be problems.
 Guys, need all the help I can get and Thanks in advance to all of you.
     Ricardo

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