2.5 inch heat riser / spacer question

[Dan B.]

Can someone please give the correct WIDTH of an original GM 2.5 " small block exhaust heat riser and F.I. spacer?? The repro spacer I have is 1 1/8 inches thick. All the original small block 2" ones I have are 1" thick and I'm curious as the exhaust studs are supposed to be the same length for all passenger side applications I thought.

The replacement studs have 1/4 " less usable thread (not threaded at the bottom all the way) and that plus the extra thickness of the spacer and the stops on the studs also going in too far, and the flange on the pipe being 1/8 thicker (side pipes) are causing the replacement stud to end up too short to catch the nut at the bottom.

I am going to make my own studs since the replacements available don't work here. Anyone installing 2.5 side pipes on stock manifolds seen this?

[Richard M.]

Re: 2.5 inch heat riser / spacer question

Dan I don't have the specs or pieces but some ideas.....

Is it possible that the studs are threaded too far into the manifold flange? I've seen that exact problem with some of the combinations of hardware and often by lowering the studs a bit solved the problem. Once the nuts catch and tightened, next time it's easier since the pipe flange tip ends are now "relaxed" from tightening the first time.

It's also possible the fiber donut is a bit larger than originals and again once it snugged up all fits better.

Rich

[Joe L.]

Re: 2.5 inch heat riser / spacer question

Can someone please give the correct WIDTH of an original GM 2.5 " small block exhaust heat riser and F.I. spacer?? The repro spacer I have is 1 1/8 inches thick. All the original small block 2" ones I have are 1" thick and I'm curious as the exhaust studs are supposed to be the same length for all passenger side applications I thought.

The replacement studs have 1/4 " less usable thread (not threaded at the bottom all the way) and that plus the extra thickness of the spacer and the stops on the studs also going in too far, and the flange on the pipe being 1/8 thicker (side pipes) are causing the replacement stud to end up too short to catch the nut at the bottom.

I am going to make my own studs since the replacements available don't work here. Anyone installing 2.5 side pipes on stock manifolds seen this?

Dan-------


Original left side studs had an overall length of 2-1/4" and the right side studs were 3-3/8". This is true for both small blocks and big blocks through at least 1974. However, exhaust manifold studs were not all thread. There was a threaded length of about 1/2" on one end and about 1-1/4" on the other end. Between the threads there was an unthreaded shank. On the longer threaded end there was a short, reduced diameter pilot and wrenching surface. The 1/2" threaded end screws into the manifold and controls the insertion depth.

If you are using studs of this configuration and lengths, there should be no problem using heat riser valves or spacers that are 1" or 1-1/8".

[Carl B.]

Re: 2.5 inch heat riser / spacer question

re exh spacer.NAPA part #ST45,Rockford Products,is the pilot end ,long stud.I sold them for my entire 42 year career with Genuine Parts Company.

[Dan B.]

Re: 2.5 inch heat riser / spacer question

Dan I don't have the specs or pieces but some ideas.....

Is it possible that the studs are threaded too far into the manifold flange? I've seen that exact problem with some of the combinations of hardware and often by lowering the studs a bit solved the problem. Once the nuts catch and tightened, next time it's easier since the pipe flange tip ends are now "relaxed" from tightening the first time.

It's also possible the fiber donut is a bit larger than originals and again once it snugged up all fits better.

Rich

Rich, Yes exactly right that was part of the problem. The replacement studs have a stop that allows them to go in about 1/4 inch too far.

Joe, thanks for that measurement of 3 3/8, the replacement studs are advertised as 3" but in reality only have 2 3/4 usable thread available. That was the other part of the problem.

So when these 2 things are combined it's a loss of 7/8 of an inch which is a no go for the nut. I decided to make my own studs out of SS and made them 3 1/2 which coincidentally was close to Joe's info. I will be using the longer brass nuts, so 3 1/2 works out well here with a reasonable amount of threads showing after the nut is tightened.

Many thanks for your replies guys!
I am sure some others may experience this issue in the future.

Here's a photo which explains it better (camera angle is a bit off) but the top stud is 3 1/2 inches and the bottom is 3" overall length:

[Dan B.]

Re: 2.5 inch heat riser / spacer question

One follow up question, is 1 1/8 " thickness correct for the repro spacer compared to original GM part?? Just curious ...........

[Joe L.]

Re: 2.5 inch heat riser / spacer question

One follow up question, is 1 1/8 " thickness correct for the repro spacer compared to original GM part?? Just curious ...........

Dan------


The thickness of original GM heat riser valves and spacers is, nominally, 1". However, this is not 1.000". They may vary by several thousandths although I would not expect as much as 1/8". Nevertheless, as I previously mentioned, the stock length studs, all other things being equal, should work just fine with either a 1" or 1-1/8" thickness.

The bottom stud in your photo looks to be about 3-1/4" in length if the ruler were placed at the very end of the stud. This is very close to the GM stock dimension and this stud should work fine all other things being equal.

I HIGHLY recommend the use of stainless steel studs and they are ABSOLUTELY the only type of studs I would use. EVER. However, I'm not talking about stainless steel studs made up from all thread, stainless steel rod stock. I'm talking about stainless steel studs of the stock configuration and length. These are available in the aftermarket.

Attached are photos of such studs installed on the right side of a big block with a reproduction spacer.


DSCN3843.jpgDSCN3844.jpg

[Dan B.]

Re: 2.5 inch heat riser / spacer question

Joe,
Thanks for that explanation and photos. Not much room to spare there.

[Joe L.]

Re: 2.5 inch heat riser / spacer question

Joe,
Thanks for that explanation and photos. Not much room to spare there.

Dan------

There does not need to be much in the way of exposed threads. Exposed threads do not accomplish anything. However, in this case, the cast stainless exhaust pipe flange plate is much thicker than an original GM flange plate. If a GM style plate were used, there would be a greater length of exposed threads. In addition, if a self-locking style nut were used, as original, there would be no lockwasher needed and that would add to the length of unused threads.

[Dan B.]

Re: 2.5 inch heat riser / spacer question

I was able to locate some other studs that measured 3 5/16 full thread (almost identical to the GM original spec Joe provided) and decided to use those with some SS washers and nuts. Done now.