I'm installing a autozone 15356 thermostat in my bbc and was wondering what size bypass hole to drill.
thanks
thanks
What size bypass holes in thermostat.
lucky3
1 - 18 of 18 Posts
Joined
·
345 Posts
I've been using two 1/8 holes with good results. They seem to help a lot with wild temp swings just prior to the thermostat opening
Joined
·
4,963 Posts
using 3/16 holes with good results
Joined
·
553 Posts
I have 4 1/8" holes. Blocked off bypass in a smallblock. Works great!
Joined
·
8,690 Posts
Don't drill ANY damned holes. If the holes were needed, the thermostat engineers would have put them there. This assumes you haven't blocked the factory-engineered coolant bypass, or if you did, you don't have an open-flow heater core coolant circuit.
Robertshaw thermostats were made with a small stamped vent, probably about 0.062 or so. 1/8" is too fukking huge, never mind two or four of 'em, or 3/16".
Might as well take the thermostat out.
Don't screw with the factory bypass, don't drill the thermostat.
Robertshaw thermostats were made with a small stamped vent, probably about 0.062 or so. 1/8" is too fukking huge, never mind two or four of 'em, or 3/16".
Might as well take the thermostat out.
Don't screw with the factory bypass, don't drill the thermostat.
Joined
·
10,919 Posts
I drill one 1/8 inch hole in mine, it helps in letting the air out of the heads when you're filling the engine with coolant for the first time.
Maybe Grouchy, I mean, Schurky hasn't seen the newer cars thermostats with the vents in them...
Maybe Grouchy, I mean, Schurky hasn't seen the newer cars thermostats with the vents in them...
Joined
·
8,690 Posts
I've seen thermostats with vents (the aforementioned Robertshaw units, with a single, tiny vent hole. It doesn't take an eighth-inch hole to vent air during coolant-fill. I've seen thermostats with "jiggle valves" that vent until the water pump pressure closes the jiggle valve entirely.I drill one 1/8 inch hole in mine, it helps in letting the air out of the heads when you're filling the engine with coolant for the first time.
Maybe Grouchy, I mean, Schurky hasn't seen the newer cars thermostats with the vents in them...
I've never seen a thermostat with enormous 1/8" or larger open "vent" holes as engineered. If you've seen something like that, post a picture, part number, and the application. So far as I know, this whole "1/8" hole(s) in the thermostat flange" thing is a shade-tree attempt to "fix" something that isn't wrong to begin with--or--it's compensation for an improper coolant fill procedure--or--it's because there's a fault in the cooling system and someone is modifying the thermostat to "cure" something else.
True enough, if you defeat the OEM bypass, and you aren't able to use the heater core coolant path as a bypass, the thermostat will need a hole or two popped into it. (but why defeat the bypass system?)
Joined
·
8,690 Posts
I put some drill bits into a Robertshaw thermostat vent stamping, to gauge it's size.
I had to use the second-smallest drill bit that I own-- #79--to get it to fit into the triangular hole. A #79 drill bit is 0.0145, just two thousanths larger than one tenth the size of an 1/8" (0.125) drill bit.
Of course, the actual area of the vent stamping is larger than a #79 drill bit due to the triangular shape--I guess it's approximately equivalent to a 1/16 or 1/32 hole, but hard to actually measure. It's damn sure smaller than a 1/8 drilling. It'll easily vent air, but wouldn't flow water very well at all.
![Image]()
![Image]()
I had to use the second-smallest drill bit that I own-- #79--to get it to fit into the triangular hole. A #79 drill bit is 0.0145, just two thousanths larger than one tenth the size of an 1/8" (0.125) drill bit.
Of course, the actual area of the vent stamping is larger than a #79 drill bit due to the triangular shape--I guess it's approximately equivalent to a 1/16 or 1/32 hole, but hard to actually measure. It's damn sure smaller than a 1/8 drilling. It'll easily vent air, but wouldn't flow water very well at all.
Joined
·
4,963 Posts
THERMOSTATS
Manufactured by Robert Shaw and modified by Stewart, these custom thermostats feature a balanced sleeve design and are constructed for high flow, high RPM applications. Strongly recommended for any performance application - must be used with Stewart Stage 2, 3 or 4 water pumps. Our Stage 2,3 and 4 Small Block Chevy water pumps do not have the factory bypass hole, therefore we put three 3/16” hole in our modified high flow thermostats, this moves the bypass to thermostat. This also allows air to escape on initial system fill making bleeding the cooling system much easier.
http://www.stewartcomponents.com/index.php?route=product/category&path=61
Joined
·
8,690 Posts
1. The thermostat wasn't engineered that way, it was "modified" for a specific, specialized purpose.
2. I've said all along that if you screw-up the OEM bypass, AND you can't use the heater core as a bypass, that the thermostat will need to be drilled.
3. Note that Stewart only mandates them with the Stage 2, 3, and 4 water pumps--NOT with a water pump having a functioning bypass! Stewart sells drilled thermostats, but tells you they aren't required if you have a functioning bypass.
1. Cuts machining time and cuts cost when turning a raw casting into a finished pump?
2. Produces 1% better coolant flow which could theoretically be useful for competition, but would be silly on street-driven vehicles?
3. Justifies selling a high-profit ($25--$27) thermostat?
Joined
·
4,963 Posts
Not arguing with you just showing what I thought you asked for. I believe your thoughts. But removing the bypass hose and not running a heater, the holes in thermostat work for me. Rather cleaner look with out by pass hose IMO.
#3. , although selling a thermostat for twenty-five bucks when you can get one for less than two bucks from rockauto isn't really justifiable to an ethical person. Only thermostat failure I've ever had was a Robertshaw in my 318 Mopar. Thermostat was in use about six months when it failed closed.
Joined
·
8,690 Posts
The Robertshaw thermostats were always sold as a "premium" brand; they have some very useful features--one of which is that when they're open, the flow area is much larger than an ordinary Stant or other brand. They're not two-buck cheapies. Tripling or quadrupling the price because you've popped three holes in them seems steep, though.
Robertshaw vs. Stant, as seen through near-boiling water. Note larger flow area on Robertshaw.
I had a sticky one on my Toronado. Given that every vehicle I ever owned except an air-cooled '67 Kraut Can had a Stant in it at one time or another, and only a few of them failed...but this was my first-ever Robertshaw. Yeah, I was disappointed. I put one in my K1500 a few months ago, and it's working fine.
At this point, the Robertshaw thermostats are discontinued. There's no listings on the Robertshaw web-site. It may be that Stant bought the tooling, 'cause I've got a Stant 'stat in a Stant box with "Stant" stamped-into the metal side of what appears to be a Robertshaw-design thermostat. I think even those are discontinued now.
I've got a heaping pile of NOS 160-degree Robertshaw thermostats to fit GM and Ford; they're good for the marine market where positive-displacement (not centrifugal) water pumps are used. The thermostat housing has a spring-loaded poppet valve to bypass lakewater coolant until the engine warms up. These engines will run at 100 or 120 degrees without a thermostat installed. Installing a thermostat without the special, marine housing and poppet-valve will ruin the neoprene impeller of a marine water pump.
Joined
·
8,349 Posts
Milodon makes good quality thermostats. I have used my self, installed or recomended it to friends and so far no one has failed and in some cases cured problems for friends with high flow waterpumps.
They look like the Robert shaw design, according to summit when I asked a while back they have Us as country as origin.
http://m.summitracing.com/parts/mil-16400
http://m.summitracing.com/parts/mil-16401
They look like the Robert shaw design, according to summit when I asked a while back they have Us as country as origin.
http://m.summitracing.com/parts/mil-16400
http://m.summitracing.com/parts/mil-16401
Joined
·
2,847 Posts
Interesting point from PRC:
It isn't often that two widely diverse problems can be resolved with the same solution. What's even more unusual is that the solution is very simple and inexpensive. The first problem is an air pocket created under the thermostat after installing a new motor or simply changing your coolant. This air pocket causes excessive heat build up in the engine until the air in the pocket is heated sufficiently to open the thermostat. This added heat is undesirable for any engine, but can be especially true for engines fitted with numerous aluminum components.
The second problem is thermal shock that may occur at the radiator when hot coolant is released by the thermostat when the ambient temperature is near 32 degrees F. Duane Davis, Owner of PRC, observed cracks developing in radiator cores in the area of the inlet in a few rare instances. He noted that this only occurred with customers located in Northern part of the United States and in Canada and only when the owner drove his car in frigid temperatures.
http://www.streetrodstuff.com/Articles/Cooling/Thermal_Challenges/
Another effect of the holes in the thermostat is that it reduces the loop gain at the crack point of the thermostat.
It isn't often that two widely diverse problems can be resolved with the same solution. What's even more unusual is that the solution is very simple and inexpensive. The first problem is an air pocket created under the thermostat after installing a new motor or simply changing your coolant. This air pocket causes excessive heat build up in the engine until the air in the pocket is heated sufficiently to open the thermostat. This added heat is undesirable for any engine, but can be especially true for engines fitted with numerous aluminum components.
The second problem is thermal shock that may occur at the radiator when hot coolant is released by the thermostat when the ambient temperature is near 32 degrees F. Duane Davis, Owner of PRC, observed cracks developing in radiator cores in the area of the inlet in a few rare instances. He noted that this only occurred with customers located in Northern part of the United States and in Canada and only when the owner drove his car in frigid temperatures.
http://www.streetrodstuff.com/Articles/Cooling/Thermal_Challenges/
Another effect of the holes in the thermostat is that it reduces the loop gain at the crack point of the thermostat.
What's interesting to me is how these problems could have continued so long without being discovered by the automotive engineers at FoMoCo, GM, and Chrysler. Apparently it was left up to the shade tree mechanics to discover not only the problems, but the very simple and inexpensive solution to both problems.
Where can I read about "loop gain at the crack point of the thermostat"? Sounds interesting.
Joined
·
2,847 Posts
The best place to start is to study valves and control systems. The crack point is the point at which the valve just begins to flow. Your basic automotive thermostat is a simple proportional control with hysteresis, and at the crack point the gain is very high since the flow goes to or from zero. If there is an auxiliary path the flow never goes to zero, and the change in flow per incremental move of the valve is reduced. Once the flow through the valve gets to many times the bypass flow, the effect is reduced.
If you had a similar valve controlling the fuel/air mixture to your engine, you would call it a throttle, and you would have a method of assuring that it never closed. If you were designing a boiler control system for a steamship, you would keep the fuel oil control valves from closing to the crack point to assure that your fires didn't go out.
1 - 18 of 18 Posts
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
-
?
