Tapping new engine mount bolt holes on stringers?

[demonboy]

Our new engine mounts need to be moved up by about 50mm, meaning we can't use the old bolt holes. Can I retap straight into the fibreglass or will I need to think about reinforcing with a steel plate, or what are the chances the whole thing is a stainless construction covered in fibreglass? A google image search shows all kinds of stringers, from steel to fibreglass to wood. The stringers are in good condition (not soggy).

 

[vyv_cox]

Our original mounts were tapped into the fibreglass but, being a Sadler, there may well be aluminium plates there incorporated into the layup. When our engine was replaced the mounts needed to be raised as well as moved, so the easiest solution was to add steel plates, 1 x 3 inches section, attached by the original holes but drilled and tapped for the new mounts.

 

[PetiteFleur]

I added a length of hardwood, fibreglassed to the bearers and then I used 10mm stainless coachbolts. Approved by the Beta engineer. The original engine had also used coachbolts.

 

[macd]

Hi, mate.
I think it depends on what's under your old bearers. When you write 'bolt holes' do you mean regular bolts (machine screws) or the sort of coach bolts PetitieFleur mentions? If the latter, the interior of the bearers will be specc'd for that method. So glassing hardwood onto the bearers and using long coachbolts (extending well into the bearers and engaging on both old bearers and new wood), would be fine. Beta specifiy the size of coachboalts for various of their engines: I can probably let you have the size for yours if you tell me it's size/weight. Our Beta 50 is held down in this way, although direct to the original bearers.

The glassing-on of the new wood needs to be substantial. I'd use epoxy and lots of cloth.

Do not rely on any fastener simple engaging on the skin of GRP. It won't be strong enough.

Vyv's approach is of course also fine, as you'd expect, but it might be easier to source hardwood than stainless where you are.

 

[PlankWalker]

Tapping into fiberglass would not be man enough for a 4 cylinder engine IMHO
I expect there would be a steel plate under the fiberglass.
Check with a magnet to locate the extent of any plate and if you are lucky it will extend under the proposed new feet, then all that's required is to drill and tap through the glass and into the steel plate.
If your not lucky, major surgery is required.

Plank

 

[Plum]

Our new engine mounts need to be moved up by about 50mm, meaning we can't use the old bolt holes. Can I retap straight into the fibreglass or will I need to think about reinforcing with a steel plate, or what are the chances the whole thing is a stainless construction covered in fibreglass? A google image search shows all kinds of stringers, from steel to fibreglass to wood. The stringers are in good condition (not soggy).

View attachment 58514

Really depends on what is inside the grp bearers. Wood? Are they hollow? Are there steel or aluminium plates under? (One of those diy metal/pipe detectors may help) and what sort of bolt did you remove, machine screw or coach bolt? More info would help.

Www.solocoastalsailing.co.uk

 

[demonboy]

Thanks for the replies.

Mac, it's a Beta 60.

 

[macd]

Mac, it's a Beta 60.

Can't seem to find the reference, but pretty sure it's 12mm.
(We have a 50: the missus likes red.)

 

[Norman_E]

I suggest a test drill in the right places, with a fairly small drill, say 5mm, to see what it meets.

 

[john_morris_uk]

If it's hard wood inside the GRP bearers , stainless studding set into holes with suitable epoxy is stronger than most people realise. I once mounted an engine this way on the recommendation of a surveyor. Perfect easy solution.

 

[demonboy]

Really depends on what is inside the grp bearers. Wood? Are they hollow? Are there steel or aluminium plates under? (One of those diy metal/pipe detectors may help) and what sort of bolt did you remove, machine screw or coach bolt? More info would help.

Www.solocoastalsailing.co.uk

I don't know what the construction is. I'm trying to find that out but originally there were steel plates underneath so we removed hex bolts, not coachbolts. Now that the engine is in position the mounts have moved up by 35mm (not 50mm as originally guessed). At the least the lower bolts should go back in through the original plate. The question is whether the plate extends long enough for the upper bolts. I can't do any magnet nest because the engine is in position unfortunately.

 

[demonboy]

I suggest a test drill in the right places, with a fairly small drill, say 5mm, to see what it meets.

Yep, gonna try that tomorrow, Norman.

 

[Plum]

I don't know what the construction is. I'm trying to find that out but originally there were steel plates underneath so we removed hex bolts, not coachbolts. Now that the engine is in position the mounts have moved up by 35mm (not 50mm as originally guessed). At the least the lower bolts should go back in through the original plate. The question is whether the plate extends long enough for the upper bolts. I can't do any magnet nest because the engine is in position unfortunately.

As you say, one set of bolts should go into new drilled and tapped holes that go into the existing steel backing plates. If your test holes for the other, forward, set of bolts reveal steel then you are in luck! If they hit just timber then you could fit coachbolts. If they go through the grp into a void then i recommend you drill, say, 40mm holes into the SIDE face of the bearer with a holesaw so you can slip in a large thick washer and a nut to bolt into. My Volvo Penta engine is bolted down in this way.

Www.solocoastalsailing.co.uk