Logo
FULL SITE MENU

What length bolts for Mercruiser starter?

B

beb123

New member
I have a Mercruiser in my Ski Centurion v-drive. Not sure exactly what engine it is - presumably the standard (5L?). I got some water in the bilge so decided to swap out the starter as it's likely to fail sooner or later. Originally it had a small form factor starter, I'd replaced it with a large one a few years ago as that was the only one that the local parts shop had. I ordered a new starter (small size) and yesterday pulled off the old one (what a PITA that was!), but found that the bolts for the old big one are about 1.2 inch longer than the new one. What is the correct size bolts to get?

The starter has two long bolts in an offset pattern. The existing bolts are about 4 3/4" long overall with the shank and threads about 4 3/8". The mounting hole on the old one is about 3 5/8" long, the hole on the new one is about 3 1/4". I can't use the old ones as the unthreaded part of the shank protrudes from the mounting hole in the starter.

thx
 
Uh, maybe 1.2 inches shorter? Good rule of thumb is to have about 3/4 of an inch of bolt going into the engine. so measure the new starter and add 3/4 inch and you'll be pretty much right on. Remember bolts are measured over the total length, not the length under the head. I highly recommend the use of 316 stainless bolts in this application especially if this boat sees salty water. Order them from Mcmaster Carr
 
o2batsea said:
Uh, maybe 1.2 inches shorter? Good rule of thumb is to have about 3/4 of an inch of bolt going into the engine. so measure the new starter and add 3/4 inch and you'll be pretty much right on. Remember bolts are measured over the total length, not the length under the head. I highly recommend the use of 316 stainless bolts in this application especially if this boat sees salty water. Order them from Mcmaster Carr
Click to expand...

Thanks... I guess what I should have asked is if it's unusual for the new starter to need a different length bolt? I'd have guessed they'd be the same even with a different form factor. My typo also, the old starter is about 3/8" longer than the new. New measures 3 1/4", so with your rule of thumb I need 4" plus the head. Old ones are 4 3/8" plus the head.

Boat never sees salt but I'll get good bolts. Appreciate the tips and link.
 
....................
beb123 said:
...... Originally it had a small form factor starter, I'd replaced it with a large one a few years ago as that was the only one that the local parts shop had.
I will assume that the original was a HTGR/PMGR starter motor, and that you replaced it with a non-gear reduction motor.


I ordered a new starter (small size) and yesterday pulled off the old one (what a PITA that was!), but found that the bolts for the old big one are about 1.2 inch longer than the new one. What is the correct size bolts to get?
OK..... there is no correct answer because there are many starter motor manufacturers, and from many areas around the world.
Each manufacturer designs their own unique nose housing..... therefor each bolt length varies some.

Please DO NOT take anyone's word on bolt length. Take your new starter motor with you to a parts store and look at their assortment of knurled starter motor bolts.
Make your selection based on the knurled area being in the correct place (it acts like an alignment dowel), and the amount of threads that protrude through the nose housing.
Also, please note that Stainless Steel bolts are not as strong as carbon steel bolts.
Click to expand...

beb123 said:
Thanks... I guess what I should have asked is if it's unusual for the new starter to need a different length bolt?
No.... not at all!


Boat never sees salt but I'll get good bolts. Appreciate the tips and link.
Use a thread sealant on the bolt threads. It will help prevent corrosion and/or rust.
Click to expand...


Also, see the image below for when measuring a bolt for it's length.
 

Attachments

  • bolt length measurement.jpg
    bolt length measurement.jpg
    52.2 KB · Views: 142
Last edited:
Aren't those special bolts with a cross hatched, enlarged shank area?

Jeff

PS: Assuming this is a bolt up(wards) starter
 
........................
fastjeff said:
Aren't those special bolts with a cross hatched, enlarged shank area? Assuming this is a bolt up(wards) starter
Yes..... the area of the bolt that is knurled acts as an alignment dowel! It's rather crude but it works!
Click to expand...
 

Attachments

  • Starter motor bolt lengths explained.jpg
    Starter motor bolt lengths explained.jpg
    58.5 KB · Views: 809
The other way is to install a bolt and snug it, measure from the block to the bolt head, then measure the starters flange.The starter MUST be longer than the measurement (ABC,XYZ)otherwise the starter will not tighten and will break the bolts
 
I think that we are both suggesting the same thing:
Install a bolt minus the starter motor.
Measure the extending bolt shank and compare that to the nose housing's bore length.

Each bolt should be able to fully tighten down on the nose housing prior to the bolt threads bottoming-out in the cylinder block.


Just keep in mind that the nose housings from one manufacturer to another may be different..... therefor they each brand may require a different bolt length.
Better yet..... when ordering a new HTGR/PMGR motor, ask for the correct bolts to be sent with it.



.
 
Last edited:
Also, see the image below for when measuring a bolt for it's length.
Click to expand...
My bad. I am not thinking SAE. I've been buying nuts and bolts for the Land Rovers lately (just got another 1968 88 2A) and all that BSF and Whitworth stuff is the total length of the bolt.
 
Last edited:
IMG_5047.jpg
The image got me thinking - here is the new starter with the old bolts. There is a knurled part that extends out, which is what made me think that the bolts are too long. But the drawing above shows a portion of the hole in the engine mount that the knurled part goes into - I'd assumed it was threaded the entire way. Maybe when I'd originally swapped the starter to the large size I'd used the old bolts from the smaller starter... so they may fit now! At any rate I'll do the real-world test by screwing the bolts in the block and measuring the exposed area. Thanks again all.
 
.........................
beb123 said:
The image got me thinking - here is the new starter with the old bolts. There is a knurled part that extends out, which is what made me think that the bolts are too long. But the drawing above shows a portion of the hole in the engine mount that the knurled part goes into
Yes.... exactly!

I'd assumed it was threaded the entire way.
No..... the first portions of both bolt bores are NOT threaded.
That area is larger and is minus any threads as to accept the doweled area of the bolt..... thus creating the alignment.

Maybe when I'd originally swapped the starter to the large size I'd used the old bolts from the smaller starter... so they may fit now! At any rate I'll do the real-world test by screwing the bolts in the block and measuring the exposed area.
Yes...... that would be best, and is what Bt Doctur and I have suggested.

Thanks again all.
Click to expand...

Also, if the bolts are too long, and if you are in a pinch........ you can add an SAE washer above the bolt head as to compensate for the excessive length.
However, in looking at your photo, it looks like that bolt is OK.

View attachment 16832
 
Back
Top