Hello all,

I'm changing the thermostat on my car and have run into a problem that I need some opinions on.

My car is completely stock and I don't think the Tstat has ever been replaced.  Some time ago I purchased the correct 180 degree Robertshaw Tstat along with a brass restrictor plate from MARLINJACK.

When I pulled the old Tstat out, I compared it to my new one, and although they look very similar the "hat" on the top of the button on the old one I pulled out is a bit smaller than my new one:


The two parts I got from MarlinJack fit together with very little clearance around the hat when it is in the hole on the restrictor plate:


I was hoping to not replace the old restrictor plate, but my concern is that if the hole in the stock restrictor plate is too small, then the new thermostat will not open fully when the larger "hat" does not fit into the smaller stock hole.  


Has anyone seen this problem and have any suggestions? Does anyone know how to get the old restrictor plate out of the block?  It seems to be a press fit like a freeze plug and I don't want to cause damage trying to pry it out.

Thanks in advance for your help!



Images (3)
Last edited by George P
Original Post

By the way, I'm not sure why my photos didn't show up in the post. Hopefully the photobucket links will work for you.

Good observation and not just replacing part and hoping it fitted

I am "reading" that you have not measure the hole in the plate, just assuming the old would have the same fit as the new pieces.

I'll assume you do note have telescoping gage to take measurement.  But you could trim cardboard to be a fit to the new plate and use it to compare the installed plate


Last edited by jfb05177

Thanks for the reply,

I have not measured the hole in the stock plate.  It is not possible to get a caliper or other measuring tool in there to do so.  However, I do like the idea of cutting a piece of cardboard precisely the size of the hat on the new Tstat, gluing it to a stick and see if it goes into the old hole. It would be bad if the little cardboard disc fell inside though!

I'll try it but my assumption is that it won't fit. 


I just pasted the three thermostat drawings I have.  the first is the measurements I took from my MJ.  notice MJ hat is slightly larger than the RShaw and Myers drawings.Dimensions for 351C thermostat r1


Images (1)

 The Cleveland small block is the only one with an internal coolant bypass. Other Ford small blocks have an external hose that is always in operation. 

 The gap between the Cleveland plate and hat has been calculated and shown to allow a similar amount of coolant flow as the always in-play bypass hose on the other small blocks.

 It was apparently never intended for the hat to completely block the passage in the plate .

 It is possible that Marlin chose to make his hat to more closely fit the opening in the plate. 

 I do not know if there is a requirement by design for some Cleveland coolant to always be re-circulated, but that is the case on the other small blocks. 

 The thermostats now being sold by Tim Meyer are, I believe, an exact match to the originals. 

 If I were you I would do one of two things :

 Contact Marlin and ask him to explain the dimensional differences.  If it turns out his intent was in fact to more fully fill the opening in the plate I would be very cautious to assume that is a proper modification.

 Order a new thermostat from Tim Meyer .

 Hope this helps you better understand some of the issues involved in your situation.



I've got the same Flea-bay gauges as JFB.  What a coincidence.  They come in handy occasionally.


Not criticizing - they were a good value for the price.  And since THEY are not calibrated (you measure them with a micrometer), they seem to be adequate for the job.


Last edited by rocky

I suspect you're over thinking it....  Clean off the gasket material.  Drop the new thermostat in boiling water.  Wait till it fully expands.  VERY CAREFULLY remove it from the boiling water and drop it into place and see how it fits.  I think it should be nicely centered and drop in smoothly.  If it doesn't then pry out the old block off plate and tap in the new block off plate, then repeat.  No sense in buying more new parts until you confirmed what you have is an issue.

rick posted:

…  No sense in buying more new parts until you confirmed what you have is an issue ...

Which it won't be.

...I always tell My customers..."If You find the Original Plate Installed in the Block, Leave IT THERE! If you pry it out you will damage the 'Press-Fit'. NO NEED TO PULL The Original Plate.

The ID in the Original Plate is LARGER than My Repro Plate, at aprox. .740" Plus .005" Minus Zero. The original 'Hat' as You call it, is at .640", by the Last One I Measured. Being that skirt is smaller, it was NOT Restricting much of a Flow.

Read this:...In Closing Off 95%-99% of the By-Pass Flow, Even More Coolant Is Diverted to the Radiator. This is a GREAT Improvement in Cooling Efficiency. 

My Repro Plates And the SKIRTS are Machined to a Target of .723", with a .001" Clearence, that is they are Matched, Exactly. Yes, there still is a Flow past the Skirt so the Thermostat may Acclimate/Monitor to/the Coolant just returning from the Radiator. Openings: 180F=.260" 195F=.130" Know that there is a Vortex with-in a Gradiant.


Last edited by marlinjack


Thank you for the response and clarification.  You are definitely an asset to the community and deserve our gratitude!

I certainly did not mean to criticize or offend you by my questions.  If I have, I apologize. 

Thanks again!

 I do not have a degree nor any training in fluid dynamics.  Therefore, I can only respond using the figures provided by Marlin and the facts as I know them.

 One would assume the Ford engineers who designed the Cleveland plate and thermostat hat DID have training and/or degrees that would allow them to professionally design and engineer the coolant bypass feature found in a Cleveland small block. 

 The Ford engineers certainly could have designed the system with a clearance greater or less than, according to Marlin’s posted figures, the .100” clearance design they chose. But they didn’t.

Marlin chose to re-design (in his words, “improve”) both pieces of the Cleveland bypass system to allow for a clearance of just .001”. 

 If just Marlin’s thermostat is installed while retaining the Ford plate, the bypass will have a clearance of .017”. 

In summary:

.100” OEM clearance

.017” clearance with OEM plate with Marlin’s thermostat

.001” clearance with Marlin’s plate and Marlin’s thermostat 

 Any of those clearances MAY perform in a satisfactory manner. Or not.

 Removal of the plate is not an easy task; I have done this in the past. Removal may cause displacement of 45 year old debris.  The plate is not subject to wear or deterioration.   Dennis’ motor is not a bare block on an engine stand but is currently in his car.  In light of these facts I would not advise him to remove the plate, as there is no need to do so. 

 This would leave Dennis with two clearance options :

.017” using Marlin’s redesigned hat (skirt)

.100” using an OEM-spec hat

 My choice would be to retain the coolant bypass system as designed by the Ford engineers, and in order to do that I would purchase the thermostat currently offered by Tim Meyer. 


Last edited by lf-tp2511

... When I Machine the Skirt for a Thermostat, KNOWING it will be used in the Original R. Plate; I Machine the Skirt to a Diameter of .745" +.003" -.000"  as plates I Have Measured were at .750"



Last edited by marlinjack

NOTE: FYI, Marlin has edited his previous post. My comments below are based on his original non-edited posting.

 Once again, I have no training or degree in fluid dynamics. If anyone involved in this discussion does have such, they have not shared that information.  I believe this discussion is best served by relying on facts and not personal opinions.

Facts are statements that can be verified.

I.e. - The Dodgers are a baseball team.

Unverified statements are opinions.

 I.e. - The Dodgers are the best baseball team.

Marlin said the OEM plates and hats “DO NOT WORK !!” 

Obviously not a fact, as tens of thousands of Clevelands around the world perform just fine using the OEM components; Marlin presents no facts to support his statement.  

Marlin said  “... we PERFECTED ( the Cleveland bypass system). Fully tested and proven.” 

PERFECTED? Tested and proven?   It is a fact that Marlin re-designed the Cleveland bypass system components, but unless, and until, data from an engineer/ testing laboratory is provided, there are no facts to verify the system was “perfected” by those changes.

  As a skilled machinist I highly doubt Marlin ever deviated from a component’s design specifications based just on his, or anyone’s, opinion. 

I do not understand why he apparently chose to do just that in regards to the Cleveland bypass system.

 All of the clearances provided by mixing and matching the OEM components and Marlin’s re-designed components may in fact perform just fine. 

 Only one of those clearances matches the OEM clearance as designed by the Ford engineers. 

That is a fact, not an opinion.



Last edited by lf-tp2511

...and My Point was, the 'Improvement' was for the Absence Of One Or Both of these Components. Not, so much, the % of a Restriction.

These are the Persons Whom have witnessed the Greatest 'Improvement' in the Clevelands' Cooling Systems.

What an Engine!!

I'll leave the debate there.


SAE doc #700080

The original paper regarding the 351C 4V, presented by F. Miller and J. Van Deun  of the Ford Motor Company to the SAE Congress convened in Detroit Michigan January 12-16, 1970.


Here is an excerpt from page 20 of this document. I believe it verifies the expectation of Ford engineers was for all coolant to be blocked from recirculation and to flow through the radiator.



Images (2)

Add Reply

Link copied to your clipboard.