Chevy 261 inline six - chasing coolant leak
 

Some of you may have seen my thread regarding moving the draft tube, at this point I am reinstalling my re-built (for the second time) 1959 261 Chevy inline six engine (read the thread if you want the gory details)

Now I have re-installed the engine, rebuilt the Stromberg two-barrel carb and have the engine timed and running nicely. However, I have a coolant leak that is bugging the hell out of me. When I first saw the leak, I pulled the radiator and used a black light flashlight and could only find coolant sitting in the bottom edge of the timing cover (front engine mount). I checked all the bolt tightness around the water pump and the timing cover and found one slightly loose bolt for the water pump, but the flashlight showed no traces of coolant coming from that area.

Hoping I had found it, I put the radiator back in and let it sit with coolant in it for several days and there was no leak. I just fired the engine back up, ran it for 15 minutes, got it up to temp and found a leak at the exact same spot at the base of the front timing cover.

My question is this, is there really a water jacket area near the front timing cover that could possibly be leaking? When I initially had the engine out a year ago and tore it down to a massive oil leak, it was coming from a cracked front main cap on the crank. With the timing cover off back then, I don't recall any water jacket access. Where could this coolant leak be coming from?

Re: Chevy 261 inline six - chasing coolant leak
 

Most of the bolt holes holding that plate behind the cam gear against the block open into the lower water jacket.. Did you put sealant on those bolts?

Take a look at the pictures in your old thread and you'll see the ones I'm talking about.

Re: Chevy 261 inline six - chasing coolant leak
 

I'm going to say put a pressure tester on it and get under it and watch to see if you can figure it out. Any leak inside the timing cover will get coolant in the oil.

I'm thinking that you may have a leak at the base of the water pump that trickles around the timing cover but is only a minute pressure leak.

The only bare block 261 photos I found on the net are those From a guy on Ebay selling a block that has 5 .040 over holes and 1 .060 over hole. eBay item number:165237459190

Looking at the images and the other images I found says that the water jacket should stop an inch or so above the crank if not more.

Re: Chevy 261 inline six - chasing coolant leak
 

Thank you both for your comments and info. Here is a pic of the installed engine and an arrow pointing to the area I am finding coolant with my blacklight flashlight.

Attachment 2420418

Here is a photo of the engine when I originally tore it out and tore it down, you can see the opening where the water pump was, and I recall that some of the bolt holes went into the water jacket. I seriously hope that you are right and one of those holes was not fully sealed by the engine builder and that is causing the coolant to leak along the timing cover and pool at the base of the timing cover.

Attachment 2420419

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I'm hoping that coolant was not finding its way into the timing cover and into the oil pan. Just to be sure, I plan on draining the oil tomorrow and looking to see if there is any coolant visible.

Thanks for the tips and reminding me I had photos of the front of the engine without the water pump and cam timing gear.

Re: Chevy 261 inline six - chasing coolant leak
 

Coolant should come out first if there is any in the pan and it hasn't mixed with the oil making a coolant/oil milkshake. Back in my broker than a church mouse days I had a beater with a cracked block and I'd jack it up every morning and drain the water out of the pan before driving it. I finally broke down and bought a can of Bar's leak and it lasted for a few months before I sold it to someone who wanted to do an engine swap in it.

I still think yours is an outside leak and probably a pressure leak and that may be because you are running more pressure in the system than a part is good for.

Re: Chevy 261 inline six - chasing coolant leak
 

I got up this morning and the first thing I did after my morning jolt of coffee was to drain the oil from the engine, looking for signs of coolant encroachment. Luckily as Jed Clampett from the Beverly Hillbillies would say, "nothin but black gold - Texas tea"

Attachment 2420487

I then drained the radiator and engine of coolant, then spent the next hour wiping up the coolant that didn't happen to hit the five gallon bucket. I have family coming to visit this afternoon, otherwise I'd pull the water pump and start my deep dive detective work.

As for pressure / boost, I suspect you're referring to the McCulloch supercharger. I didn't even have it running during my engine tests, one variable at a time, although even at full boost its only putting out about 8 psi.

More to come as the detective work progresses.

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Re: Chevy 261 inline six - chasing coolant leak
 

Given that I need to purchase two new water pump gaskets, one for the pump to the backing plate and one for the backing plate to the block. What are your thoughts on buying a new water pump? I really don't want to do this again anytime soon.

Re: Chevy 261 inline six - chasing coolant leak
 

You're going about it all wrong. You're assuming a gasket is at fault.. Assuming is not good diagnostic practice.. Draining the oil is ok, but before draining the coolant, you should conduct a pressure test of the cooling system.. This is done without the engine running. About 95% of the time, the leak will expose itself.

The pressure mr48chev is talking about is the cooling system pressure not boost pressure...

Re: Chevy 261 inline six - chasing coolant leak
 

I did try a pressure test last week but because I have an unusual/custom radiator, I couldn't get a good seal on the radiator cap.

Also, with the water pump off, I noted that all the holes for bolts to secure the water pump and backing plate are sealed by the original casting and not open to the water jacket. The only water access would be through the pump shaft itself.

Re: Chevy 261 inline six - chasing coolant leak
 

Correction, there is one hole that has access to the water jacket beyond the opening for the water pump shaft itself. See photos below:


Attachment 2420501

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Re: Chevy 261 inline six - chasing coolant leak
 

I've continued my investigation with the blacklight flashlight looking for signs of the leak origin. Since the coolant was pooling around the foot of the timing cover/front engine mount, I decided to remove the harmonic balancer/pulley to get a better look. Here is what I see -

Attachment 2420503

Thoughts?

Re: Chevy 261 inline six - chasing coolant leak
 

Water runs down hill.. Look higher.. Do whatever is necessary to put pressure on the cooling system.... Fabricate an adaptor from a radiator cap that fits the radiator.. Put an air line fitting in an old radiator hose and use a regulated air supply to introduce pressure.. With all due respect, your technique is just wandering around in the dark...

Re: Chevy 261 inline six - chasing coolant leak
 

I have some larger diameter rubber plugs showing up today to allow me to better do a pressure test.

Re: Chevy 261 inline six - chasing coolant leak
 

No offense taken, but I have attempted to eliminate one variable at a time.

I did attempt a pressure test, but when it failed I ordered rubber stoppers on Friday, that expand by the bolt threw them, so that I could pressure test properly.

While those were on order, I decided to eliminate the possibility that coolant had found its way into the block and the oil galleries. That mean draining the oil. Since it was clear, I moved on.

I drained the coolant from the radiator and block to get a better look at the water pump, since it is north of the timing cover and area where coolant was pooling. With the water pump and mounting plate removed, I don't see any obvious signs of a leak.

That brought me to removing the harmonic balancer to see if there was anything more obvious there, which was the photo of coolant around the bolts from the oil pan that seal the timing cover.

Since there isn't any water jacket access near the timing cover, as you suggest look higher, hence back to the water pump, since the thermostat housing and gasket is dry as a bone and there are zero leaks at any hose (radiator, thermostat, or heater)

Re: Chevy 261 inline six - chasing coolant leak
 

I'll make one more comment and then leave you to your own devices.. The cooling system must be fully intact, meaning no components are removed.. Hoses, radiator, water pump, etc, all must be in place.. If done properly, a cooling system test will reveal even a loose hose clamp.

Re: Chevy 261 inline six - chasing coolant leak
 

Yep, I understand that as I've done pressure testing on other cars where I replaced components, that's why I own the pressure testing setup, but always had a standard radiator that sealed properly. The plan is to remount the water pump with new gaskets and sealant, then remount the radiator and hoses to do another pressure test. More to come....

Re: Chevy 261 inline six - chasing coolant leak
 

as you have probably figured out from previous posts, the best way to figure out if you have a coolant leak, and where it's coming from, is to assemble your cooling system, pressure it up to a couple lbs more than the rad cap says and then walk away for awhile, like 15-20 minutes. get a decent tester and try it on something that you know is in good operating condition so you know the tester doesn't leak down. your system should hold that pressure for that time. if it doesn't then the first thing to check is the connection of the tester at the rad, to ensure it isn't allowing a leakdown. after that look at any and all connections for hoses, any gasket connection, the rad itself, etc. keep pumping the tester up and leaving the time for the leaks to appear. check the rad core closely for any damp spots, the underside of all the the connections for anything the same color as your coolant. if all is tight and you find no leaks, visually, then start looking internally, assuming your tester is still leaking down. look for any coolant that would show up in the oil pan. oil floats on coolant so the coolant will leak out of the oil pan plug first. hopefully that doesn't happen. after that test, check the cylinders. pull the spark plugs, place a rag or paper towel over the hole and tape it there so the air blowing out of the holes won't blow the rag/paper away, and then crank it over. if there is coolant in the cylinders it should show a spot on the rag/paper. if you still can't find evidence of a leak then place a big piece of cardboard under the engine and rad area, leave the oil plug out, ensure the rad is full, put the pressure tester on and pressure it up, walk away. set your alarm for an hour and recheck the tester. pump it back up as required. keep an eye on the cardboard for drops. hopefully you find something easy. if not there is always the dye trick. its a dye that goes in the coolant, you run the engine, then use a special light to show up any coolant that may have leaked out.

Re: Chevy 261 inline six - chasing coolant leak
 

sorry, I didn't realize you had already used the dye.

Re: Chevy 261 inline six - chasing coolant leak
 

when you install the water pump make sure to use sealant, not silicone, on the bolt threads if they go into coolant. I like to use aviation form a gasket on anything that touches coolant. any other bolts should be lubed or locktited if needed. no dry bolts should go into anything when assembling. either lube, sealant or loctite.
hopefully you find the cause. man, you have patience for this old truck. it's gonna be awesome for you to get out on the road with it and simply enjoy it.

Re: Chevy 261 inline six - chasing coolant leak
 

Yesterday, my son came over and as I was explaining the leak issue to him, I noticed that the underside of the thermostat housing, lit up brightly with my blacklight. You may not be able to see in the photo below, but since I have a bracket to hold the McCulloch supercharger, it is designed to mount directly to the head and then the thermostat housing mounts to it (using longer bolts to go through both the thermostat housing and the supercharger bracket into the head)

Attachment 2420598

Since it clearly showed symptoms of leaking I decided to take it apart too. Boy am I glad I did. While there is evidence of the engine builder using a paper gasket and sealant between the supercharger bracket and the head, all he used was a very thin layer of sealant between the bracket and the thermostat. When I took the thermostat off of the bracket/head, it was 75% metal to metal with the sealant having been squished out.

Fortunately, in my bag of "stuff" I found the correct size gasket and reinstalled the thermostat and the water pump this morning. There is now a gasket and a liberal amount of sealant between the thermostat and the supercharger bracket, as well as between the water pump and the mounting bracket and the mounting bracket and the engine block.

I'm letting it set overnight, will put the radiator back in tomorrow along with coolant and do a pressure test. Fingers crossed.


As for my patience with this truck, it is running thin..... I have been working on this truck for three years now and have signed up to take it to three car shows starting in mid-May. I did the same last year, but with the engine being rebuilt, that didn't happen. I'm hoping for better luck this year.

Re: Chevy 261 inline six - chasing coolant leak
 

sooo glad you found the leak and got it sealed up. it's hard to believe the builder didn't use a second gasket there but stuff happens and you just gotta let it go. I really hope thats it for leaks now and I'm sure you do to! maybe install the vibration damper/front pulley and anything else that is apart and needs sealing and then run some hot water over the front of the engine, top to bottom, to get rid of any traces of old coolant that can fool you later when doing your pressure test. blow it off and wipe it down so as not to leave water spots that can fool you or mark up the shiny paint. then install the rad etc
if you start to get frustrated with something like this sometimes its a good idea to simply walk away and come back to it in a few days. try to put it out of your mind so you can "cool off" and not do something you will regret later. sometimes we get tunnel vision when we are upset and we can easily overlook the obvious or simply write it off as already good in that area. when you come back to it try to think logically about the problem from a "start over" perspective, sorta like you are taking over a job from somebody else. start a list of things to check and go from there. with leaks I always start at the drip site and work my way upwards until there are no wet or damp spots. shiny new paint can hide leaks because stuff runs off new paint without much of a trace, it doesn't absorb like bare cast iron does. in the past I have used small pieces of paper towel placed at gasket joint areas on painted engines. these absorb the coolant or oil as it runs past and the liquid shows up on the paper towel pretty well so you can see it better. hopefully you won't have to do that. I suggest to clean it, dry it, assemble the cooling system part of things, fill it and burp it, then pressure test it. go have a coffee break for 15 minutes and come back to check the tester, assume you will have a leakdown someplace so you go into it with that mindset instead of getting frustrated if there is a leakdown. go after it systematically from top to bottom and don't forget to check the rad core etc. anything that has been apart runs the chance of having a new leak due to mechanical damage or simply not liking the being bolted up and flexed each time.
anyway, keep us posted with how it works out. you got this.

Re: Chevy 261 inline six - chasing coolant leak
 

Like you suggest, to find the culprit (short of the upcoming pressure test) I investigated all the areas where a leak could come from. As someone said, water runs down hill. So I started looking at the location of the puddle (the foot of the timing cover). Since the water jacket doesn't extend down to the cam and crank, I decided the one way to eliminate the timing cover itself from consideration was to drain the oil and look for any signs of water in the oiling system. Since it was completely clear, I moved upwards.

The next area was the water pump and all attendant hoses. Every hose clamp was dry, so I had to eliminate the water pump mounting to the backing plate and the backing plate to the block. It was sealed pretty darn tight and examination of both gaskets didn't reveal any smoking guns (or water leaks).

It was explaining the situation to my son that resulted in me looking closer at the thermostat housing. When I was able to see the underside and coolant staining, I then decided to remove it from the supercharger bracket. The gasket behind the bracket and its sealant is visible all the way around it, so I didn't suspect it. But when the thermostat housing came off of the supercharger bracket easily and I could see the lack of sealant or gasket between the two, especially on the underside, was when I realized the likely issue.

Luckily in my stash of excess gaskets and stuff, I found the appropriate gasket for the thermostat to the bracket or block. I used plenty of sealant between all gaskets and the engine block. I have all my fingers crossed (and my toes too) that when I install the radiator and coolant later today that the issue can be put to rest. But I will placing pieces of paper towel where I can to spot any possible leaks.